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  • doc/generic_types/generic_types.tex

    rf674479 ra0fc78a  
    228228\end{lstlisting}
    229229Within the block, the nested version of @<@ performs @>@ and this local version overrides the built-in @<@ so it is passed to @qsort@.
    230 Hence, programmers can easily form local environments, adding and modifying appropriate functions, to maximize reuse of other existing functions and types.
     230Hence, programmers can easily form a local environments, adding and modifying appropriate functions, to maximize reuse of other existing functions and types.
    231231
    232232Finally, \CFA allows variable overloading:

  • doc/rob_thesis/cfa-format.tex

    rf674479 ra0fc78a  
    7272  morecomment=[n]{/+}{+/},
    7373  morecomment=[n][\color{blue}]{/++}{+/},
    74   % Options
    75   sensitive=true
    76 }
    77 
    78 \lstdefinelanguage{rust}{
    79   % Keywords
    80   morekeywords=[1]{
    81     abstract, alignof, as, become, box,
    82     break, const, continue, crate, do,
    83     else, enum, extern, false, final,
    84     fn, for, if, impl, in,
    85     let, loop, macro, match, mod,
    86     move, mut, offsetof, override, priv,
    87     proc, pub, pure, ref, return,
    88     Self, self, sizeof, static, struct,
    89     super, trait, true,  type, typeof,
    90     unsafe, unsized, use, virtual, where,
    91     while, yield
    92   },
    93   % Strings
    94   morestring=[b]{"},
    95   % Comments
    96   comment=[l]{//},
    97   morecomment=[s]{/*}{*/},
    9874  % Options
    9975  sensitive=true
    … …  
    184160}{}
    185161
    186 \lstnewenvironment{rustcode}[1][]{
    187   \lstset{
    188     language = rust,
    189     style=defaultStyle,
    190     #1
    191   }
    192 }{}
    193 
    194162\newcommand{\zero}{\lstinline{zero_t}\xspace}
    195163\newcommand{\one}{\lstinline{one_t}\xspace}

  • doc/rob_thesis/conclusions.tex

    rf674479 ra0fc78a  
    33%======================================================================
    44
    5 \section{Constructors and Destructors}
    6 \CFA supports the RAII idiom using constructors and destructors.
    7 There are many engineering challenges in introducing constructors and destructors, partially since \CFA is not an object-oriented language.
    8 By making use of managed types, \CFA programmers are afforded an extra layer of safety and ease of use in comparison to C programmers.
    9 While constructors and destructors provide a sensible default behaviour, \CFA allows experienced programmers to declare unmanaged objects to take control of object management for performance reasons.
    10 Constructors and destructors as named functions fit the \CFA polymorphism model perfectly, allowing polymorphic code to use managed types seamlessly.
    11 
    12 \section{Tuples}
    13 \CFA can express functions with multiple return values in a way that is simple, concise, and safe.
    14 The addition of multiple-return-value functions naturally requires a way to use multiple return values, which begets tuple types.
    15 Tuples provide two useful notions of assignment: multiple assignment, allowing simple, yet expressive assignment between multiple variables, and mass assignment, allowing a lossless assignment of a single value across multiple variables.
    16 Tuples have a flexible structure that allows the \CFA type-system to decide how to restructure tuples, making it syntactically simple to pass tuples between functions.
    17 Tuple types can be combined with polymorphism and tuple conversions can apply during assertion inference to produce a cohesive feel.
    18 
    19 \section{Variadic Functions}
    20 Type-safe variadic functions of a similar feel to variadic templates are added to \CFA.
    21 The new variadic functions can express complicated recursive algorithms.
    22 Unlike variadic templates, it is possible to write @new@ as a library routine and to separately compile @ttype@ polymorphic functions.
    23 Variadic functions are statically type checked and provide a user experience that is consistent with that of tuples and polymorphic functions.
    24 
    25 \section{Future Work}
    26 \subsection{Constructors and Destructors}
    27 Both \CC and Rust support move semantics, which expand the user's control of memory management by providing the ability to transfer ownership of large data, rather than forcing potentially expensive copy semantics.
    28 \CFA currently does not support move semantics, partially due to the complexity of the model.
    29 The design space is currently being explored with the goal of finding an alternative to move semantics that provides necessary performance benefits, while reducing the amount of repetition required to create a new type, along with the cognitive burden placed on the user.
    30 
    31 Exception handling is among the features expected to be added to \CFA in the near future.
    32 For exception handling to properly interact with the rest of the language, it must ensure all RAII guarantees continue to be met.
    33 That is, when an exception is raised, it must properly unwind the stack by calling the destructors for any objects that live between the raise and the handler.
    34 This can be accomplished either by augmenting the translator to properly emit code that executes the destructors, or by switching destructors to hook into the GCC @cleanup@ attribute \cite[6.32.1]{GCCExtensions}.
    35 
    36 The @cleanup@ attribute, which is attached to a variable declaration, takes a function name as an argument and schedules that routine to be executed when the variable goes out of scope.
    37 \begin{cfacode}
    38 struct S { int x; };
    39 void __dtor_S(struct S *);
    40 {
    41   __attribute__((cleanup(__dtor_S))) struct S s;
    42 } // calls __dtor_S(&s)
    43 \end{cfacode}
    44 This mechanism is known and understood by GCC, so that the destructor is properly called in any situation where a variable goes out of scope, including function returns, branches, and built-in GCC exception handling mechanisms using libunwind.
    45 
    46 A caveat of this approach is that the @cleanup@ attribute only permits a name that refers to a function that consumes a single argument of type @T *@ for a variable of type @T@.
    47 This means that any destructor that consumes multiple arguments (e.g., because it is polymorphic) or any destructor that is a function pointer (e.g., because it is an assertion parameter) must be called through a local thunk.
    48 For example,
    49 \begin{cfacode}
    50 forall(otype T)
    51 struct Box {
    52   T x;
    53 };
    54 forall(otype T) void ^?{}(Box(T) * x);
    55 
    56 forall(otype T)
    57 void f(T x) {
    58   T y = x;
    59   Box(T) z = { x };
    60 }
    61 \end{cfacode}
    62 currently generates the following
    63 \begin{cfacode}
    64 void _dtor_BoxT(  // consumes more than 1 parameter due to assertions
    65   void (*_adapter_PTT)(void (*)(), void *, void *),
    66   void (*_adapter_T_PTT)(void (*)(), void *, void *, void *),
    67   long unsigned int _sizeof_T,
    68   long unsigned int _alignof_T,
    69   void *(*_assign_T_PTT)(void *, void *),
    70   void (*_ctor_PT)(void *),
    71   void (*_ctor_PTT)(void *, void *),
    72   void (*_dtor_PT)(void *),
    73   void *x
    74 );
    75 
    76 void f(
    77   void (*_adapter_PTT)(void (*)(), void *, void *),
    78   void (*_adapter_T_PTT)(void (*)(), void *, void *, void *),
    79   long unsigned int _sizeof_T,
    80   long unsigned int _alignof_T,
    81   void *(*_assign_TT)(void *, void *),
    82   void (*_ctor_T)(void *),
    83   void (*_ctor_TT)(void *, void *),
    84   void (*_dtor_T)(void *),
    85   void *x
    86 ){
    87   void *y = __builtin_alloca(_sizeof_T);
    88   // constructor call elided
    89 
    90   // generic layout computation elided
    91   long unsigned int _sizeof_BoxT = ...;
    92   void *z = __builtin_alloca(_sizeof_BoxT);
    93   // constructor call elided
    94 
    95   _dtor_BoxT(  // ^?{}(&z); -- _dtor_BoxT has > 1 arguments
    96     _adapter_PTT,
    97     _adapter_T_PTT,
    98     _sizeof_T,
    99     _alignof_T,
    100     _assign_TT,
    101     _ctor_T,
    102     _ctor_TT,
    103     _dtor_T,
    104     z
    105   );
    106   _dtor_T(y);  // ^?{}(&y); -- _dtor_T is a function pointer
    107 }
    108 \end{cfacode}
    109 Further to this point, every distinct array type will require a thunk for its destructor, where array destructor code is currently inlined, since array destructors hard code the length of the array.
    110 
    111 For function call temporaries, new scopes have to be added for destructor ordering to remain consistent.
    112 In particular, the translator currently destroys argument and return value temporary objects as soon as the statement they were created for ends.
    113 In order for this behaviour to be maintained, new scopes have to be added around every statement that contains a function call.
    114 Since a nested expression can raise an exception, care must be taken when destroying temporary objects.
    115 One way to achieve this is to split statements at every function call, to provide the correct scoping to destroy objects as necessary.
    116 For example,
    117 \begin{cfacode}
    118 struct S { ... };
    119 void ?{}(S *, S);
    120 void ^?{}(S *);
    121 
    122 S f();
    123 S g(S);
    124 
    125 g(f());
    126 \end{cfacode}
    127 would generate
    128 \begin{cfacode}
    129 struct S { ... };
    130 void _ctor_S(struct S *, struct S);
    131 void _dtor_S(struct S *);
    132 
    133 {
    134   __attribute__((cleanup(_dtor_S))) struct S _tmp1 = f();
    135   __attribute__((cleanup(_dtor_S))) struct S _tmp2 =
    136     (_ctor_S(&_tmp2, _tmp1), _tmp2);
    137   __attribute__((cleanup(_dtor_S))) struct S _tmp3 = g(_tmp2);
    138 } // destroy _tmp3, _tmp2, _tmp1
    139 \end{cfacode}
    140 Note that destructors must be registered after the temporary is fully initialized, since it is possible for initialization expressions to raise exceptions, and a destructor should never be called on an uninitialized object.
    141 This requires a slightly strange looking initializer for constructor calls, where a comma expression is used to produce the value of the object being initialized, after the constructor call, conceptually bitwise copying the initialized data into itself.
    142 Since this copy is wholly unnecessary, it is easily optimized away.
    143 
    144 A second approach is to attach an accompanying boolean to every temporary that records whether the object contains valid data, and thus whether the value should be destructed.
    145 \begin{cfacode}
    146 struct S { ... };
    147 void _ctor_S(struct S *, struct S);
    148 void _dtor_S(struct S *);
    149 
    150 struct __tmp_bundle_S {
    151   bool valid;
    152   struct S value;
    153 };
    154 
    155 void _dtor_tmpS(struct __tmp_bundle_S * ret) {
    156   if (ret->valid) {
    157     _dtor_S(&ret->value);
    158   }
    159 }
    160 
    161 {
    162   __attribute__((cleanup(_dtor_tmpS))) struct __tmp_bundle_S _tmp1 = { 0 };
    163   __attribute__((cleanup(_dtor_tmpS))) struct __tmp_bundle_S _tmp2 = { 0 };
    164   __attribute__((cleanup(_dtor_tmpS))) struct __tmp_bundle_S _tmp3 = { 0 };
    165   _tmp2.value = g(
    166     (_ctor_S(
    167       &_tmp2.value,
    168       (_tmp1.value = f(), _tmp1.valid = 1, _tmp1.value)
    169     ), _tmp2.valid = 1, _tmp2.value)
    170   ), _tmp3.valid = 1, _tmp3.value;
    171 } // destroy _tmp3, _tmp2, _tmp1
    172 \end{cfacode}
    173 In particular, the boolean is set immediately after argument construction and immediately after return value copy.
    174 The boolean is checked as a part of the @cleanup@ routine, forwarding to the object's destructor if the object is valid.
    175 One such type and @cleanup@ routine needs to be generated for every type used in a function parameter or return value.
    176 
    177 The former approach generates much simpler code, however splitting expressions requires care to ensure that expression evaluation order does not change.
    178 Expression ordering has to be performed by a full compiler, so it is possible that the latter approach would be more suited to the \CFA prototype, whereas the former approach is clearly the better option in a full compiler.
    179 More investigation is needed to determine whether the translator's current design can easily handle proper expression ordering.
    180 
    181 As discussed in Section \ref{s:implicit_copy_construction}, return values are destructed with a different @this@ pointer than they are constructed with.
    182 This problem can be easily fixed once a full \CFA compiler is built, since it would have full control over the call/return mechanism.
    183 In particular, since the callee is aware of where it needs to place the return value, it can construct the return value directly, rather than bitwise copy the internal data.
    184 
    185 Currently, the special functions are always auto-generated, except for generic types where the type parameter does not have assertions for the corresponding operation.
    186 For example,
    187 \begin{cfacode}
    188 forall(dtype T | sized(T) | { void ?{}(T *); })
    189 struct S { T x; };
    190 \end{cfacode}
    191 will only auto-generate the default constructor for @S@, since the member @x@ is missing the other 3 special functions.
    192 Once deleted functions have been added, function generation can make use of this information to disable generation of special functions when a member has a deleted function.
    193 For example,
    194 \begin{cfacode}
    195 struct A {};
    196 void ?{}(A *) = delete;
    197 struct S { A x; };  // does not generate void ?{}(S *);
    198 \end{cfacode}
    199 
    200 Unmanaged objects and their interactions with the managed \CFA environment are an open problem that deserves greater attention.
    201 In particular, the interactions between unmanaged objects and copy semantics are subtle and can easily lead to errors.
    202 It is possible that the compiler should mark some of these situations as errors by default, and possibly conditionally emit warnings for some situations.
    203 Another possibility is to construct, destruct, and assign unmanaged objects using the intrinsic and auto-generated functions.
    204 A more thorough examination of the design space for this problem is required.
    205 
    206 Currently, the \CFA translator does not support any warnings.
    207 Ideally, the translator should support optional warnings in the case where it can detect that an object has been constructed twice.
    208 For example, forwarding constructor calls are guaranteed to initialize the entire object, so redundant constructor calls can cause problems such as memory leaks, while looking innocuous to a novice user.
    209 \begin{cfacode}
    210 struct B { ... };
    211 struct A {
    212         B x, y, z;
    213 };
    214 void ?{}(A * a, B x) {
    215         // y, z implicitly default constructed
    216         (&a->x){ ... }; // explicitly construct x
    217 } // constructs an entire A
    218 void ?{}(A * a) {
    219         (&a->y){}; // initialize y
    220         a{ (B){ ... } }; // forwarding constructor call
    221                          // initializes entire object, including y
    222 }
    223 \end{cfacode}
    224 
    225 Finally, while constructors provide a mechanism for establishing invariants, there is currently no mechanism for maintaining invariants without resorting to opaque types.
    226 That is, structure fields can be accessed and modified by any block of code without restriction, so while it's possible to ensure that an object is initially set to a valid state, it isn't possible to ensure that it remains in a consistent state throughout its lifetime.
    227 A popular technique for ensuring consistency in object-oriented programming languages is to provide access modifiers such as @private@, which provides compile-time checks that only privileged code accesses private data.
    228 This approach could be added to \CFA, but it requires an idiomatic way of specifying what code is privileged.
    229 One possibility is to tie access control into an eventual module system.
    230 
    231 \subsection{Tuples}
    232 Named result values are planned, but not yet implemented.
    233 This feature ties nicely into named tuples, as seen in D and Swift.
    234 
    235 Currently, tuple flattening and structuring conversions are 0-cost.
    236 This makes tuples conceptually very simple to work with, but easily causes unnecessary ambiguity in situations where the type system should be able to differentiate between alternatives.
    237 Adding an appropriate cost function to tuple conversions will allow tuples to interact with the rest of the programming language more cohesively.
    238 
    239 \subsection{Variadic Functions}
    240 Use of @ttype@ functions currently relies heavily on recursion.
    241 \CC has opened variadic templates up so that recursion isn't strictly necessary in some cases, and it would be interesting to see if any such cases can be applied to \CFA.
    242 
    243 \CC supports variadic templated data types, making it possible to express arbitrary length tuples, arbitrary parameter function objects, and more with generic types.
    244 Currently, \CFA does not support @ttype@-parameter generic types, though there does not appear to be a technical reason that it cannot.
    245 Notably, opening up support for this makes it possible to implement the exit form of scope guard (see section \ref{s:ResMgmt}), making it possible to call arbitrary functions at scope exit in idiomatic \CFA.
     5Conclusion paragraphs.

  • doc/rob_thesis/ctordtor.tex

    rf674479 ra0fc78a  
    33%======================================================================
    44
    5 % TODO now: as an experiment, implement Andrei Alexandrescu's ScopeGuard http://www.drdobbs.com/cpp/generic-change-the-way-you-write-excepti/184403758?pgno=2
     5% TODO: discuss move semantics; they haven't been implemented, but could be. Currently looking at alternative models. (future work)
     6
     7% TODO: as an experiment, implement Andrei Alexandrescu's ScopeGuard http://www.drdobbs.com/cpp/generic-change-the-way-you-write-excepti/184403758?pgno=2
    68% doesn't seem possible to do this without allowing ttype on generic structs?
    79
     10% If a Cforall constructor is in scope, C style initialization is
     11% disabled by default.
     12% * initialization rule: if any constructor is in scope for type T, try
     13%   to find a matching constructor for the call. If there are no
     14%   constructors in scope for type T, then attempt to fall back on
     15%   C-style initialization.
     16% + if this rule was not in place, it would be easy to accidentally
     17%   use C-style initialization in certain cases, which could lead to
     18%   subtle errors [2]
     19% - this means we need special syntax if we want to allow users to force
     20%   a C-style initialization (to give users more control)
     21% - two different declarations in the same scope can be implicitly
     22%   initialized differently. That is, there may be two objects of type
     23%   T that are initialized differently because there is a constructor
     24%   definition between them. This is not technically specific to
     25%   constructors.
     26
     27% C-style initializers can be accessed with @= syntax
     28% + provides a way to get around the requirement of using a constructor
     29%   (for advanced programmers only)
     30% - can break invariants in the type => unsafe
     31% * provides a way of asserting that a variable is an instance of a
     32%   C struct (i.e. a POD struct), and so will not be implicitly
     33%   destructed (this can be useful at times, maybe mitigates the need
     34%   for move semantics?) [3]
     35% + can modernize a code base one step at a time
     36
     37% Cforall constructors can be used in expressions to initialize any
     38% piece of memory.
     39% + malloc() { ... } calls the appropriate constructor on the newly
     40%   allocated space; the argument is moved into the constructor call
     41%   without taking its address [4]
     42% - with the above form, there is still no way to ensure that
     43%   dynamically allocated objects are constructed. To resolve this,
     44%   we might want a stronger "new" function which always calls the
     45%   constructor, although how we accomplish that is currently still
     46%   unresolved (compiler magic vs. better variadic functions?)
     47% + This can be used as a placement syntax [5]
     48% - can call the constructor on an object more than once, which could
     49%   cause resource leaks and reinitialize const fields (can try to
     50%   detect and prevent this in some cases)
     51%   * compiler always tries to implicitly insert a ctor/dtor pair for
     52%     non-@= objects.
     53%     * For POD objects, this will resolve to an autogenerated or
     54%       intrinsic function.
     55%     * Intrinsic functions are not automatically called. Autogenerated
     56%       are, because they may call a non-autogenerated function.
     57%     * destructors are automatically inserted at appropriate branches
     58%       (e.g. return, break, continue, goto) and at the end of the block
     59%       in which they are declared.
     60%   * For @= objects, the compiler never tries to interfere and insert
     61%     constructor and destructor calls for that object. Copy constructor
     62%     calls do not count, because the object is not the target of the copy
     63%     constructor.
     64
     65% A constructor is declared with the name ?{}
     66% + combines the look of C initializers with the precedent of ?() being
     67%   the name for the function call operator
     68% + it is possible to easily search for all constructors in a project
     69%   and immediately know that a function is a constructor by seeing the
     70%   name "?{}"
     71
     72% A destructor is declared with the name ^?{}
     73% + name mirrors a constructor's name, with an extra symbol to
     74%   distinguish it
     75% - the symbol '~' cannot be used due to parsing conflicts with the
     76%   unary '~' (bitwise negation) operator - this conflict exists because
     77%   we want to allow users to write ^x{}; to destruct x, rather than
     78%   ^?{}(&x);
     79
     80% The first argument of a constructor must be a pointer. The constructed
     81% type is the base type of the pointer. E.g. void ?{}(T *) is a default
     82% constructor for a T.
     83% + can name the argument whatever you like, so not constrained by
     84%   language keyword "this" or "self", etc.
     85% - have to explicitly qualify all object members to initialize them
     86%   (e.g. this->x = 0, rather than just x = 0)
     87
     88% Destructors can take arguments other than just the destructed pointer
     89% * open research problem: not sure how useful this is
     90
     91% Pointer constructors
     92% + can construct separately compiled objects (opaque types) [6]
     93% + orthogonal design, follows directly from the definition of the first
     94%   argument of a constructor
     95% - may require copy constructor or move constructor (or equivalent)
     96%   for correct implementation, which may not be obvious to everyone
     97% + required feature for the prelude to specify as much behavior as possible
     98%   (similar to pointer assignment operators in this respect)
     99
     100% Designations can only be used for C-style initialization
     101% * designation for constructors is equivalent to designation for any
     102%   general function call. Since a function prototype can be redeclared
     103%   many times, with arguments named differently each time (or not at
     104%   all!), this is considered to be an undesirable feature. We could
     105%   construct some set of rules to allow this behaviour, but it is
     106%   probably more trouble than it's worth, and no matter what we choose,
     107%   it is not likely to be obvious to most people.
     108
     109% Constructing an anonymous member [7]
     110% + same as with calling any other function on an anonymous member
     111%   (implicit conversion by the compiler)
     112% - may be some cases where this is ambiguous => clarify with a cast
     113%   (try to design APIs to avoid sharing function signatures between
     114%   composed types to avoid this)
     115
     116% Default Constructors and Destructors are called implicitly
     117% + cannot forget to construct or destruct an object
     118% - requires special syntax to specify that an object is not to be
     119%   constructed (@=)
     120% * an object will not be implicitly constructed OR destructed if
     121%   explicitly initialized like a C object (@= syntax)
     122% * an object will be destructed if there are no constructors in scope
     123%   (even though it is initialized like a C object) [8]
     124
     125% An object which changes from POD type to non POD type will not change
     126% the semantics of a type containing it by composition
     127% * That is, constructors will not be regenerated at the point where
     128%   an object changes from POD type to non POD type, because this could
     129%   cause a cascade of constructors being regenerated for many other
     130%   types. Further, there is precedence for this behaviour in other
     131%   facets of Cforall's design, such as how nested functions interact.
     132% * This behaviour can be simplified in a language without declaration
     133%   before use, because a type can be classified as POD or non POD
     134%   (rather than potentially changing between the two at some point) at
     135%   at the global scope (which is likely the most common case)
     136% * [9]
     137
     138% Changes to polymorphic type classes
     139% * dtype and ftype remain the same
     140% * forall(otype T) is currently essentially the same as
     141%   forall(dtype T | { @size(T); void ?=?(T *, T); }).
     142%   The big addition is that you can declare an object of type T, rather
     143%   than just a pointer to an object of type T since you know the size,
     144%   and you can assign into a T.
     145%   * this definition is changed to add default constructor and
     146%     destructor declarations, to remain consistent with what type meant
     147%     before the introduction of constructors and destructors.
     148%     * that is, forall(type T) is now essentially the same as
     149%       forall(dtype T | { @size(T); void ?=?(T *, T);
     150%                          void ?{}(T *); void ^?{}(T *); })
     151%     + this is required to make generic types work correctly in
     152%       polymorphic functions
     153%     ? since declaring a constructor invalidates the autogenerated
     154%       routines, it is possible for a type to have constructors, but
     155%       not default constructors. That is, it might be the case that
     156%       you want to write a polymorphic function for a type which has
     157%       a size, but non-default constructors? Some options:
     158%       * declaring a constructor as a part of the assertions list for
     159%         a type declaration invalidates the default, so
     160%         forall(otype T | { void ?{}(T *, int); })
     161%         really means
     162%         forall(dtype T | { @size(T); void ?=?(T *, T);
     163%                            void ?{}(T *, int); void ^?{}(T *); })
     164%       * force users to fully declare the assertions list like the
     165%         above in this case (this seems very undesirable)
     166%       * add another type class with the current desugaring of type
     167%         (just size and assignment)
     168%       * provide some way of subtracting from an existing assertions
     169%         list (this might be useful to have in general)
     170
     171% Implementation issues:
     172% Changes to prelude/autogen or built in defaults?
     173% * pointer ctors/dtors [prelude]
     174%   * other pointer type routines are declared in the prelude, and this
     175%     doesn't seem like it should be any different
     176% * basic type ctors/dtors [prelude]
     177%   * other basic type routines are declared in the prelude, and this
     178%     doesn't seem like it should be any different
     179% ? aggregate types [undecided, but leaning towards autogenerate]
     180%   * prelude
     181%     * routines specific to aggregate types cannot be predeclared in
     182%       the prelude because we don't know the name of every
     183%       aggregate type in the entire program
     184%   * autogenerate
     185%     + default assignment operator is already autogenerated for
     186%       aggregate types
     187%       * this seems to lead us in the direction of autogenerating,
     188%         because we may have a struct which contains other objects
     189%         that require construction [10]. If we choose not to
     190%         autogenerate in this case, then objects which are part of
     191%         other objects by composition will not be constructed unless
     192%         a constructor for the outer type is explicitly defined
     193%       * in this case, we would always autogenerate the appropriate
     194%         constructor(s) for an aggregate type, but just like with
     195%         basic types, pointer types, and enum types, the constructor
     196%         call can be elided when when it is not necessary.
     197%     + constructors will have to be explicitly autogenerated
     198%       in the case where they are required for a polymorphic function,
     199%       when no user defined constructor is in scope, which may make it
     200%       easiest to always autogenerate all appropriate constructors
     201%     - n+2 constructors would have to be generated for a POD type
     202%       * one constructor for each number of valid arguments [0, n],
     203%         plus the copy constructor
     204%         * this is taking a simplified approach: in C, it is possible
     205%           to omit the enclosing braces in a declaration, which would
     206%           lead to a combinatorial explosion of generated constructors.
     207%           In the interest of keeping things tractable, Cforall may be
     208%           incompatible with C in this case. [11]
     209%       * for non-POD types, only autogenerate the default and copy
     210%         constructors
     211%       * alternative: generate only the default constructor and
     212%         special case initialization for any other constructor when
     213%         only the autogenerated one exists
     214%         - this is not very sensible, as by the previous point, these
     215%           constructors may be needed for polymorphic functions
     216%           anyway.
     217%     - must somehow distinguish in resolver between autogenerated and
     218%       user defined constructors (autogenerated should never be chosen
     219%       when a user defined option exists [check first parameter], even
     220%       if full signature differs) (this may also have applications
     221%       to other autogenerated routines?)
     222%     - this scheme does not naturally support designation (i.e. general
     223%       functions calls do not support designation), thus these cases
     224%       will have to be treated specially in either case
     225%   * defaults
     226%     * i.e. hardcode a new set of rules for some "appropriate" default
     227%       behaviour for
     228%     + when resolving an initialization expression, explicitly check to
     229%       see if any constructors are in scope. If yes, attempt to resolve
     230%       to a constructor, and produce an error message if a match is not
     231%       found. If there are no constructors in scope, resolve to
     232%       initializing each field individually (C-style)
     233%     + does not attempt to autogenerate constructors for POD types,
     234%       which can be seen as a space optimization for the program
     235%       binary
     236%     - as stated previously, a polymorphic routine may require these
     237%       autogenerated constructors, so this doesn't seem like a big win,
     238%       because this leads to more complicated logic and tracking of
     239%       which constructors have already been generated
     240%     - even though a constructor is not explicitly declared or used
     241%       polymorphically, we might still need one for all uses of a
     242%       struct (e.g. in the case of composition).
     243%   * the biggest tradeoff in autogenerating vs. defaulting appears to
     244%     be in where and how the special code to check if constructors are
     245%     present is handled. It appears that there are more reasons to
     246%     autogenerate than not.
     247
     248% --- examples
     249% [1] As an example of using constructors polymorphically, consider a
     250% slight modification on the foldl example I put on the mailing list a
     251% few months ago:
     252
     253% context iterable(type collection, type element, type iterator) {
     254%   void ?{}(iterator *, collection); // used to be makeIterator, but can
     255%                             // idiomatically use constructor
     256%   int hasNext(iterator);
     257%   iterator ++?(iterator *);
     258%   lvalue element *?(iterator);
     259% };
     260
     261
     262% forall(type collection, type element, type result, type iterator
     263%   | iterable(collection, element, iterator))
     264% result foldl(collection c, result acc,
     265%     result (*reduce)(result, element)) {
     266%   iterator it = { c };
     267%   while (hasNext(it)) {
     268%     acc = reduce(acc, *it);
     269%     ++it;
     270%   }
     271%   return acc;
     272% }
     273
     274% Now foldl makes use of the knowledge that the iterator type has a
     275% single argument constructor which takes the collection to iterate
     276% over. This pattern allows polymorphic code to look more natural
     277% (constructors are generally preferred to named initializer/creation
     278% routines, e.g. "makeIterator")
     279
     280% [2] An example of some potentially dangerous code that we don't want
     281% to let easily slip through the cracks - if this is really what you
     282% want, then use @= syntax for the second declaration to quiet the
     283% compiler.
     284
     285% struct A { int x, y, z; }
     286% ?{}(A *, int);
     287% ?{}(A *, int, int, int);
     288
     289% A a1 = { 1 };         // uses ?{}(A *, int);
     290% A a2 = { 2, 3 };      // C-style initialization -> no invariants!
     291% A a3 = { 4, 5, 6 };   // uses ?{}(A *, int, int, int);
     292
     293% [3] Since @= syntax creates a C object (essentially a POD, as far as
     294% the compiler is concerned), the object will not be destructed
     295% implicitly when it leaves scope, nor will it be copy constructed when
     296% it is returned. In this case, a memcpy should be equivalent to a move.
     297
     298% // Box.h
     299% struct Box;
     300% void ?{}(Box **, int};
     301% void ^?{}(Box **);
     302% Box * make_fortytwo();
     303
     304% // Box.cfa
     305% Box * make_fortytwo() {
     306%   Box *b @= {};
     307%   (&b){ 42 }; // construct explicitly
     308%   return b; // no destruction, essentially a move?
     309% }
     310
     311% [4] Cforall's typesafe malloc can be composed with constructor
     312% expressions. It is possible for a user to define their own functions
     313% similar to malloc and achieve the same effects (e.g. Aaron's example
     314% of an arena allocator)
     315
     316% // CFA malloc
     317% forall(type T)
     318% T * malloc() { return (T *)malloc(sizeof(T)); }
     319
     320% struct A { int x, y, z; };
     321% void ?{}(A *, int);
     322
     323% int foo(){
     324%   ...
     325%   // desugars to:
     326%   // A * a = ?{}(malloc(), 123);
     327%   A * a = malloc() { 123 };
     328%   ...
     329% }
     330
     331% [5] Aaron's example of combining function calls with constructor
     332% syntax to perform an operation similar to C++'s std::vector::emplace
     333% (i.e. to construct a new element in place, without the need to
     334% copy)
     335
     336% forall(type T)
     337% struct vector {
     338%   T * elem;
     339%   int len;
     340%   ...
     341% };
     342
     343% ...
     344% forall(type T)
     345% T * vector_new(vector(T) * v) {
     346%   // reallocate if needed
     347%   return &v->elem[len++];
     348% }
     349
     350% int main() {
     351%   vector(int) * v = ...
     352%   vector_new(v){ 42 };  // add element to the end of vector
     353% }
     354
     355% [6] Pointer Constructors. It could be useful to use the existing
     356% constructor syntax even more uniformly for ADTs. With this, ADTs can
     357% be initialized in the same manor as any other object in a polymorphic
     358% function.
     359
     360% // vector.h
     361% forall(type T) struct vector;
     362% forall(type T) void ?{}(vector(T) **);
     363% // adds an element to the end
     364% forall(type T) vector(T) * ?+?(vector(T) *, T);
     365
     366% // vector.cfa
     367% // don't want to expose the implementation to the user and/or don't
     368% // want to recompile the entire program if the struct definition
     369% // changes
     370
     371% forall(type T) struct vector {
     372%   T * elem;
     373%   int len;
     374%   int capacity;
     375% };
     376
     377% forall(type T) void resize(vector(T) ** v) { ... }
     378
     379% forall(type T) void ?{}(vector(T) ** v) {
     380%   vector(T) * vect = *v = malloc();
     381%   vect->capacity = 10;
     382%   vect->len = 0;
     383%   vect->elem = malloc(vect->capacity);
     384% }
     385
     386% forall(type T) vector(T) * ?+?(vector(T) *v, T elem) {
     387%   if (v->len == v->capacity) resize(&v);
     388%   v->elem[v->len++] = elem;
     389% }
     390
     391% // main.cfa
     392% #include "adt.h"
     393% forall(type T | { T ?+?(T, int); }
     394% T sumRange(int lower, int upper) {
     395%   T x;    // default construct
     396%   for (int i = lower; i <= upper; i++) {
     397%     x = x + i;
     398%   }
     399%   return x;
     400% }
     401
     402% int main() {
     403%   vector(int) * numbers = sumRange(1, 10);
     404%   // numbers is now a vector containing [1..10]
     405
     406%   int sum = sumRange(1, 10);
     407%   // sum is now an int containing the value 55
     408% }
     409
     410% [7] The current proposal is to use the plan 9 model of inheritance.
     411% Under this model, all of the members of an unnamed struct instance
     412% become members of the containing struct. In addition, an object
     413% can be passed as an argument to a function expecting one of its
     414% base structs.
     415
     416% struct Point {
     417%   double x;
     418%   double y;
     419% };
     420
     421% struct ColoredPoint {
     422%   Point;        // anonymous member (no identifier)
     423%                 // => a ColoredPoint has an x and y of type double
     424%   int color;
     425% };
     426
     427% ColoredPoint cp = ...;
     428% cp.x = 10.3;    // x from Point is accessed directly
     429% cp.color = 0x33aaff; // color is accessed normally
     430% foo(cp);        // cp can be used directly as a Point
     431
     432% void ?{}(Point *p, double x, double y) {
     433%   p->x = x;
     434%   p->y = y;
     435% }
     436
     437% void ?{}(ColoredPoint *cp, double x, double y, int color) {
     438%   (&cp){ x, y };  // unambiguous, no ?{}(ColoredPoint*,double,double)
     439%   cp->color = color;
     440% }
     441
     442% struct Size {
     443%   double width;
     444%   double height;
     445% };
     446
     447% void ?{}(Size *s, double w, double h) {
     448%   p->width = w;
     449%   p->height = h;
     450% }
     451
     452% struct Foo {
     453%   Point;
     454%   Size;
     455% }
     456
     457% ?{}(Foo &f, double x, double y, double w, double h) {
     458%   // (&F,x,y) is ambiguous => is it ?{}(Point*,double,double) or
     459%   // ?{}(Size*,double,double)? Solve with a cast:
     460%   ((Point*)&F){ x, y };
     461%   ((Size*)&F){ w, h };
     462% }
     463
     464% [8] Destructors will be called on objects that were not constructed.
     465
     466% struct A { ... };
     467% ^?{}(A *);
     468% {
     469%   A x;
     470%   A y @= {};
     471% } // x is destructed, even though it wasn't constructed
     472%   // y is not destructed, because it is explicitly a C object
     473
     474
     475% [9] A type's constructor is generated at declaration time using
     476% current information about an object's members. This is analogous to
     477% the treatment of other operators. For example, an object's assignment
     478% operator will not change to call the override of a member's assignment
     479% operator unless the object's assignment is also explicitly overridden.
     480% This problem can potentially be treated differently in Do, since each
     481% compilation unit is passed over at least twice (once to gather
     482% symbol information, once to generate code - this is necessary to
     483% achieve the "No declarations" goal)
     484
     485% struct A { ... };
     486% struct B { A x; };
     487% ...
     488% void ?{}(A *);  // from this point on, A objects will be constructed
     489% B b1;           // b1 and b1.x are both NOT constructed, because B
     490%                 // objects are not constructed
     491% void ?{}(B *);  // from this point on, B objects will be constructed
     492% B b2;           // b2 and b2.x are both constructed
     493
     494% struct C { A x; };
     495% // implicit definition of ?{}(C*), because C is not a POD type since
     496% // it contains a non-POD type by composition
     497% C c;            // c and c.x are both constructed
     498
     499% [10] Requiring construction by composition
     500
     501% struct A {
     502%   ...
     503% };
     504
     505% // declared ctor disables default c-style initialization of
     506% // A objects; A is no longer a POD type
     507% void ?{}(A *);
     508
     509% struct B {
     510%   A x;
     511% };
     512
     513% // B objects can not be C-style initialized, because A objects
     514% // must be constructed => B objects are transitively not POD types
     515% B b; // b.x must be constructed, but B is not constructible
     516%      // => must autogenerate ?{}(B *) after struct B definition,
     517%      // which calls ?{}(&b.x)
     518
     519% [11] Explosion in the number of generated constructors, due to strange
     520% C semantics.
     521
     522% struct A { int x, y; };
     523% struct B { A u, v, w; };
     524
     525% A a = { 0, 0 };
     526
     527% // in C, you are allowed to do this
     528% B b1 = { 1, 2, 3, 4, 5, 6 };
     529% B b2 = { 1, 2, 3 };
     530% B b3 = { a, a, a };
     531% B b4 = { a, 5, 4, a };
     532% B b5 = { 1, 2, a, 3 };
     533
     534% // we want to disallow b1, b2, b4, and b5 in Cforall.
     535% // In particular, we will autogenerate these constructors:
     536% void ?{}(A *);             // default/0 parameters
     537% void ?{}(A *, int);        // 1 parameter
     538% void ?{}(A *, int, int);   // 2 parameters
     539% void ?{}(A *, const A *);  // copy constructor
     540
     541% void ?{}(B *);             // default/0 parameters
     542% void ?{}(B *, A);          // 1 parameter
     543% void ?{}(B *, A, A);       // 2 parameters
     544% void ?{}(B *, A, A, A);    // 3 parameters
     545% void ?{}(B *, const B *);  // copy constructor
     546
     547% // we will not generate constructors for every valid combination
     548% // of members in C. For example, we will not generate
     549% void ?{}(B *, int, int, int, int, int, int);   // b1 would need this
     550% void ?{}(B *, int, int, int);                  // b2 would need this
     551% void ?{}(B *, A, int, int, A);                 // b4 would need this
     552% void ?{}(B *, int, int, A, int);               // b5 would need this
     553% // and so on
     554
     555
     556
     557% TODO: talk somewhere about compound literals?
     558
    8559Since \CFA is a true systems language, it does not provide a garbage collector.
    9 As well, \CFA is not an object-oriented programming language, i.e., structures cannot have routine members.
     560As well, \CFA is not an object-oriented programming language, i.e. structures cannot have routine members.
    10561Nevertheless, one important goal is to reduce programming complexity and increase safety.
    11562To that end, \CFA provides support for implicit pre/post-execution of routines for objects, via constructors and destructors.
    12563
     564% TODO: this is old. remove or refactor
     565% Manual resource management is difficult.
     566% Part of the difficulty results from not having any guarantees about the current state of an object.
     567% Objects can be internally composed of pointers that may reference resources which may or may not need to be manually released, and keeping track of that state for each object can be difficult for the end user.
     568
     569% Constructors and destructors provide a mechanism to bookend the lifetime of an object, allowing the designer of a type to establish invariants for objects of that type.
     570% Constructors guarantee that object initialization code is run before the object can be used, while destructors provide a mechanism that is guaranteed to be run immediately before an object's lifetime ends.
     571% Constructors and destructors can help to simplify resource management when used in a disciplined way.
     572% In particular, when all resources are acquired in a constructor, and all resources are released in a destructor, no resource leaks are possible.
     573% This pattern is a popular idiom in several languages, such as \CC, known as RAII (Resource Acquisition Is Initialization).
     574
    13575This chapter details the design of constructors and destructors in \CFA, along with their current implementation in the translator.
    14 Generated code samples have been edited for clarity and brevity.
     576Generated code samples have been edited to provide comments for clarity and to save on space.
    15577
    16578\section{Design Criteria}
    … …  
    30592Next, @x@ is assigned the value of @y@.
    31593In the last line, @z@ is implicitly initialized to 0 since it is marked @static@.
    32 The key difference between assignment and initialization being that assignment occurs on a live object (i.e., an object that contains data).
     594The key difference between assignment and initialization being that assignment occurs on a live object (i.e. an object that contains data).
    33595It is important to note that this means @x@ could have been used uninitialized prior to being assigned, while @y@ could not be used uninitialized.
    34 Use of uninitialized variables yields undefined behaviour, which is a common source of errors in C programs.
    35 
    36 Initialization of a declaration is strictly optional, permitting uninitialized variables to exist.
    37 Furthermore, declaration initialization is limited to expressions, so there is no way to insert arbitrary code before a variable is live, without delaying the declaration.
    38 Many C compilers give good warnings for uninitialized variables most of the time, but they cannot in all cases.
    39 \begin{cfacode}
    40 int f(int *);  // output parameter: never reads, only writes
    41 int g(int *);  // input parameter: never writes, only reads,
    42                // so requires initialized variable
     596Use of uninitialized variables yields undefined behaviour, which is a common source of errors in C programs. % TODO: *citation*
     597
     598Declaration initialization is insufficient, because it permits uninitialized variables to exist and because it does not allow for the insertion of arbitrary code before the variable is live.
     599Many C compilers give good warnings most of the time, but they cannot in all cases.
     600\begin{cfacode}
     601int f(int *);  // never reads the parameter, only writes
     602int g(int *);  // reads the parameter - expects an initialized variable
    43603
    44604int x, y;
    45605f(&x);  // okay - only writes to x
    46 g(&y);  // uses y uninitialized
    47 \end{cfacode}
    48 Other languages are able to give errors in the case of uninitialized variable use, but due to backwards compatibility concerns, this is not the case in \CFA.
    49 
    50 In C, constructors and destructors are often mimicked by providing routines that create and tear down objects, where the tear down function is typically only necessary if the type modifies the execution environment.
     606g(&y);  // will use y uninitialized
     607\end{cfacode}
     608Other languages are able to give errors in the case of uninitialized variable use, but due to backwards compatibility concerns, this cannot be the case in \CFA.
     609
     610In C, constructors and destructors are often mimicked by providing routines that create and teardown objects, where the teardown function is typically only necessary if the type modifies the execution environment.
    51611\begin{cfacode}
    52612struct array_int {
    … …  
    54614};
    55615struct array_int create_array(int sz) {
    56   return (struct array_int) { calloc(sizeof(int)*sz) };
     616  return (struct array_int) { malloc(sizeof(int)*sz) };
    57617}
    58618void destroy_rh(struct resource_holder * rh) {
    … …  
    74634Furthermore, even with this idiom it is easy to make mistakes, such as forgetting to destroy an object or destroying it multiple times.
    75635
    76 A constructor provides a way of ensuring that the necessary aspects of object initialization is performed, from setting up invariants to providing compile- and run-time checks for appropriate initialization parameters.
     636A constructor provides a way of ensuring that the necessary aspects of object initialization is performed, from setting up invariants to providing compile-time checks for appropriate initialization parameters.
    77637This goal is achieved through a guarantee that a constructor is called implicitly after every object is allocated from a type with associated constructors, as part of an object's definition.
    78638Since a constructor is called on every object of a managed type, it is impossible to forget to initialize such objects, as long as all constructors perform some sensible form of initialization.
    79639
    80640In \CFA, a constructor is a function with the name @?{}@.
    81 Like other operators in \CFA, the name represents the syntax used to call the constructor, e.g., @struct S = { ... };@.
    82641Every constructor must have a return type of @void@ and at least one parameter, the first of which is colloquially referred to as the \emph{this} parameter, as in many object-oriented programming-languages (however, a programmer can give it an arbitrary name).
    83642The @this@ parameter must have a pointer type, whose base type is the type of object that the function constructs.
    … …  
    96655
    97656In C, if the user creates an @Array@ object, the fields @data@ and @len@ are uninitialized, unless an explicit initializer list is present.
    98 It is the user's responsibility to remember to initialize both of the fields to sensible values, since there are no implicit checks for invalid values or reasonable defaults.
     657It is the user's responsibility to remember to initialize both of the fields to sensible values.
    99658In \CFA, the user can define a constructor to handle initialization of @Array@ objects.
    100659
    … …  
    112671This constructor initializes @x@ so that its @length@ field has the value 10, and its @data@ field holds a pointer to a block of memory large enough to hold 10 @int@s, and sets the value of each element of the array to 0.
    113672This particular form of constructor is called the \emph{default constructor}, because it is called on an object defined without an initializer.
    114 In other words, a default constructor is a constructor that takes a single argument: the @this@ parameter.
    115 
    116 In \CFA, a destructor is a function much like a constructor, except that its name is \lstinline!^?{}! and it take only one argument.
     673In other words, a default constructor is a constructor that takes a single argument, the @this@ parameter.
     674
     675In \CFA, a destructor is a function much like a constructor, except that its name is \lstinline!^?{}!.
    117676A destructor for the @Array@ type can be defined as such.
    118677\begin{cfacode}
    … …  
    121680}
    122681\end{cfacode}
    123 The destructor is automatically called at deallocation for all objects of type @Array@.
    124 Hence, the memory associated with an @Array@ is automatically freed when the object's lifetime ends.
     682Since the destructor is automatically called at deallocation for all objects of type @Array@, the memory associated with an @Array@ is automatically freed when the object's lifetime ends.
    125683The exact guarantees made by \CFA with respect to the calling of destructors are discussed in section \ref{sub:implicit_dtor}.
    126684
    … …  
    133691\end{cfacode}
    134692By the previous definition of the default constructor for @Array@, @x@ and @y@ are initialized to valid arrays of length 10 after their respective definitions.
    135 On line 2, @z@ is initialized with the value of @x@, while on line 3, @y@ is assigned the value of @x@.
     693On line 3, @z@ is initialized with the value of @x@, while on line @4@, @y@ is assigned the value of @x@.
    136694The key distinction between initialization and assignment is that a value to be initialized does not hold any meaningful values, whereas an object to be assigned might.
    137695In particular, these cases cannot be handled the same way because in the former case @z@ does not currently own an array, while @y@ does.
    … …  
    154712The first function is called a \emph{copy constructor}, because it constructs its argument by copying the values from another object of the same type.
    155713The second function is the standard copy-assignment operator.
    156 The four functions (default constructor, destructor, copy constructor, and assignment operator) are special in that they safely control the state of most objects.
     714These four functions are special in that they control the state of most objects.
    157715
    158716It is possible to define a constructor that takes any combination of parameters to provide additional initialization options.
    159 For example, a reasonable extension to the array type would be a constructor that allocates the array to a given initial capacity and initializes the elements of the array to a given @fill@ value.
     717For example, a reasonable extension to the array type would be a constructor that allocates the array to a given initial capacity and initializes the array to a given @fill@ value.
    160718\begin{cfacode}
    161719void ?{}(Array * arr, int capacity, int fill) {
    … …  
    171729Array x, y = { 20, 0xdeadbeef }, z = y;
    172730\end{cfacode}
    173 
    174731In \CFA, constructor calls look just like C initializers, which allows them to be inserted into legacy C code with minimal code changes, and also provides a very simple syntax that veteran C programmers are familiar with.
    175732One downside of reusing C initialization syntax is that it isn't possible to determine whether an object is constructed just by looking at its declaration, since that requires knowledge of whether the type is managed at that point.
    … …  
    191748Destructors are implicitly called in reverse declaration-order so that objects with dependencies are destructed before the objects they are dependent on.
    192749
    193 \subsection{Calling Syntax}
    194 \label{sub:syntax}
     750\subsection{Syntax}
     751\label{sub:syntax} % TODO: finish this section
    195752There are several ways to construct an object in \CFA.
    196753As previously introduced, every variable is automatically constructed at its definition, which is the most natural way to construct an object.
    … …  
    216773A * y = malloc();  // copy construct: ?{}(&y, malloc())
    217774
    218 ?{}(&x);    // explicit construct x, second construction
    219 ?{}(y, x);  // explit construct y from x, second construction
    220 ^?{}(&x);   // explicit destroy x, in different order
     775?{}(&x);    // explicit construct x
     776?{}(y, x);  // explit construct y from x
     777^?{}(&x);   // explicit destroy x
    221778^?{}(y);    // explicit destroy y
    222779
    … …  
    224781// implicit ^?{}(&x);
    225782\end{cfacode}
    226 Calling a constructor or destructor directly is a flexible feature that allows complete control over the management of storage.
     783Calling a constructor or destructor directly is a flexible feature that allows complete control over the management of a piece of storage.
    227784In particular, constructors double as a placement syntax.
    228785\begin{cfacode}
    … …  
    247804Finally, constructors and destructors support \emph{operator syntax}.
    248805Like other operators in \CFA, the function name mirrors the use-case, in that the first $N$ arguments fill in the place of the question mark.
    249 This syntactic form is similar to the new initialization syntax in \CCeleven, except that it is used in expression contexts, rather than declaration contexts.
    250806\begin{cfacode}
    251807struct A { ... };
    … …  
    266822Destructor operator syntax is actually an statement, and requires parentheses for symmetry with constructor syntax.
    267823
    268 One of these three syntactic forms should appeal to either C or \CC programmers using \CFA.
    269 
    270 \subsection{Constructor Expressions}
    271 In \CFA, it is possible to use a constructor as an expression.
    272 Like other operators, the function name @?{}@ matches its operator syntax.
    273 For example, @(&x){}@ calls the default constructor on the variable @x@, and produces @&x@ as a result.
    274 A key example for this capability is the use of constructor expressions to initialize the result of a call to standard C routine @malloc@.
    275 \begin{cfacode}
    276 struct X { ... };
    277 void ?{}(X *, double);
    278 X * x = malloc(sizeof(X)){ 1.5 };
    279 \end{cfacode}
    280 In this example, @malloc@ dynamically allocates storage and initializes it using a constructor, all before assigning it into the variable @x@.
    281 If this extension is not present, constructing dynamically allocated objects is much more cumbersome, requiring separate initialization of the pointer and initialization of the pointed-to memory.
    282 \begin{cfacode}
    283 X * x = malloc(sizeof(X));
    284 x{ 1.5 };
    285 \end{cfacode}
    286 Not only is this verbose, but it is also more error prone, since this form allows maintenance code to easily sneak in between the initialization of @x@ and the initialization of the memory that @x@ points to.
    287 This feature is implemented via a transformation producing the value of the first argument of the constructor, since constructors do not themselves have a return value.
    288 Since this transformation results in two instances of the subexpression, care is taken to allocate a temporary variable to hold the result of the subexpression in the case where the subexpression may contain side effects.
    289 The previous example generates the following code.
    290 \begin{cfacode}
    291 struct X *_tmp_ctor;
    292 struct X *x = ?{}(  // construct result of malloc
    293   _tmp_ctor=malloc(sizeof(struct X)), // store result of malloc
    294   1.5
    295 ), _tmp_ctor; // produce constructed result of malloc
    296 \end{cfacode}
    297 It should be noted that this technique is not exclusive to @malloc@, and allows a user to write a custom allocator that can be idiomatically used in much the same way as a constructed @malloc@ call.
    298 
    299 It is also possible to use operator syntax with destructors.
    300 Unlike constructors, operator syntax with destructors is a statement and thus does not produce a value, since the destructed object is invalidated by the use of a destructor.
    301 For example, \lstinline!^(&x){}! calls the destructor on the variable @x@.
    302 
    303824\subsection{Function Generation}
    304 In \CFA, every type is defined to have the core set of four special functions described previously.
     825In \CFA, every type is defined to have the core set of four functions described previously.
    305826Having these functions exist for every type greatly simplifies the semantics of the language, since most operations can simply be defined directly in terms of function calls.
    306827In addition to simplifying the definition of the language, it also simplifies the analysis that the translator must perform.
    … …  
    312833There are several options for user-defined types: structures, unions, and enumerations.
    313834To aid in ease of use, the standard set of four functions is automatically generated for a user-defined type after its definition is completed.
    314 By auto-generating these functions, it is ensured that legacy C code continues to work correctly in every context where \CFA expects these functions to exist, since they are generated for every complete type.
     835By auto-generating these functions, it is ensured that legacy C code will continue to work correctly in every context where \CFA expects these functions to exist, since they are generated for every complete type.
    315836
    316837The generated functions for enumerations are the simplest.
    317 Since enumerations in C are essentially just another integral type, the generated functions behave in the same way that the built-in functions for the basic types work.
     838Since enumerations in C are essentially just another integral type, the generated functions behave in the same way that the builtin functions for the basic types work.
     839% TODO: examples for enums
    318840For example, given the enumeration
    319841\begin{cfacode}
    … …  
    328850}
    329851void ?{}(enum Colour *_dst, enum Colour _src){
    330   *_dst=_src;  // bitwise copy
     852  (*_dst)=_src;  // bitwise copy
    331853}
    332854void ^?{}(enum Colour *_dst){
    … …  
    334856}
    335857enum Colour ?=?(enum Colour *_dst, enum Colour _src){
    336   return *_dst=_src; // bitwise copy
     858  return (*_dst)=_src; // bitwise copy
    337859}
    338860\end{cfacode}
    339861In the future, \CFA will introduce strongly-typed enumerations, like those in \CC.
    340 The existing generated routines are sufficient to express this restriction, since they are currently set up to take in values of that enumeration type.
     862The existing generated routines will be sufficient to express this restriction, since they are currently set up to take in values of that enumeration type.
    341863Changes related to this feature only need to affect the expression resolution phase, where more strict rules will be applied to prevent implicit conversions from integral types to enumeration types, but should continue to permit conversions from enumeration types to @int@.
    342 In this way, it is still possible to add an @int@ to an enumeration, but the resulting value is an @int@, meaning it cannot be reassigned to an enumeration without a cast.
     864In this way, it will still be possible to add an @int@ to an enumeration, but the resulting value will be an @int@, meaning that it won't be possible to reassign the value into an enumeration without a cast.
    343865
    344866For structures, the situation is more complicated.
    345 Given a structure @S@ with members @M$_0$@, @M$_1$@, ... @M$_{N-1}$@, each function @f@ in the standard set calls \lstinline{f(s->M$_i$, ...)} for each @$i$@.
    346 That is, a default constructor for @S@ default constructs the members of @S@, the copy constructor copy constructs them, and so on.
    347 For example, given the structure definition
     867For a structure @S@ with members @M$_0$@, @M$_1$@, ... @M$_{N-1}$@, each function @f@ in the standard set calls \lstinline{f(s->M$_i$, ...)} for each @$i$@.
     868That is, a default constructor for @S@ default constructs the members of @S@, the copy constructor with copy construct them, and so on.
     869For example given the struct definition
    348870\begin{cfacode}
    349871struct A {
    … …  
    371893}
    372894\end{cfacode}
    373 It is important to note that the destructors are called in reverse declaration order to prevent conflicts in the event there are dependencies among members.
     895It is important to note that the destructors are called in reverse declaration order to resolve conflicts in the event there are dependencies among members.
    374896
    375897In addition to the standard set, a set of \emph{field constructors} is also generated for structures.
    376 The field constructors are constructors that consume a prefix of the structure's member-list.
     898The field constructors are constructors that consume a prefix of the struct's member list.
    377899That is, $N$ constructors are built of the form @void ?{}(S *, T$_{\text{M}_0}$)@, @void ?{}(S *, T$_{\text{M}_0}$, T$_{\text{M}_1}$)@, ..., @void ?{}(S *, T$_{\text{M}_0}$, T$_{\text{M}_1}$, ..., T$_{\text{M}_{N-1}}$)@, where members are copy constructed if they have a corresponding positional argument and are default constructed otherwise.
    378 The addition of field constructors allows structures in \CFA to be used naturally in the same ways as used in C (i.e., to initialize any prefix of the structure), e.g., @A a0 = { b }, a1 = { b, c }@.
     900The addition of field constructors allows structs in \CFA to be used naturally in the same ways that they could be used in C (i.e. to initialize any prefix of the struct), e.g., @A a0 = { b }, a1 = { b, c }@.
    379901Extending the previous example, the following constructors are implicitly generated for @A@.
    380902\begin{cfacode}
    … …  
    389911\end{cfacode}
    390912
    391 For unions, the default constructor and destructor do nothing, as it is not obvious which member, if any, should be constructed.
     913For unions, the default constructor and destructor do nothing, as it is not obvious which member if any should be constructed.
    392914For copy constructor and assignment operations, a bitwise @memcpy@ is applied.
    393915In standard C, a union can also be initialized using a value of the same type as its first member, and so a corresponding field constructor is generated to perform a bitwise @memcpy@ of the object.
    394 An alternative to this design is to always construct and destruct the first member of a union, to match with the C semantics of initializing the first member of the union.
     916An alterantive to this design is to always construct and destruct the first member of a union, to match with the C semantics of initializing the first member of the union.
    395917This approach ultimately feels subtle and unsafe.
    396918Another option is to, like \CC, disallow unions from containing members that are themselves managed types.
    … …  
    425947
    426948% This feature works in the \CFA model, since constructors are simply special functions and can be called explicitly, unlike in \CC. % this sentence isn't really true => placement new
    427 In \CCeleven, unions may have managed members, with the caveat that if there are any members with a user-defined operation, then that operation is not implicitly defined, forcing the user to define the operation if necessary.
     949In \CCeleven, this restriction has been loosened to allow unions with managed members, with the caveat that any if there are any members with a user-defined operation, then that operation is not implicitly defined, forcing the user to define the operation if necessary.
    428950This restriction could easily be added into \CFA once \emph{deleted} functions are added.
    429951
    … …  
    448970Here, @&s@ and @&s2@ are cast to unqualified pointer types.
    449971This mechanism allows the same constructors and destructors to be used for qualified objects as for unqualified objects.
    450 This applies only to implicitly generated constructor calls.
    451 Hence, explicitly re-initializing qualified objects with a constructor requires an explicit cast.
    452 
    453 As discussed in Section \ref{sub:c_background}, compound literals create unnamed objects.
    454 This mechanism can continue to be used seamlessly in \CFA with managed types to create temporary objects.
    455 The object created by a compound literal is constructed using the provided brace-enclosed initializer-list, and is destructed at the end of the scope it is used in.
    456 For example,
    457 \begin{cfacode}
    458 struct A { int x; };
    459 void ?{}(A *, int, int);
    460 {
    461   int x = (A){ 10, 20 }.x;
    462 }
    463 \end{cfacode}
    464 is equivalent to
    465 \begin{cfacode}
    466 struct A { int x, y; };
    467 void ?{}(A *, int, int);
    468 {
    469   A _tmp;
    470   ?{}(&_tmp, 10, 20);
    471   int x = _tmp.x;
    472   ^?{}(&tmp);
    473 }
    474 \end{cfacode}
     972Since this applies only to implicitly generated constructor calls, the language does not allow qualified objects to be re-initialized with a constructor without an explicit cast.
    475973
    476974Unlike \CC, \CFA provides an escape hatch that allows a user to decide at an object's definition whether it should be managed or not.
    … …  
    486984A a2 @= { 0 };  // unmanaged
    487985\end{cfacode}
    488 In this example, @a1@ is a managed object, and thus is default constructed and destructed at the start/end of @a1@'s lifetime, while @a2@ is an unmanaged object and is not implicitly constructed or destructed.
    489 Instead, @a2->x@ is initialized to @0@ as if it were a C object, because of the explicit initializer.
    490 
    491 In addition to freedom, \ateq provides a simple path to migrating legacy C code to \CFA, in that objects can be moved from C-style initialization to \CFA gradually and individually.
     986In this example, @a1@ is a managed object, and thus is default constructed and destructed at the end of @a1@'s lifetime, while @a2@ is an unmanaged object and is not implicitly constructed or destructed.
     987Instead, @a2->x@ is initialized to @0@ as if it were a C object, due to the explicit initializer.
     988Existing constructors are ignored when \ateq is used, so that any valid C initializer is able to initialize the object.
     989
     990In addition to freedom, \ateq provides a simple path to migrating legacy C code to Cforall, in that objects can be moved from C-style initialization to \CFA gradually and individually.
    492991It is worth noting that the use of unmanaged objects can be tricky to get right, since there is no guarantee that the proper invariants are established on an unmanaged object.
    493992It is recommended that most objects be managed by sensible constructors and destructors, except where absolutely necessary.
    494993
    495 When a user declares any constructor or destructor, the corresponding intrinsic/generated function and all field constructors for that type are hidden, so that they are not found during expression resolution until the user-defined function goes out of scope.
    496 Furthermore, if the user declares any constructor, then the intrinsic/generated default constructor is also hidden, precluding default construction.
    497 These semantics closely mirror the rule for implicit declaration of constructors in \CC, wherein the default constructor is implicitly declared if there is no user-declared constructor \cite[p.~186]{ANSI98:C++}.
     994When the user declares any constructor or destructor, the corresponding intrinsic/generated function and all field constructors for that type are hidden, so that they will not be found during expression resolution unless the user-defined function goes out of scope.
     995Furthermore, if the user declares any constructor, then the intrinsic/generated default constructor is also hidden, making it so that objects of a type may not be default constructable.
     996This closely mirrors the rule for implicit declaration of constructors in \CC, wherein the default constructor is implicitly declared if there is no user-declared constructor. % TODO: cite C++98 page 186??
    498997\begin{cfacode}
    499998struct S { int x, y; };
    … …  
    5021001  S s0, s1 = { 0 }, s2 = { 0, 2 }, s3 = s2;  // okay
    5031002  {
    504     void ?{}(S * s, int i) { s->x = i*2; } // locally hide autogen constructors
     1003    void ?{}(S * s, int i) { s->x = i*2; }
    5051004    S s4;  // error
    5061005    S s5 = { 3 };  // okay
    … …  
    5151014When defining a constructor or destructor for a struct @S@, any members that are not explicitly constructed or destructed are implicitly constructed or destructed automatically.
    5161015If an explicit call is present, then that call is taken in preference to any implicitly generated call.
    517 A consequence of this rule is that it is possible, unlike \CC, to precisely control the order of construction and destruction of sub-objects on a per-constructor basis, whereas in \CC sub-object initialization and destruction is always performed based on the declaration order.
     1016A consequence of this rule is that it is possible, unlike \CC, to precisely control the order of construction and destruction of subobjects on a per-constructor basis, whereas in \CC subobject initialization and destruction is always performed based on the declaration order.
    5181017\begin{cfacode}
    5191018struct A {
    … …  
    5341033}
    5351034\end{cfacode}
    536 Finally, it is illegal for a sub-object to be explicitly constructed for the first time after it is used for the first time.
     1035Finally, it is illegal for a subobject to be explicitly constructed for the first time after it is used for the first time.
    5371036If the translator cannot be reasonably sure that an object is constructed prior to its first use, but is constructed afterward, an error is emitted.
    538 More specifically, the translator searches the body of a constructor to ensure that every sub-object is initialized.
     1037More specifically, the translator searches the body of a constructor to ensure that every subobject is initialized.
    5391038\begin{cfacode}
    5401039void ?{}(A * a, double x) {
    … …  
    5431042}
    5441043\end{cfacode}
    545 However, if the translator sees a sub-object used within the body of a constructor, but does not see a constructor call that uses the sub-object as the target of a constructor, then the translator assumes the object is to be implicitly constructed (copy constructed in a copy constructor and default constructed in any other constructor).
     1044However, if the translator sees a subobject used within the body of a constructor, but does not see a constructor call that uses the subobject as the target of a constructor, then the translator assumes the object is to be implicitly constructed (copy constructed in a copy constructor and default constructed in any other constructor).
    5461045\begin{cfacode}
    5471046void ?{}(A * a) {
    … …  
    5591058} // z, y, w implicitly destructed, in this order
    5601059\end{cfacode}
    561 If at any point, the @this@ parameter is passed directly as the target of another constructor, then it is assumed that constructor handles the initialization of all of the object's members and no implicit constructor calls are added.
     1060If at any point, the @this@ parameter is passed directly as the target of another constructor, then it is assumed that constructor handles the initialization of all of the object's members and no implicit constructor calls are added. % TODO: confirm that this is correct. It might be possible to get subtle errors if you initialize some members then call another constructor... -- in fact, this is basically always wrong. if anything, I should check that such a constructor does not initialize any members, otherwise it'll always initialize the member twice (once locally, once by the called constructor).
    5621061To override this rule, \ateq can be used to force the translator to trust the programmer's discretion.
    5631062This form of \ateq is not yet implemented.
    … …  
    5651064Despite great effort, some forms of C syntax do not work well with constructors in \CFA.
    5661065In particular, constructor calls cannot contain designations (see \ref{sub:c_background}), since this is equivalent to allowing designations on the arguments to arbitrary function calls.
     1066In C, function prototypes are permitted to have arbitrary parameter names, including no names at all, which may have no connection to the actual names used at function definition.
     1067Furthermore, a function prototype can be repeated an arbitrary number of times, each time using different names.
    5671068\begin{cfacode}
    5681069// all legal forward declarations in C
    … …  
    5751076f(b:10, a:20, c:30);  // which parameter is which?
    5761077\end{cfacode}
    577 In C, function prototypes are permitted to have arbitrary parameter names, including no names at all, which may have no connection to the actual names used at function definition.
    578 Furthermore, a function prototype can be repeated an arbitrary number of times, each time using different names.
    5791078As a result, it was decided that any attempt to resolve designated function calls with C's function prototype rules would be brittle, and thus it is not sensible to allow designations in constructor calls.
    580 
    581 In addition, constructor calls do not support unnamed nesting.
    582 \begin{cfacode}
    583 struct B { int x; };
    584 struct C { int y; };
    585 struct A { B b; C c; };
    586 void ?{}(A *, B);
    587 void ?{}(A *, C);
    588 
    589 A a = {
    590   { 10 },  // construct B? - invalid
    591 };
    592 \end{cfacode}
    593 In C, nesting initializers means that the programmer intends to initialize sub-objects with the nested initializers.
    594 The reason for this omission is to both simplify the mental model for using constructors, and to make initialization simpler for the expression resolver.
    595 If this were allowed, it would be necessary for the expression resolver to decide whether each argument to the constructor call could initialize to some argument in one of the available constructors, making the problem highly recursive and potentially much more expensive.
    596 That is, in the previous example the line marked as an error could mean construct using @?{}(A *, B)@ or with @?{}(A *, C)@, since the inner initializer @{ 10 }@ could be taken as an intermediate object of type @B@ or @C@.
    597 In practice, however, there could be many objects that can be constructed from a given @int@ (or, indeed, any arbitrary parameter list), and thus a complete solution to this problem would require fully exploring all possibilities.
    598 
    599 More precisely, constructor calls cannot have a nesting depth greater than the number of array components in the type of the initialized object, plus one.
     1079% Many other languages do allow named arguments, such as Python and Scala, but they do not allow multiple arbitrarily named forward declarations of a function.
     1080
     1081In addition, constructor calls cannot have a nesting depth greater than the number of array components in the type of the initialized object, plus one.
    6001082For example,
    6011083\begin{cfacode}
    … …  
    6141096}
    6151097\end{cfacode}
     1098% TODO: in CFA if the array dimension is empty, no object constructors are added -- need to fix this.
    6161099The body of @A@ has been omitted, since only the constructor interfaces are important.
    617 
     1100In C, having a greater nesting depth means that the programmer intends to initialize subobjects with the nested initializer.
     1101The reason for this omission is to both simplify the mental model for using constructors, and to make initialization simpler for the expression resolver.
     1102If this were allowed, it would be necessary for the expression resolver to decide whether each argument to the constructor call could initialize to some argument in one of the available constructors, making the problem highly recursive and potentially much more expensive.
     1103That is, in the previous example the line marked as an error could mean construct using @?{}(A *, A, A)@, since the inner initializer @{ 11 }@ could be taken as an intermediate object of type @A@ constructed with @?{}(A *, int)@.
     1104In practice, however, there could be many objects that can be constructed from a given @int@ (or, indeed, any arbitrary parameter list), and thus a complete solution to this problem would require fully exploring all possibilities.
    6181105It should be noted that unmanaged objects can still make use of designations and nested initializers in \CFA.
    619 It is simple to overcome this limitation for managed objects by making use of compound literals, so that the arguments to the constructor call are explicitly typed.
    6201106
    6211107\subsection{Implicit Destructors}
    … …  
    6411127    if (i == 2) return; // destruct x, y
    6421128  } // destruct y
    643 } // destruct x
    644 \end{cfacode}
     1129}
     1130\end{cfacode}
     1131
     1132%% having this feels excessive, but it's here if necessary
     1133% This procedure generates the following code.
     1134% \begin{cfacode}
     1135% void f(int i){
     1136%   struct A x;
     1137%   ?{}(&x);
     1138%   {
     1139%     struct A y;
     1140%     ?{}(&y);
     1141%     {
     1142%       struct A z;
     1143%       ?{}(&z);
     1144%       {
     1145%         if ((i==0)!=0) {
     1146%           ^?{}(&z);
     1147%           ^?{}(&y);
     1148%           ^?{}(&x);
     1149%           return;
     1150%         }
     1151%       }
     1152%       if (((i==1)!=0) {
     1153%           ^?{}(&z);
     1154%           ^?{}(&y);
     1155%           ^?{}(&x);
     1156%           return ;
     1157%       }
     1158%       ^?{}(&z);
     1159%     }
     1160
     1161%     if ((i==2)!=0) {
     1162%       ^?{}(&y);
     1163%       ^?{}(&x);
     1164%       return;
     1165%     }
     1166%     ^?{}(&y);
     1167%   }
     1168
     1169%   ^?{}(&x);
     1170% }
     1171% \end{cfacode}
    6451172
    6461173The next example illustrates the use of simple continue and break statements and the manner that they interact with implicit destructors.
    … …  
    6561183\end{cfacode}
    6571184Since a destructor call is automatically inserted at the end of the block, nothing special needs to happen to destruct @x@ in the case where control reaches the end of the loop.
    658 In the case where @i@ is @2@, the continue statement runs the loop update expression and attempts to begin the next iteration of the loop.
    659 Since continue is a C statement, which does not understand destructors, it is transformed into a @goto@ statement that branches to the end of the loop, just before the block's destructors, to ensure that @x@ is destructed.
     1185In the case where @i@ is @2@, the continue statement runs the loop update expression and attemps to begin the next iteration of the loop.
     1186Since continue is a C statement, which does not understand destructors, a destructor call is added just before the continue statement to ensure that @x@ is destructed.
    6601187When @i@ is @3@, the break statement moves control to just past the end of the loop.
    661 Unlike the previous case, the destructor for @x@ cannot be reused, so a destructor call for @x@ is inserted just before the break statement.
    662 
    663 \CFA also supports labeled break and continue statements, which allow more precise manipulation of control flow.
    664 Labeled break and continue allow the programmer to specify which control structure to target by using a label attached to a control structure.
     1188Like the previous case, a destructor call for @x@ is inserted just before the break statement.
     1189
     1190\CFA also supports labelled break and continue statements, which allow more precise manipulation of control flow.
     1191Labelled break and continue allow the programmer to specify which control structure to target by using a label attached to a control structure.
    6651192\begin{cfacode}[emph={L1,L2}, emphstyle=\color{red}]
    6661193L1: for (int i = 0; i < 10; i++) {
    6671194  A x;
    668   for (int j = 0; j < 10; j++) {
     1195  L2: for (int j = 0; j < 10; j++) {
    6691196    A y;
    670     if (i == 1) {
     1197    if (j == 0) {
     1198      continue;    // destruct y
     1199    } else if (j == 1) {
     1200      break;       // destruct y
     1201    } else if (i == 1) {
    6711202      continue L1; // destruct y
    6721203    } else if (i == 2) {
    … …  
    6771208\end{cfacode}
    6781209The statement @continue L1@ begins the next iteration of the outer for-loop.
    679 Since the semantics of continue require the loop update expression to execute, control branches to the end of the outer for loop, meaning that the block destructor for @x@ can be reused, and it is only necessary to generate the destructor for @y@.
    680 Break, on the other hand, requires jumping out of both loops, so the destructors for both @x@ and @y@ are generated and inserted before the @break L1@ statement.
     1210Since the semantics of continue require the loop update expression to execute, control branches to the \emph{end} of the outer for loop, meaning that the block destructor for @x@ can be reused, and it is only necessary to generate the destructor for @y@.
     1211Break, on the other hand, requires jumping out of the loop, so the destructors for both @x@ and @y@ are generated and inserted before the @break L1@ statement.
    6811212
    6821213Finally, an example which demonstrates goto.
    … …  
    7251256}
    7261257\end{cfacode}
    727 All break and continue statements are implemented in \CFA in terms of goto statements, so the more constrained forms are precisely governed by these rules.
     1258Labelled break and continue are implemented in \CFA in terms of goto statements, so the more constrained forms are precisely goverened by these rules.
    7281259
    7291260The next example demonstrates the error case.
    … …  
    7421273
    7431274\subsection{Implicit Copy Construction}
    744 \label{s:implicit_copy_construction}
    7451275When a function is called, the arguments supplied to the call are subject to implicit copy construction (and destruction of the generated temporary), and the return value is subject to destruction.
    7461276When a value is returned from a function, the copy constructor is called to pass the value back to the call site.
    747 Exempt from these rules are intrinsic and built-in functions.
     1277Exempt from these rules are intrinsic and builtin functions.
    7481278It should be noted that unmanaged objects are subject to copy constructor calls when passed as arguments to a function or when returned from a function, since they are not the \emph{target} of the copy constructor call.
    749 That is, since the parameter is not marked as an unmanaged object using \ateq, it will be copy constructed if it is returned by value or passed as an argument to another function, so to guarantee consistent behaviour, unmanaged objects must be copy constructed when passed as arguments.
    7501279This is an important detail to bear in mind when using unmanaged objects, and could produce unexpected results when mixed with objects that are explicitly constructed.
    7511280\begin{cfacode}
    … …  
    7551284void ^?{}(A *);
    7561285
    757 A identity(A x) { // pass by value => need local copy
    758   return x;       // return by value => make call-site copy
     1286A f(A x) {
     1287  return x;
    7591288}
    7601289
    7611290A y, z @= {};
    762 identity(y);  // copy construct y into x
    763 identity(z);  // copy construct z into x
     1291identity(y);
     1292identity(z);
    7641293\end{cfacode}
    7651294Note that @z@ is copy constructed into a temporary variable to be passed as an argument, which is also destructed after the call.
     1295A special syntactic form, such as a variant of \ateq, could be implemented to specify at the call site that an argument should not be copy constructed, to regain some control for the C programmer.
    7661296
    7671297This generates the following
    7681298\begin{cfacode}
    7691299struct A f(struct A x){
    770   struct A _retval_f;    // return value
    771   ?{}((&_retval_f), x);  // copy construct return value
     1300  struct A _retval_f;
     1301  ?{}((&_retval_f), x);
    7721302  return _retval_f;
    7731303}
    7741304
    7751305struct A y;
    776 ?{}(&y);                 // default construct
    777 struct A z = { 0 };      // C default
    778 
    779 struct A _tmp_cp1;       // argument 1
    780 struct A _tmp_cp_ret0;   // return value
    781 _tmp_cp_ret0=f(
    782   (?{}(&_tmp_cp1, y) , _tmp_cp1)  // argument is a comma expression
    783 ), _tmp_cp_ret0;         // return value for cascading
    784 ^?{}(&_tmp_cp_ret0);     // destruct return value
    785 ^?{}(&_tmp_cp1);         // destruct argument 1
    786 
    787 struct A _tmp_cp2;       // argument 1
    788 struct A _tmp_cp_ret1;   // return value
    789 _tmp_cp_ret1=f(
    790   (?{}(&_tmp_cp2, z), _tmp_cp2)  // argument is a common expression
    791 ), _tmp_cp_ret1;         // return value for cascading
    792 ^?{}(&_tmp_cp_ret1);     // destruct return value
    793 ^?{}(&_tmp_cp2);         // destruct argument 1
     1306?{}(&y);
     1307struct A z = { 0 };
     1308
     1309struct A _tmp_cp1;     // argument 1
     1310struct A _tmp_cp_ret0; // return value
     1311_tmp_cp_ret0=f((?{}(&_tmp_cp1, y) , _tmp_cp1)), _tmp_cp_ret0;
     1312^?{}(&_tmp_cp_ret0);   // return value
     1313^?{}(&_tmp_cp1);       // argument 1
     1314
     1315struct A _tmp_cp2;     // argument 1
     1316struct A _tmp_cp_ret1; // return value
     1317_tmp_cp_ret1=f((?{}(&_tmp_cp2, z), _tmp_cp2)), _tmp_cp_ret1;
     1318^?{}(&_tmp_cp_ret1);   // return value
     1319^?{}(&_tmp_cp2);       // argument 1
    7941320^?{}(&y);
    7951321\end{cfacode}
    7961322
    797 A special syntactic form, such as a variant of \ateq, can be implemented to specify at the call site that an argument should not be copy constructed, to regain some control for the C programmer.
    798 \begin{cfacode}
    799 identity(z@);  // do not copy construct argument
    800                // - will copy construct/destruct return value
    801 A@ identity_nocopy(A @ x) {  // argument not copy constructed or destructed
    802   return x;  // not copy constructed
    803              // return type marked @ => not destructed
    804 }
    805 \end{cfacode}
    806 It should be noted that reference types will allow specifying that a value does not need to be copied, however reference types do not provide a means of preventing implicit copy construction from uses of the reference, so the problem is still present when passing or returning the reference by value.
    807 
    808 A known issue with this implementation is that the argument and return value temporaries are not guaranteed to have the same address for their entire lifetimes.
    809 In the previous example, since @_retval_f@ is allocated and constructed in @f@, then returned by value, the internal data is bitwise copied into the caller's stack frame.
     1323A known issue with this implementation is that the return value of a function is not guaranteed to have the same address for its entire lifetime.
     1324Specifically, since @_retval_f@ is allocated and constructed in @f@ then returned by value, the internal data is bitwise copied into the caller's stack frame.
    8101325This approach works out most of the time, because typically destructors need to only access the fields of the object and recursively destroy.
    811 It is currently the case that constructors and destructors that use the @this@ pointer as a unique identifier to store data externally do not work correctly for return value objects.
    812 Thus, it is currently not safe to rely on an object's @this@ pointer to remain constant throughout execution of the program.
     1326It is currently the case that constructors and destructors which use the @this@ pointer as a unique identifier to store data externally will not work correctly for return value objects.
     1327Thus is it not safe to rely on an object's @this@ pointer to remain constant throughout execution of the program.
    8131328\begin{cfacode}
    8141329A * external_data[32];
    … …  
    8261341  }
    8271342}
    828 
    829 A makeA() {
    830   A x;  // stores &x in external_data
    831   return x;
    832 }
    833 makeA();  // return temporary has a different address than x
    834 // equivalent to:
    835 //   A _tmp;
    836 //   _tmp = makeA(), _tmp;
    837 //   ^?{}(&_tmp);
    8381343\end{cfacode}
    8391344In the above example, a global array of pointers is used to keep track of all of the allocated @A@ objects.
    840 Due to copying on return, the current object being destructed does not exist in the array if an @A@ object is ever returned by value from a function, such as in @makeA@.
    841 
    842 This problem could be solved in the translator by changing the function signatures so that the return value is moved into the parameter list.
     1345Due to copying on return, the current object being destructed will not exist in the array if an @A@ object is ever returned by value from a function.
     1346
     1347This problem could be solved in the translator by mutating the function signatures so that the return value is moved into the parameter list.
    8431348For example, the translator could restructure the code like so
    8441349\begin{cfacode}
    … …  
    8581363\end{cfacode}
    8591364This transformation provides @f@ with the address of the return variable so that it can be constructed into directly.
    860 It is worth pointing out that this kind of signature rewriting already occurs in polymorphic functions that return by value, as discussed in \cite{Bilson03}.
     1365It is worth pointing out that this kind of signature rewriting already occurs in polymorphic functions which return by value, as discussed in \cite{Bilson03}.
    8611366A key difference in this case is that every function would need to be rewritten like this, since types can switch between managed and unmanaged at different scope levels, e.g.
    8621367\begin{cfacode}
    8631368struct A { int v; };
    864 A x; // unmanaged, since only trivial constructors are available
     1369A x; // unmanaged
    8651370{
    8661371  void ?{}(A * a) { ... }
    … …  
    8701375A z; // unmanaged
    8711376\end{cfacode}
    872 Hence there is not enough information to determine at function declaration whether a type is managed or not, and thus it is the case that all signatures have to be rewritten to account for possible copy constructor and destructor calls.
     1377Hence there is not enough information to determine at function declaration to determine whether a type is managed or not, and thus it is the case that all signatures have to be rewritten to account for possible copy constructor and destructor calls.
    8731378Even with this change, it would still be possible to declare backwards compatible function prototypes with an @extern "C"@ block, which allows for the definition of C-compatible functions within \CFA code, however this would require actual changes to the way code inside of an @extern "C"@ function is generated as compared with normal code generation.
    874 Furthermore, it is not possible to overload C functions, so using @extern "C"@ to declare functions is of limited use.
    875 
    876 It would be possible to regain some control by adding an attribute to structs that specifies whether they can be managed or not (perhaps \emph{manageable} or \emph{unmanageable}), and to emit an error in the case that a constructor or destructor is declared for an unmanageable type.
     1379Furthermore, it isn't possible to overload C functions, so using @extern "C"@ to declare functions is of limited use.
     1380
     1381It would be possible to regain some control by adding an attribute to structs which specifies whether they can be managed or not (perhaps \emph{manageable} or \emph{unmanageable}), and to emit an error in the case that a constructor or destructor is declared for an unmanageable type.
    8771382Ideally, structs should be manageable by default, since otherwise the default case becomes more verbose.
    8781383This means that in general, function signatures would have to be rewritten, and in a select few cases the signatures would not be rewritten.
    … …  
    8871392\end{cfacode}
    8881393An alternative is to instead make the attribute \emph{identifiable}, which states that objects of this type use the @this@ parameter as an identity.
    889 This strikes more closely to the visible problem, in that only types marked as identifiable would need to have the return value moved into the parameter list, and every other type could remain the same.
     1394This strikes more closely to the visibile problem, in that only types marked as identifiable would need to have the return value moved into the parameter list, and every other type could remain the same.
    8901395Furthermore, no restrictions would need to be placed on whether objects can be constructed.
    8911396\begin{cfacode}
    … …  
    8971402\end{cfacode}
    8981403
    899 Ultimately, both of these are patchwork solutions.
    900 Since a real compiler has full control over its calling conventions, it can seamlessly allow passing the return parameter without outwardly changing the signature of a routine.
    901 As such, it has been decided that this issue is not currently a priority and will be fixed when a full \CFA compiler is implemented.
     1404Ultimately, this is the type of transformation that a real compiler would make when generating assembly code.
     1405Since a compiler has full control over its calling conventions, it can seamlessly allow passing the return parameter without outwardly changing the signature of a routine.
     1406As such, it has been decided that this issue is not currently a priority.
    9021407
    9031408\section{Implementation}
    9041409\subsection{Array Initialization}
    905 Arrays are a special case in the C type-system.
     1410Arrays are a special case in the C type system.
    9061411C arrays do not carry around their size, making it impossible to write a standalone \CFA function that constructs or destructs an array while maintaining the standard interface for constructors and destructors.
    9071412Instead, \CFA defines the initialization and destruction of an array recursively.
    … …  
    10201525By default, objects within a translation unit are constructed in declaration order, and destructed in the reverse order.
    10211526The default order of construction of objects amongst translation units is unspecified.
     1527% TODO: not yet implemented, but g++ provides attribute init_priority, which allows specifying the order of global construction on a per object basis
     1528%   https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Attributes.html#C_002b_002b-Attributes
     1529% suggestion: implement this in CFA by picking objects with a specified priority and pulling them into their own init functions (could even group them by priority level -> map<int, list<ObjectDecl*>>) and pull init_priority forward into constructor and destructor attributes with the same priority level
    10221530It is, however, guaranteed that any global objects in the standard library are initialized prior to the initialization of any object in the user program.
    10231531
    1024 This feature is implemented in the \CFA translator by grouping every global constructor call into a function with the GCC attribute \emph{constructor}, which performs most of the heavy lifting \cite[6.31.1]{GCCExtensions}.
     1532This feature is implemented in the \CFA translator by grouping every global constructor call into a function with the GCC attribute \emph{constructor}, which performs most of the heavy lifting. % CITE: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes
    10251533A similar function is generated with the \emph{destructor} attribute, which handles all global destructor calls.
    10261534At the time of writing, initialization routines in the library are specified with priority \emph{101}, which is the highest priority level that GCC allows, whereas initialization routines in the user's code are implicitly given the default priority level, which ensures they have a lower priority than any code with a specified priority level.
    1027 This mechanism allows arbitrarily complicated initialization to occur before any user code runs, making it possible for library designers to initialize their modules without requiring the user to call specific startup or tear-down routines.
     1535This mechanism allows arbitrarily complicated initialization to occur before any user code runs, making it possible for library designers to initialize their modules without requiring the user to call specific startup or teardown routines.
    10281536
    10291537For example, given the following global declarations.
    … …  
    10511559\end{cfacode}
    10521560
    1053 %   https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Attributes.html#C_002b_002b-Attributes
    1054 % suggestion: implement this in CFA by picking objects with a specified priority and pulling them into their own init functions (could even group them by priority level -> map<int, list<ObjectDecl*>>) and pull init_priority forward into constructor and destructor attributes with the same priority level
    1055 GCC provides an attribute @init_priority@, which allows specifying the relative priority for initialization of global objects on a per-object basis in \CC.
    1056 A similar attribute can be implemented in \CFA by pulling marked objects into global constructor/destructor-attribute functions with the specified priority.
    1057 For example,
    1058 \begin{cfacode}
    1059 struct A { ... };
    1060 void ?{}(A *, int);
    1061 void ^?{}(A *);
    1062 __attribute__((init_priority(200))) A x = { 123 };
    1063 \end{cfacode}
    1064 would generate
    1065 \begin{cfacode}
    1066 A x;
    1067 __attribute__((constructor(200))) __init_x() {
    1068   ?{}(&x, 123);  // construct x with priority 200
    1069 }
    1070 __attribute__((destructor(200))) __destroy_x() {
    1071   ?{}(&x);       // destruct x with priority 200
    1072 }
    1073 \end{cfacode}
    1074 
    10751561\subsection{Static Local Variables}
    10761562In standard C, it is possible to mark variables that are local to a function with the @static@ storage class.
    1077 Unlike normal local variables, a @static@ local variable is defined to live for the entire duration of the program, so that each call to the function has access to the same variable with the same address and value as it had in the previous call to the function.
    1078 Much like global variables, in C @static@ variables can only be initialized to a \emph{compile-time constant value} so that a compiler is able to create storage for the variable and initialize it at compile-time.
     1563Unlike normal local variables, a @static@ local variable is defined to live for the entire duration of the program, so that each call to the function has access to the same variable with the same address and value as it had in the previous call to the function. % TODO: mention dynamic loading caveat??
     1564Much like global variables, in C @static@ variables must be initialized to a \emph{compile-time constant value} so that a compiler is able to create storage for the variable and initialize it before the program begins running.
    10791565
    10801566Yet again, this rule is too restrictive for a language with constructors and destructors.
    … …  
    10871573Construction of @static@ local objects is implemented via an accompanying @static bool@ variable, which records whether the variable has already been constructed.
    10881574A conditional branch checks the value of the companion @bool@, and if the variable has not yet been constructed then the object is constructed.
    1089 The object's destructor is scheduled to be run when the program terminates using @atexit@ \footnote{When using the dynamic linker, it is possible to dynamically load and unload a shared library. Since glibc 2.2.3 \cite{atexit}, functions registered with @atexit@ within the shared library are called when unloading the shared library. As such, static local objects can be destructed using this mechanism even in shared libraries on Linux systems.}, and the companion @bool@'s value is set so that subsequent invocations of the function do not reconstruct the object.
     1575The object's destructor is scheduled to be run when the program terminates using @atexit@, and the companion @bool@'s value is set so that subsequent invocations of the function will not reconstruct the object.
    10901576Since the parameter to @atexit@ is a parameter-less function, some additional tweaking is required.
    10911577First, the @static@ variable must be hoisted up to global scope and uniquely renamed to prevent name clashes with other global objects.
    … …  
    10931579Finally, the newly generated function is registered with @atexit@, instead of registering the destructor directly.
    10941580Since @atexit@ calls functions in the reverse order in which they are registered, @static@ local variables are guaranteed to be destructed in the reverse order that they are constructed, which may differ between multiple executions of the same program.
     1581
    10951582Extending the previous example
    10961583\begin{cfacode}
    … …  
    11431630\end{cfacode}
    11441631
    1145 \subsection{Polymorphism}
    1146 As mentioned in section \ref{sub:polymorphism}, \CFA currently has 3 type-classes that are used to designate polymorphic data types: @otype@, @dtype@, and @ftype@.
    1147 In previous versions of \CFA, @otype@ was syntactic sugar for @dtype@ with known size/alignment information and an assignment function.
    1148 That is,
    1149 \begin{cfacode}
    1150 forall(otype T)
    1151 void f(T);
    1152 \end{cfacode}
    1153 was equivalent to
    1154 \begin{cfacode}
    1155 forall(dtype T | sized(T) | { T ?=?(T *, T); })
    1156 void f(T);
    1157 \end{cfacode}
    1158 This allows easily specifying constraints that are common to all complete object types very simply.
    1159 
    1160 Now that \CFA has constructors and destructors, more of a complete object's behaviour can be specified by than was previously possible.
    1161 As such, @otype@ has been augmented to include assertions for a default constructor, copy constructor, and destructor.
    1162 That is, the previous example is now equivalent to
    1163 \begin{cfacode}
    1164 forall(dtype T | sized(T) | { T ?=?(T *, T); void ?{}(T *); void ?{}(T *, T); void ^?{}(T *); })
    1165 void f(T);
    1166 \end{cfacode}
    1167 This allows @f@'s body to create and destroy objects of type @T@, and pass objects of type @T@ as arguments to other functions, following the normal \CFA rules.
    1168 A point of note here is that objects can be missing default constructors (and eventually other functions through deleted functions), so it is important for \CFA programmers to think carefully about the operations needed by their function, as to not over-constrain the acceptable parameter types.
     1632\subsection{Constructor Expressions}
     1633In \CFA, it is possible to use a constructor as an expression.
     1634Like other operators, the function name @?{}@ matches its operator syntax.
     1635For example, @(&x){}@ calls the default constructor on the variable @x@, and produces @&x@ as a result.
     1636The significance of constructors as expressions rather than as statements is that the result of a constructor expression can be used as part of a larger expression.
     1637A key example is the use of constructor expressions to initialize the result of a call to standard C routine @malloc@.
     1638\begin{cfacode}
     1639struct X { ... };
     1640void ?{}(X *, double);
     1641X * x = malloc(sizeof(X)){ 1.5 };
     1642\end{cfacode}
     1643In this example, @malloc@ dynamically allocates storage and initializes it using a constructor, all before assigning it into the variable @x@.
     1644If this extension is not present, constructing dynamically allocated objects is much more cumbersome, requiring separate initialization of the pointer and initialization of the pointed-to memory.
     1645\begin{cfacode}
     1646X * x = malloc(sizeof(X));
     1647x{ 1.5 };
     1648\end{cfacode}
     1649Not only is this verbose, but it is also more error prone, since this form allows maintenance code to easily sneak in between the initialization of @x@ and the initialization of the memory that @x@ points to.
     1650This feature is implemented via a transformation produceing the value of the first argument of the constructor, since constructors do not themslves have a return value.
     1651Since this transformation results in two instances of the subexpression, care is taken to allocate a temporary variable to hold the result of the subexpression in the case where the subexpression may contain side effects.
     1652The previous example generates the following code.
     1653\begin{cfacode}
     1654struct X *_tmp_ctor;
     1655struct X *x = ?{}((_tmp_ctor=((_tmp_cp_ret0=
     1656  malloc(sizeof(struct X))), _tmp_cp_ret0))), 1.5), _tmp_ctor);
     1657\end{cfacode}
     1658It should be noted that this technique is not exclusive to @malloc@, and allows a user to write a custom allocator that can be idiomatically used in much the same way as a constructed @malloc@ call.
     1659
     1660It is also possible to use operator syntax with destructors.
     1661Unlike constructors, operator syntax with destructors is a statement and thus does not produce a value, since the destructed object is invalidated by the use of a destructor.
     1662For example, \lstinline!^(&x){}! calls the destructor on the variable @x@.

  • doc/rob_thesis/intro.tex

    rf674479 ra0fc78a  
    55\section{\CFA Background}
    66\label{s:background}
    7 \CFA \footnote{Pronounced ``C-for-all'', and written \CFA or Cforall.} is a modern non-object-oriented extension to the C programming language.
     7\CFA is a modern extension to the C programming language.
    88As it is an extension of C, there is already a wealth of existing C code and principles that govern the design of the language.
    99Among the goals set out in the original design of \CFA, four points stand out \cite{Bilson03}.
    … …  
    1616Therefore, these design principles must be kept in mind throughout the design and development of new language features.
    1717In order to appeal to existing C programmers, great care must be taken to ensure that new features naturally feel like C.
    18 The remainder of this section describes some of the important new features that currently exist in \CFA, to give the reader the necessary context in which the new features presented in this thesis must dovetail.
     18The remainder of this section describes some of the important new features that currently exist in \CFA, to give the reader the necessary context in which the new features presented in this thesis must dovetail. % TODO: harmonize with?
    1919
    2020\subsection{C Background}
    … …  
    2929A a1 = { 1, .y:7, 6 };
    3030A a2[4] = { [2]:a0, [0]:a1, { .z:3 } };
    31 // equivalent to
     31// equvialent to
    3232// A a0 = { 0, 8, 0, 1 };
    3333// A a1 = { 1, 0, 7, 6 };
    … …  
    3636Designations allow specifying the field to initialize by name, rather than by position.
    3737Any field not explicitly initialized is initialized as if it had static storage duration \cite[p.~141]{C11}.
    38 A designator specifies the current object for initialization, and as such any undesignated sub-objects pick up where the last initialization left off.
    39 For example, in the initialization of @a1@, the initializer of @y@ is @7@, and the unnamed initializer @6@ initializes the next sub-object, @z@.
    40 Later initializers override earlier initializers, so a sub-object for which there is more than one initializer is only initialized by its last initializer.
    41 These semantics can be seen in the initialization of @a0@, where @x@ is designated twice, and thus initialized to @8@.
    42 Note that in \CFA, designations use a colon separator, rather than an equals sign as in C, because this syntax is one of the few places that conflicts with the new language features.
     38A designator specifies the current object for initialization, and as such any undesignated subobjects pick up where the last initialization left off.
     39For example, in the initialization of @a1@, the initializer of @y@ is @7@, and the unnamed initializer @6@ initializes the next subobject, @z@.
     40Later initializers override earlier initializers, so a subobject for which there is more than one initializer is only initailized by its last initializer.
     41This can be seen in the initialization of @a0@, where @x@ is designated twice, and thus initialized to @8@.
     42Note that in \CFA, designations use a colon separator, rather than an equals sign as in C.
    4343
    4444C also provides \emph{compound literal} expressions, which provide a first-class mechanism for creating unnamed objects.
    … …  
    9191
    9292There are times when a function should logically return multiple values.
    93 Since a function in standard C can only return a single value, a programmer must either take in additional return values by address, or the function's designer must create a wrapper structure to package multiple return-values.
     93Since a function in standard C can only return a single value, a programmer must either take in additional return values by address, or the function's designer must create a wrapper structure t0 package multiple return-values.
    9494\begin{cfacode}
    9595int f(int * ret) {        // returns a value through parameter ret
    … …  
    102102\end{cfacode}
    103103The former solution is awkward because it requires the caller to explicitly allocate memory for $n$ result variables, even if they are only temporary values used as a subexpression, or even not used at all.
    104 The latter approach:
    105104\begin{cfacode}
    106105struct A {
    … …  
    113112... res3.x ... res3.y ... // use result values
    114113\end{cfacode}
    115 requires the caller to either learn the field names of the structure or learn the names of helper routines to access the individual return values.
     114The latter approach requires the caller to either learn the field names of the structure or learn the names of helper routines to access the individual return values.
    116115Both solutions are syntactically unnatural.
    117116
    118 In \CFA, it is possible to directly declare a function returning multiple values.
    119 This extension provides important semantic information to the caller, since return values are only for output.
    120 \begin{cfacode}
    121 [int, int] f() {       // no new type
     117In \CFA, it is possible to directly declare a function returning mutliple values.
     118This provides important semantic information to the caller, since return values are only for output.
     119\begin{cfacode}
     120[int, int] f() {       // don't need to create a new type
    122121  return [123, 37];
    123122}
    124123\end{cfacode}
    125 However, the ability to return multiple values is useless without a syntax for accepting the results from the function.
    126 
     124However, the ability to return multiple values requires a syntax for accepting the results from a function.
    127125In standard C, return values are most commonly assigned directly into local variables, or are used as the arguments to another function call.
    128126\CFA allows both of these contexts to accept multiple return values.
    … …  
    150148  g(f());             // selects (2)
    151149  \end{cfacode}
    152 In this example, the only possible call to @f@ that can produce the two @int@s required for assigning into the variables @x@ and @y@ is the second option.
    153 A similar reasoning holds calling the function @g@.
     150In this example, the only possible call to @f@ that can produce the two @int@s required by @g@ is the second option.
     151A similar reasoning holds for assigning into multiple variables.
    154152
    155153In \CFA, overloading also applies to operator names, known as \emph{operator overloading}.
    … …  
    168166  bool ?<?(A x, A y);
    169167  \end{cfacode}
    170 Notably, the only difference is syntax.
     168Notably, the only difference in this example is syntax.
    171169Most of the operators supported by \CC for operator overloading are also supported in \CFA.
    172170Of notable exception are the logical operators (e.g. @||@), the sequence operator (i.e. @,@), and the member-access operators (e.g. @.@ and \lstinline{->}).
    … …  
    174172Finally, \CFA also permits overloading variable identifiers.
    175173This feature is not available in \CC.
    176   \begin{cfacode}
     174  \begin{cfacode} % TODO: pick something better than x? max, zero, one?
    177175  struct Rational { int numer, denom; };
    178176  int x = 3;               // (1)
    … …  
    188186In this example, there are three definitions of the variable @x@.
    189187Based on the context, \CFA attempts to choose the variable whose type best matches the expression context.
    190 When used judiciously, this feature allows names like @MAX@, @MIN@, and @PI@ to apply across many types.
    191188
    192189Finally, the values @0@ and @1@ have special status in standard C.
    … …  
    200197}
    201198\end{cfacode}
    202 Every if- and iteration-statement in C compares the condition with @0@, and every increment and decrement operator is semantically equivalent to adding or subtracting the value @1@ and storing the result.
     199Every if statement in C compares the condition with @0@, and every increment and decrement operator is semantically equivalent to adding or subtracting the value @1@ and storing the result.
    203200Due to these rewrite rules, the values @0@ and @1@ have the types \zero and \one in \CFA, which allow for overloading various operations that connect to @0@ and @1@ \footnote{In the original design of \CFA, @0@ and @1@ were overloadable names \cite[p.~7]{cforall}.}.
    204 The types \zero and \one have special built-in implicit conversions to the various integral types, and a conversion to pointer types for @0@, which allows standard C code involving @0@ and @1@ to work as normal.
     201The types \zero and \one have special built in implicit conversions to the various integral types, and a conversion to pointer types for @0@, which allows standard C code involving @0@ and @1@ to work as normal.
    205202  \begin{cfacode}
    206203  // lvalue is similar to returning a reference in C++
    … …  
    296293This capability allows specifying the same set of assertions in multiple locations, without the repetition and likelihood of mistakes that come with manually writing them out for each function declaration.
    297294
    298 An interesting application of return-type resolution and polymorphism is with type-safe @malloc@.
    299 \begin{cfacode}
    300 forall(dtype T | sized(T))
    301 T * malloc() {
    302   return (T*)malloc(sizeof(T)); // call C malloc
    303 }
    304 int * x = malloc();     // malloc(sizeof(int))
    305 double * y = malloc();  // malloc(sizeof(double))
    306 
    307 struct S { ... };
    308 S * s = malloc();       // malloc(sizeof(S))
    309 \end{cfacode}
    310 The built-in trait @sized@ ensures that size and alignment information for @T@ is available in the body of @malloc@ through @sizeof@ and @_Alignof@ expressions respectively.
    311 In calls to @malloc@, the type @T@ is bound based on call-site information, allowing \CFA code to allocate memory without the potential for errors introduced by manually specifying the size of the allocated block.
    312 
    313295\section{Invariants}
    314 An \emph{invariant} is a logical assertion that is true for some duration of a program's execution.
     296% TODO: discuss software engineering benefits of ctor/dtors: {pre/post} conditions, invariants
     297% an important invariant is the state of the environment (memory, resources)
     298% some objects pass their contract to the object user
     299An \emph{invariant} is a logical assertion that true for some duration of a program's execution.
    315300Invariants help a programmer to reason about code correctness and prove properties of programs.
    316301
    317302In object-oriented programming languages, type invariants are typically established in a constructor and maintained throughout the object's lifetime.
    318 These assertions are typically achieved through a combination of access control modifiers and a restricted interface.
     303This is typically achieved through a combination of access control modifiers and a restricted interface.
    319304Typically, data which requires the maintenance of an invariant is hidden from external sources using the \emph{private} modifier, which restricts reads and writes to a select set of trusted routines, including member functions.
    320305It is these trusted routines that perform all modifications to internal data in a way that is consistent with the invariant, by ensuring that the invariant holds true at the end of the routine call.
    … …  
    322307In C, the @assert@ macro is often used to ensure invariants are true.
    323308Using @assert@, the programmer can check a condition and abort execution if the condition is not true.
    324 This powerful tool forces the programmer to deal with logical inconsistencies as they occur.
     309This is a powerful tool that forces the programmer to deal with logical inconsistencies as they occur.
    325310For production, assertions can be removed by simply defining the preprocessor macro @NDEBUG@, making it simple to ensure that assertions are 0-cost for a performance intensive application.
    326311\begin{cfacode}
    … …  
    369354\end{dcode}
    370355The D compiler is able to assume that assertions and invariants hold true and perform optimizations based on those assumptions.
    371 Note, these invariants are internal to the type's correct behaviour.
    372 
    373 Types also have external invariants with the state of the execution environment, including the heap, the open-file table, the state of global variables, etc.
    374 Since resources are finite and shared (concurrency), it is important to ensure that objects clean up properly when they are finished, restoring the execution environment to a stable state so that new objects can reuse resources.
     356
     357An important invariant is the state of the execution environment, including the heap, the open file table, the state of global variables, etc.
     358Since resources are finite, it is important to ensure that objects clean up properly when they are finished, restoring the execution environment to a stable state so that new objects can reuse resources.
    375359
    376360\section{Resource Management}
    … …  
    382366The program stack grows and shrinks automatically with each function call, as needed for local variables.
    383367However, whenever a program needs a variable to outlive the block it is created in, the storage must be allocated dynamically with @malloc@ and later released with @free@.
    384 This pattern is extended to more complex objects, such as files and sockets, which can also outlive the block where they are created, and thus require their own resource management.
    385 Once allocated storage escapes\footnote{In garbage collected languages, such as Java, escape analysis \cite{Choi:1999:EAJ:320385.320386} is used to determine when dynamically allocated objects are strictly contained within a function, which allows the optimizer to allocate them on the stack.} a block, the responsibility for deallocating the storage is not specified in a function's type, that is, that the return value is owned by the caller.
     368This pattern is extended to more complex objects, such as files and sockets, which also outlive the block where they are created, but at their core is resource management.
     369Once allocated storage escapes a block, the responsibility for deallocating the storage is not specified in a function's type, that is, that the return value is owned by the caller.
    386370This implicit convention is provided only through documentation about the expectations of functions.
    387371
    388372In other languages, a hybrid situation exists where resources escape the allocation block, but ownership is precisely controlled by the language.
    389 This pattern requires a strict interface and protocol for a data structure, consisting of a pre-initialization and a post-termination call, and all intervening access is done via interface routines.
    390 This kind of encapsulation is popular in object-oriented programming languages, and like the stack, it takes care of a significant portion of resource management cases.
     373This pattern requires a strict interface and protocol for a data structure, where the protocol consists of a pre-initialization and a post-termination call, and all intervening access is done via interface routines.
     374This kind of encapsulation is popular in object-oriented programming languages, and like the stack, it contains a significant portion of resource management cases.
    391375
    392376For example, \CC directly supports this pattern through class types and an idiom known as RAII \footnote{Resource Acquisition is Initialization} by means of constructors and destructors.
    … …  
    396380On the other hand, destructors provide a simple mechanism for tearing down an object and resetting the environment in which the object lived.
    397381RAII ensures that if all resources are acquired in a constructor and released in a destructor, there are no resource leaks, even in exceptional circumstances.
    398 A type with at least one non-trivial constructor or destructor is henceforth referred to as a \emph{managed type}.
    399 In the context of \CFA, a non-trivial constructor is either a user defined constructor or an auto-generated constructor that calls a non-trivial constructor.
    400 
    401 For the remaining resource ownership cases, programmer must follow a brittle, explicit protocol for freeing resources or an implicit protocol implemented via the programming language.
     382A type with at least one non-trivial constructor or destructor will henceforth be referred to as a \emph{managed type}.
     383In the context of \CFA, a non-trivial constructor is either a user defined constructor or an auto generated constructor that calls a non-trivial constructor.
     384
     385For the remaining resource ownership cases, programmer must follow a brittle, explicit protocol for freeing resources or an implicit porotocol implemented via the programming language.
    402386
    403387In garbage collected languages, such as Java, resources are largely managed by the garbage collector.
    … …  
    405389There are many kinds of resources that the garbage collector does not understand, such as sockets, open files, and database connections.
    406390In particular, Java supports \emph{finalizers}, which are similar to destructors.
    407 Sadly, finalizers are only guaranteed to be called before an object is reclaimed by the garbage collector \cite[p.~373]{Java8}, which may not happen if memory use is not contentious.
    408 Due to operating-system resource-limits, this is unacceptable for many long running programs.
    409 Instead, the paradigm in Java requires programmers to manually keep track of all resources \emph{except} memory, leading many novices and experts alike to forget to close files, etc.
    410 Complicating the picture, uncaught exceptions can cause control flow to change dramatically, leaking a resource that appears on first glance to be released.
     391Sadly, finalizers come with far fewer guarantees, to the point where a completely conforming JVM may never call a single finalizer. % TODO: citation JVM spec; http://stackoverflow.com/a/2506514/2386739
     392Due to operating system resource limits, this is unacceptable for many long running tasks. % TODO: citation?
     393Instead, the paradigm in Java requires programmers manually keep track of all resource \emph{except} memory, leading many novices and experts alike to forget to close files, etc.
     394Complicating the picture, uncaught exceptions can cause control flow to change dramatically, leaking a resource which appears on first glance to be closed.
    411395\begin{javacode}
    412396void write(String filename, String msg) throws Exception {
    … …  
    419403}
    420404\end{javacode}
    421 Any line in this program can throw an exception, which leads to a profusion of finally blocks around many function bodies, since it is not always clear when an exception may be thrown.
     405Any line in this program can throw an exception.
     406This leads to a profusion of finally blocks around many function bodies, since it isn't always clear when an exception may be thrown.
    422407\begin{javacode}
    423408public void write(String filename, String msg) throws Exception {
    … …  
    437422\end{javacode}
    438423In Java 7, a new \emph{try-with-resources} construct was added to alleviate most of the pain of working with resources, but ultimately it still places the burden squarely on the user rather than on the library designer.
    439 Furthermore, for complete safety this pattern requires nested objects to be declared separately, otherwise resources that can throw an exception on close can leak nested resources \cite{TryWithResources}.
     424Furthermore, for complete safety this pattern requires nested objects to be declared separately, otherwise resources which can throw an exception on close can leak nested resources. % TODO: cite oracle article http://www.oracle.com/technetwork/articles/java/trywithresources-401775.html?
    440425\begin{javacode}
    441426public void write(String filename, String msg) throws Exception {
    442   try (  // try-with-resources
     427  try (
    443428    FileOutputStream out = new FileOutputStream(filename);
    444429    FileOutputStream log = new FileOutputStream("log.txt");
    … …  
    449434}
    450435\end{javacode}
    451 Variables declared as part of a try-with-resources statement must conform to the @AutoClosable@ interface, and the compiler implicitly calls @close@ on each of the variables at the end of the block.
    452 Depending on when the exception is raised, both @out@ and @log@ are null, @log@ is null, or both are non-null, therefore, the cleanup for these variables at the end is appropriately guarded and conditionally executed to prevent null-pointer exceptions.
    453 
    454 While Rust \cite{Rust} does not enforce the use of a garbage collector, it does provide a manual memory management environment, with a strict ownership model that automatically frees allocated memory and prevents common memory management errors.
    455 In particular, a variable has ownership over its associated value, which is freed automatically when the owner goes out of scope.
    456 Furthermore, values are \emph{moved} by default on assignment, rather than copied, which invalidates the previous variable binding.
    457 \begin{rustcode}
    458 struct S {
    459   x: i32
    460 }
    461 let s = S { x: 123 };
    462 let z = s;           // move, invalidate s
    463 println!("{}", s.x); // error, s has been moved
    464 \end{rustcode}
    465 Types can be made copyable by implementing the @Copy@ trait.
    466 
    467 Rust allows multiple unowned views into an object through references, also known as borrows, provided that a reference does not outlive its referent.
    468 A mutable reference is allowed only if it is the only reference to its referent, preventing data race errors and iterator invalidation errors.
    469 \begin{rustcode}
    470 let mut x = 10;
    471 {
    472   let y = &x;
    473   let z = &x;
    474   println!("{} {}", y, z); // prints 10 10
    475 }
    476 {
    477   let y = &mut x;
    478   // let z1 = &x;     // not allowed, have mutable reference
    479   // let z2 = &mut x; // not allowed, have mutable reference
    480   *y = 5;
    481   println!("{}", y); // prints 5
    482 }
    483 println!("{}", x); // prints 5
    484 \end{rustcode}
    485 Since references are not owned, they do not release resources when they go out of scope.
    486 There is no runtime cost imposed on these restrictions, since they are enforced at compile-time.
    487 
    488 Rust provides RAII through the @Drop@ trait, allowing arbitrary code to execute when the object goes out of scope, allowing Rust programs to automatically clean up auxiliary resources much like a \CC program.
    489 \begin{rustcode}
    490 struct S {
    491   name: &'static str
    492 }
    493 
    494 impl Drop for S {  // RAII for S
    495   fn drop(&mut self) {
    496     println!("dropped {}", self.name);
    497   }
    498 }
    499 
    500 {
    501   let x = S { name: "x" };
    502   let y = S { name: "y" };
    503 } // prints "dropped y" "dropped x"
    504 \end{rustcode}
     436On the other hand, the Java compiler generates more code if more resources are declared, meaning that users must be more familiar with each type and library designers must provide better documentation.
    505437
    506438% D has constructors and destructors that are worth a mention (under classes) https://dlang.org/spec/spec.html
    … …  
    510442The programming language, D, also manages resources with constructors and destructors \cite{D}.
    511443In D, @struct@s are stack allocated and managed via scoping like in \CC, whereas @class@es are managed automatically by the garbage collector.
    512 Like Java, using the garbage collector means that destructors are called indeterminately, requiring the use of finally statements to ensure dynamically allocated resources that are not managed by the garbage collector, such as open files, are cleaned up.
     444Like Java, using the garbage collector means that destructors may never be called, requiring the use of finally statements to ensure dynamically allocated resources that are not managed by the garbage collector, such as open files, are cleaned up.
    513445Since D supports RAII, it is possible to use the same techniques as in \CC to ensure that resources are released in a timely manner.
    514 Finally, D provides a scope guard statement, which allows an arbitrary statement to be executed at normal scope exit with \emph{success}, at exceptional scope exit with \emph{failure}, or at normal and exceptional scope exit with \emph{exit}. % https://dlang.org/spec/statement.html#ScopeGuardStatement
    515 It has been shown that the \emph{exit} form of the scope guard statement can be implemented in a library in \CC \cite{ExceptSafe}.
    516 
    517 To provide managed types in \CFA, new kinds of constructors and destructors are added to \CFA and discussed in Chapter 2.
     446Finally, D provides a scope guard statement, which allows an arbitrary statement to be executed at normal scope exit with \emph{success}, at exceptional scope exit with \emph{failure}, or at normal and exceptional scope exit with \emph{exit}. % cite? https://dlang.org/spec/statement.html#ScopeGuardStatement
     447It has been shown that the \emph{exit} form of the scope guard statement can be implemented in a library in \CC. % cite: http://www.drdobbs.com/184403758
     448
     449% TODO: discussion of lexical scope vs. dynamic
     450% see Peter's suggestions
     451% RAII works in both cases. Guaranteed to work in stack case, works in heap case if root is deleted (but it's dangerous to rely on this, because of exceptions)
    518452
    519453\section{Tuples}
    520454\label{s:Tuples}
    521 In mathematics, tuples are finite-length sequences which, unlike sets, are ordered and allow duplicate elements.
    522 In programming languages, tuples provide fixed-sized heterogeneous lists of elements.
     455In mathematics, tuples are finite-length sequences which, unlike sets, allow duplicate elements.
     456In programming languages, tuples are a construct that provide fixed-sized heterogeneous lists of elements.
    523457Many programming languages have tuple constructs, such as SETL, \KWC, ML, and Scala.
    524458
    … …  
    528462Adding tuples to \CFA has previously been explored by Esteves \cite{Esteves04}.
    529463
    530 The design of tuples in \KWC took much of its inspiration from SETL \cite{SETL}.
     464The design of tuples in \KWC took much of its inspiration from SETL.
    531465SETL is a high-level mathematical programming language, with tuples being one of the primary data types.
    532466Tuples in SETL allow a number of operations, including subscripting, dynamic expansion, and multiple assignment.
    … …  
    536470\begin{cppcode}
    537471tuple<int, int, int> triple(10, 20, 30);
    538 get<1>(triple); // access component 1 => 20
     472get<1>(triple); // access component 1 => 30
    539473
    540474tuple<int, double> f();
    … …  
    548482Tuples are simple data structures with few specific operations.
    549483In particular, it is possible to access a component of a tuple using @std::get<N>@.
    550 Another interesting feature is @std::tie@, which creates a tuple of references, allowing assignment of the results of a tuple-returning function into separate local variables, without requiring a temporary variable.
     484Another interesting feature is @std::tie@, which creates a tuple of references, which allows assigning the results of a tuple-returning function into separate local variables, without requiring a temporary variable.
    551485Tuples also support lexicographic comparisons, making it simple to write aggregate comparators using @std::tie@.
    552486
    553 There is a proposal for \CCseventeen called \emph{structured bindings} \cite{StructuredBindings}, that introduces new syntax to eliminate the need to pre-declare variables and use @std::tie@ for binding the results from a function call.
     487There is a proposal for \CCseventeen called \emph{structured bindings}, that introduces new syntax to eliminate the need to pre-declare variables and use @std::tie@ for binding the results from a function call. % TODO: cite http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0144r0.pdf
    554488\begin{cppcode}
    555489tuple<int, double> f();
    … …  
    566500Structured bindings allow unpacking any struct with all public non-static data members into fresh local variables.
    567501The use of @&@ allows declaring new variables as references, which is something that cannot be done with @std::tie@, since \CC references do not support rebinding.
    568 This extension requires the use of @auto@ to infer the types of the new variables, so complicated expressions with a non-obvious type must be documented with some other mechanism.
     502This extension requires the use of @auto@ to infer the types of the new variables, so complicated expressions with a non-obvious type must documented with some other mechanism.
    569503Furthermore, structured bindings are not a full replacement for @std::tie@, as it always declares new variables.
    570504
    571505Like \CC, D provides tuples through a library variadic template struct.
    572506In D, it is possible to name the fields of a tuple type, which creates a distinct type.
    573 % http://dlang.org/phobos/std_typecons.html
    574 \begin{dcode}
     507\begin{dcode} % TODO: cite http://dlang.org/phobos/std_typecons.html
    575508Tuple!(float, "x", float, "y") point2D;
    576 Tuple!(float, float) float2;  // different type from point2D
     509Tuple!(float, float) float2;  // different types
    577510
    578511point2D[0]; // access first element
    … …  
    588521The @expand@ method produces the components of the tuple as a list of separate values, making it possible to call a function that takes $N$ arguments using a tuple with $N$ components.
    589522
    590 Tuples are a fundamental abstraction in most functional programming languages, such as Standard ML \cite{sml}.
     523Tuples are a fundamental abstraction in most functional programming languages, such as Standard ML.
    591524A function in SML always accepts exactly one argument.
    592525There are two ways to mimic multiple argument functions: the first through currying and the second by accepting tuple arguments.
    … …  
    602535Tuples are a foundational tool in SML, allowing the creation of arbitrarily complex structured data types.
    603536
    604 Scala, like \CC, provides tuple types through the standard library \cite{Scala}.
     537Scala, like \CC, provides tuple types through the standard library.
    605538Scala provides tuples of size 1 through 22 inclusive through generic data structures.
    606539Tuples support named access and subscript access, among a few other operations.
    … …  
    614547\end{scalacode}
    615548In Scala, tuples are primarily used as simple data structures for carrying around multiple values or for returning multiple values from a function.
    616 The 22-element restriction is an odd and arbitrary choice, but in practice it does not cause problems since large tuples are uncommon.
     549The 22-element restriction is an odd and arbitrary choice, but in practice it doesn't cause problems since large tuples are uncommon.
    617550Subscript access is provided through the @productElement@ method, which returns a value of the top-type @Any@, since it is impossible to receive a more precise type from a general subscripting method due to type erasure.
    618551The disparity between named access beginning at @_1@ and subscript access starting at @0@ is likewise an oddity, but subscript access is typically avoided since it discards type information.
    … …  
    620553
    621554
    622 \Csharp also has tuples, but has similarly strange limitations, allowing tuples of size up to 7 components. % https://msdn.microsoft.com/en-us/library/system.tuple(v=vs.110).aspx
     555\Csharp has similarly strange limitations, allowing tuples of size up to 7 components. % TODO: cite https://msdn.microsoft.com/en-us/library/system.tuple(v=vs.110).aspx
    623556The officially supported workaround for this shortcoming is to nest tuples in the 8th component.
    624557\Csharp allows accessing a component of a tuple by using the field @Item$N$@ for components 1 through 7, and @Rest@ for the nested tuple.
    625558
    626 In Python \cite{Python}, tuples are immutable sequences that provide packing and unpacking operations.
     559
     560% TODO: cite 5.3 https://docs.python.org/3/tutorial/datastructures.html
     561In Python, tuples are immutable sequences that provide packing and unpacking operations.
    627562While the tuple itself is immutable, and thus does not allow the assignment of components, there is nothing preventing a component from being internally mutable.
    628563The components of a tuple can be accessed by unpacking into multiple variables, indexing, or via field name, like D.
    629564Tuples support multiple assignment through a combination of packing and unpacking, in addition to the common sequence operations.
    630565
    631 Swift \cite{Swift}, like D, provides named tuples, with components accessed by name, index, or via extractors.
     566% TODO: cite https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Types.html#//apple_ref/doc/uid/TP40014097-CH31-ID448
     567Swift, like D, provides named tuples, with components accessed by name, index, or via extractors.
    632568Tuples are primarily used for returning multiple values from a function.
    633569In Swift, @Void@ is an alias for the empty tuple, and there are no single element tuples.
    634 
    635 Tuples comparable to those described above are added to \CFA and discussed in Chapter 3.
    636570
    637571\section{Variadic Functions}
    … …  
    647581printf("%d %g %c %s", 10, 3.5, 'X', "a string");
    648582\end{cfacode}
    649 Through the use of a format string, C programmers can communicate argument type information to @printf@, allowing C programmers to print any of the standard C data types.
     583Through the use of a format string, @printf@ allows C programmers to print any of the standard C data types.
    650584Still, @printf@ is extremely limited, since the format codes are specified by the C standard, meaning users cannot define their own format codes to extend @printf@ for new data types or new formatting rules.
    651585
    … …  
    707641A parameter pack matches 0 or more elements, which can be types or expressions depending on the context.
    708642Like other templates, variadic template functions rely on an implicit set of constraints on a type, in this example a @print@ routine.
    709 That is, it is possible to use the @f@ routine on any type provided there is a corresponding @print@ routine, making variadic templates fully open to extension, unlike variadic functions in C.
     643That is, it is possible to use the @f@ routine any any type provided there is a corresponding @print@ routine, making variadic templates fully open to extension, unlike variadic functions in C.
    710644
    711645Recent \CC standards (\CCfourteen, \CCseventeen) expand on the basic premise by allowing variadic template variables and providing convenient expansion syntax to remove the need for recursion in some cases, amongst other things.
    … …  
    738672Unfortunately, Java's use of nominal inheritance means that types must explicitly inherit from classes or interfaces in order to be considered a subclass.
    739673The combination of these two issues greatly restricts the usefulness of variadic functions in Java.
    740 
    741 Type-safe variadic functions are added to \CFA and discussed in Chapter 4.

  • doc/rob_thesis/thesis-frontpgs.tex

    rf674479 ra0fc78a  
    7676\begin{center}\textbf{Abstract}\end{center}
    7777
    78 \CFA is a modern, non-object-oriented extension of the C programming language.
    79 This thesis introduces two fundamental language features: tuples and constructors/destructors, as well as improved variadic functions.
    80 While these features exist in prior programming languages, the contribution of this work is engineering these features into a highly complex type system.
     78% \CFA is a modern extension to the C programming language.
     79% Some of the features of \CFA include parametric polymorphism, overloading, and .
     80TODO
    8181
    8282\cleardoublepage

  • doc/rob_thesis/thesis.tex

    rf674479 ra0fc78a  
    6868\documentclass[letterpaper,12pt,titlepage,oneside,final]{book}
    6969
    70 % For PDF, suitable for double-sided printing, change the PrintVersion variable below
    71 % to "true" and use this \documentclass line instead of the one above:
    72 % \documentclass[letterpaper,12pt,titlepage,openright,twoside,final]{book}
    73 
    7470\usepackage[T1]{fontenc}                                % allow Latin1 (extended ASCII) characters
    7571\usepackage{textcomp}
    7672% \usepackage[utf8]{inputenc}
    77 % \usepackage[latin1]{inputenc}
     73\usepackage[latin1]{inputenc}
    7874\usepackage{fullpage,times,comment}
    7975% \usepackage{epic,eepic}
    … …  
    9692
    9793\interfootnotelinepenalty=10000
     94
     95% For PDF, suitable for double-sided printing, change the PrintVersion variable below
     96% to "true" and use this \documentclass line instead of the one above:
     97%\documentclass[letterpaper,12pt,titlepage,openright,twoside,final]{book}
    9898
    9999% Some LaTeX commands I define for my own nomenclature.
    … …  
    225225\input{tuples}
    226226
    227 \input{variadic}
    228 
    229227\input{conclusions}
    230228
    … …  
    284282\addcontentsline{toc}{chapter}{\textbf{References}}
    285283
    286 \bibliography{cfa,thesis}
     284\bibliography{cfa}
    287285% Tip 5: You can create multiple .bib files to organize your references.
    288286% Just list them all in the \bibliogaphy command, separated by commas (no spaces).

  • doc/rob_thesis/tuples.tex

    rf674479 ra0fc78a  
    22\chapter{Tuples}
    33%======================================================================
     4
     5\section{Introduction}
     6% TODO: named return values are not currently implemented in CFA - tie in with named tuples? (future work)
     7% TODO: no passing input parameters by assignment, instead will have reference types => this is not a very C-like model and greatly complicates syntax for likely little gain (and would cause confusion with already supported return-by-rerefence)
     8% TODO: tuples are allowed in expressions, exact meaning is defined by operator overloading (e.g. can add tuples). An important caveat to note is that it is currently impossible to allow adding two triples but prevent adding a pair with a quadruple (single flattening/structuring conversions are implicit, only total number of components matters). May be able to solve this with more nuanced conversion rules (future work)
     9% TODO: benefits (conclusion) by Till: reduced number of variables and statements; no specified order of execution for multiple assignment (more optimzation freedom); can store parameter lists in variable; MRV routines (natural code); more convenient assignment statements; simple and efficient access of record fields; named return values more legible and efficient in use of storage
    410
    511\section{Multiple-Return-Value Functions}
    … …  
    814This restriction results in code which emulates functions with multiple return values by \emph{aggregation} or by \emph{aliasing}.
    915In the former situation, the function designer creates a record type that combines all of the return values into a single type.
    10 For example, consider a function returning the most frequently occurring letter in a string, and its frequency.
    11 This example is complex enough to illustrate that an array is insufficient, since arrays are homogeneous, and demonstrates a potential pitfall that exists with aliasing.
     16For example, consider a function returning the most frequently occuring letter in a string, and its frequency.
     17% TODO: consider simplifying the example!
     18%   Two things I like about this example:
     19%   * it uses different types to illustrate why an array is insufficient (this is not necessary, but is nice)
     20%   * it's complicated enough to show the uninitialized pitfall that exists in the aliasing example.
     21%   Still, it may be a touch too complicated. Is there a simpler example with these two properties?
    1222\begin{cfacode}
    1323struct mf_ret {
    … …  
    6373const char * str = "hello world";
    6474char ch;                            // pre-allocate return value
    65 int freq = most_frequent(str, &ch); // pass return value as out parameter
     75int freq = most_frequent(str, &ch); // pass return value as parameter
    6676printf("%s -- %d %c\n", str, freq, ch);
    6777\end{cfacode}
    68 Notably, using this approach, the caller is directly responsible for allocating storage for the additional temporary return values, which complicates the call site with a sequence of variable declarations leading up to the call.
     78Notably, using this approach, the caller is directly responsible for allocating storage for the additional temporary return values.
     79This complicates the call site with a sequence of variable declarations leading up to the call.
    6980Also, while a disciplined use of @const@ can give clues about whether a pointer parameter is going to be used as an out parameter, it is not immediately obvious from only the routine signature whether the callee expects such a parameter to be initialized before the call.
    7081Furthermore, while many C routines that accept pointers are designed so that it is safe to pass @NULL@ as a parameter, there are many C routines that are not null-safe.
    … …  
    7990The expression resolution phase of the \CFA translator ensures that the correct form is used depending on the values being returned and the return type of the current function.
    8091A multiple-returning function with return type @T@ can return any expression that is implicitly convertible to @T@.
    81 Using the running example, the @most_frequent@ function can be written using multiple return values as such,
     92Using the running example, the @most_frequent@ function can be written in using multiple return values as such,
    8293\begin{cfacode}
    8394[int, char] most_frequent(const char * str) {
    … …  
    98109}
    99110\end{cfacode}
    100 This approach provides the benefits of compile-time checking for appropriate return statements as in aggregation, but without the required verbosity of declaring a new named type, which precludes the bug seen with out parameters.
     111This approach provides the benefits of compile-time checking for appropriate return statements as in aggregation, but without the required verbosity of declaring a new named type.
    101112
    102113The addition of multiple-return-value functions necessitates a syntax for accepting multiple values at the call-site.
    … …  
    125136In this case, there is only one option for a function named @most_frequent@ that takes a string as input.
    126137This function returns two values, one @int@ and one @char@.
    127 There are four options for a function named @process@, but only two that accept two arguments, and of those the best match is (3), which is also an exact match.
     138There are four options for a function named @process@, but only two which accept two arguments, and of those the best match is (3), which is also an exact match.
    128139This expression first calls @most_frequent("hello world")@, which produces the values @3@ and @'l'@, which are fed directly to the first and second parameters of (3), respectively.
    129140
    … …  
    137148The previous expression has 3 \emph{components}.
    138149Each component in a tuple expression can be any \CFA expression, including another tuple expression.
     150% TODO: Tuple expressions can appear anywhere that any other expression can appear (...?)
    139151The order of evaluation of the components in a tuple expression is unspecified, to allow a compiler the greatest flexibility for program optimization.
    140152It is, however, guaranteed that each component of a tuple expression is evaluated for side-effects, even if the result is not used.
    141153Multiple-return-value functions can equivalently be called \emph{tuple-returning functions}.
     154% TODO: does this statement still apply, and if so to what extent?
     155%   Tuples are a compile-time phenomenon and have little to no run-time presence.
    142156
    143157\subsection{Tuple Variables}
    … …  
    152166These variables can be used in any of the contexts where a tuple expression is allowed, such as in the @printf@ function call.
    153167As in the @process@ example, the components of the tuple value are passed as separate parameters to @printf@, allowing very simple printing of tuple expressions.
    154 One way to access the individual components is with a simple assignment, as in previous examples.
     168If the individual components are required, they can be extracted with a simple assignment, as in previous examples.
    155169\begin{cfacode}
    156170int freq;
    … …  
    240254\label{s:TupleAssignment}
    241255An assignment where the left side of the assignment operator has a tuple type is called tuple assignment.
    242 There are two kinds of tuple assignment depending on whether the right side of the assignment operator has a tuple type or a non-tuple type, called \emph{Multiple} and \emph{Mass} Assignment, respectively.
     256There are two kinds of tuple assignment depending on whether the right side of the assignment operator has a tuple type or a non-tuple type, called Multiple and Mass Assignment, respectively.
    243257\begin{cfacode}
    244258int x;
    … …  
    258272A mass assignment assigns the value $R$ to each $L_i$.
    259273For a mass assignment to be valid, @?=?(&$L_i$, $R$)@ must be a well-typed expression.
    260 These semantics differ from C cascading assignment (e.g. @a=b=c@) in that conversions are applied to $R$ in each individual assignment, which prevents data loss from the chain of conversions that can happen during a cascading assignment.
     274This differs from C cascading assignment (e.g. @a=b=c@) in that conversions are applied to $R$ in each individual assignment, which prevents data loss from the chain of conversions that can happen during a cascading assignment.
    261275For example, @[y, x] = 3.14@ performs the assignments @y = 3.14@ and @x = 3.14@, which results in the value @3.14@ in @y@ and the value @3@ in @x@.
    262276On the other hand, the C cascading assignment @y = x = 3.14@ performs the assignments @x = 3.14@ and @y = x@, which results in the value @3@ in @x@, and as a result the value @3@ in @y@ as well.
    … …  
    274288These semantics allow cascading tuple assignment to work out naturally in any context where a tuple is permitted.
    275289These semantics are a change from the original tuple design in \KWC \cite{Till89}, wherein tuple assignment was a statement that allows cascading assignments as a special case.
    276 Restricting tuple assignment to statements was an attempt to to fix what was seen as a problem with assignment, wherein it can be used in many different locations, such as in function-call argument position.
     290This decision wa made in an attempt to fix what was seen as a problem with assignment, wherein it can be used in many different locations, such as in function-call argument position.
    277291While permitting assignment as an expression does introduce the potential for subtle complexities, it is impossible to remove assignment expressions from \CFA without affecting backwards compatibility.
    278292Furthermore, there are situations where permitting assignment as an expression improves readability by keeping code succinct and reducing repetition, and complicating the definition of tuple assignment puts a greater cognitive burden on the user.
    … …  
    301315void ?{}(S *, S);      // (4)
    302316
    303 [S, S] x = [3, 6.28];  // uses (2), (3), specialized constructors
    304 [S, S] y;              // uses (1), (1), default constructor
    305 [S, S] z = x.0;        // uses (4), (4), copy constructor
    306 \end{cfacode}
    307 In this example, @x@ is initialized by the multiple constructor calls @?{}(&x.0, 3)@ and @?{}(&x.1, 6.28)@, while @y@ is initialized by two default constructor calls @?{}(&y.0)@ and @?{}(&y.1)@.
     317[S, S] x = [3, 6.28];  // uses (2), (3)
     318[S, S] y;              // uses (1), (1)
     319[S, S] z = x.0;        // uses (4), (4)
     320\end{cfacode}
     321In this example, @x@ is initialized by the multiple constructor calls @?{}(&x.0, 3)@ and @?{}(&x.1, 6.28)@, while @y@ is initilaized by two default constructor calls @?{}(&y.0)@ and @?{}(&y.1)@.
    308322@z@ is initialized by mass copy constructor calls @?{}(&z.0, x.0)@ and @?{}(&z.1, x.0)@.
    309323Finally, @x@, @y@, and @z@ are destructed, i.e. the calls @^?{}(&x.0)@, @^?{}(&x.1)@, @^?{}(&y.0)@, @^?{}(&y.1)@, @^?{}(&z.0)@, and @^?{}(&z.1)@.
    … …  
    325339S s = t;
    326340\end{cfacode}
    327 The initialization of @s@ with @t@ works by default because @t@ is flattened into its components, which satisfies the generated field constructor @?{}(S *, int, double)@ to initialize the first two values.
     341The initialization of @s@ with @t@ works by default, because @t@ is flattened into its components, which satisfies the generated field constructor @?{}(S *, int, double)@ to initialize the first two values.
    328342
    329343\section{Member-Access Tuple Expression}
    … …  
    340354Then the type of @a.[x, y, z]@ is @[T_x, T_y, T_z]@.
    341355
    342 Since tuple index expressions are a form of member-access expression, it is possible to use tuple-index expressions in conjunction with member tuple expressions to manually restructure a tuple (e.g., rearrange components, drop components, duplicate components, etc.).
     356Since tuple index expressions are a form of member-access expression, it is possible to use tuple-index expressions in conjunction with member tuple expressions to manually restructure a tuple (e.g. rearrange components, drop components, duplicate components, etc.).
    343357\begin{cfacode}
    344358[int, int, long, double] x;
    … …  
    370384Since \CFA permits these tuple-access expressions using structures, unions, and tuples, \emph{member tuple expression} or \emph{field tuple expression} is more appropriate.
    371385
    372 It is possible to extend member-access expressions further.
     386It could be possible to extend member-access expressions further.
    373387Currently, a member-access expression whose member is a name requires that the aggregate is a structure or union, while a constant integer member requires the aggregate to be a tuple.
    374388In the interest of orthogonal design, \CFA could apply some meaning to the remaining combinations as well.
    … …  
    384398z.y;  // ???
    385399\end{cfacode}
    386 One possibility is for @s.1@ to select the second member of @s@.
     400One possiblity is for @s.1@ to select the second member of @s@.
    387401Under this interpretation, it becomes possible to not only access members of a struct by name, but also by position.
    388402Likewise, it seems natural to open this mechanism to enumerations as well, wherein the left side would be a type, rather than an expression.
    389 One benefit of this interpretation is familiarity, since it is extremely reminiscent of tuple-index expressions.
     403One benefit of this interpretation is familiar, since it is extremely reminiscent of tuple-index expressions.
    390404On the other hand, it could be argued that this interpretation is brittle in that changing the order of members or adding new members to a structure becomes a brittle operation.
    391 This problem is less of a concern with tuples, since modifying a tuple affects only the code that directly uses the tuple, whereas modifying a structure has far reaching consequences for every instance of the structure.
    392 
    393 As for @z.y@, one interpretation is to extend the meaning of member tuple expressions.
     405This problem is less of a concern with tuples, since modifying a tuple affects only the code which directly uses that tuple, whereas modifying a structure has far reaching consequences with every instance of the structure.
     406
     407As for @z.y@, a natural interpretation would be to extend the meaning of member tuple expressions.
    394408That is, currently the tuple must occur as the member, i.e. to the right of the dot.
    395409Allowing tuples to the left of the dot could distribute the member across the elements of the tuple, in much the same way that member tuple expressions distribute the aggregate across the member tuple.
    396410In this example, @z.y@ expands to @[z.0.y, z.1.y]@, allowing what is effectively a very limited compile-time field-sections map operation, where the argument must be a tuple containing only aggregates having a member named @y@.
    397 It is questionable how useful this would actually be in practice, since structures often do not have names in common with other structures, and further this could cause maintainability issues in that it encourages programmers to adopt very simple naming conventions to maximize the amount of overlap between different types.
     411It is questionable how useful this would actually be in practice, since generally structures are not designed to have names in common with other structures, and further this could cause maintainability issues in that it encourages programmers to adopt very simple naming conventions, to maximize the amount of overlap between different types.
    398412Perhaps more useful would be to allow arrays on the left side of the dot, which would likewise allow mapping a field access across the entire array, producing an array of the contained fields.
    399413The immediate problem with this idea is that C arrays do not carry around their size, which would make it impossible to use this extension for anything other than a simple stack allocated array.
    400414
    401 Supposing this feature works as described, it would be necessary to specify an ordering for the expansion of member-access expressions versus member-tuple expressions.
     415Supposing this feature works as described, it would be necessary to specify an ordering for the expansion of member access expressions versus member tuple expressions.
    402416\begin{cfacode}
    403417struct { int x, y; };
    … …  
    412426\end{cfacode}
    413427Depending on exactly how the two tuples are combined, different results can be achieved.
    414 As such, a specific ordering would need to be imposed to make this feature useful.
    415 Furthermore, this addition moves a member-tuple expression's meaning from being clear statically to needing resolver support, since the member name needs to be distributed appropriately over each member of the tuple, which could itself be a tuple.
    416 
    417 A second possibility is for \CFA to have named tuples, as they exist in Swift and D.
    418 \begin{cfacode}
    419 typedef [int x, int y] Point2D;
    420 Point2D p1, p2;
    421 p1.x + p1.y + p2.x + p2.y;
    422 p1.0 + p1.1 + p2.0 + p2.1;  // equivalent
    423 \end{cfacode}
    424 In this simpler interpretation, a tuple type carries with it a list of possibly empty identifiers.
    425 This approach fits naturally with the named return-value feature, and would likely go a long way towards implementing it.
    426 
    427 Ultimately, the first two extensions introduce complexity into the model, with relatively little perceived benefit, and so were dropped from consideration.
    428 Named tuples are a potentially useful addition to the language, provided they can be parsed with a reasonable syntax.
    429 
     428As such, a specific ordering would need to be imposed in order for this feature to be useful.
     429Furthermore, this addition moves a member tuple expression's meaning from being clear statically to needing resolver support, since the member name needs to be distributed appropriately over each member of the tuple, which could itself be a tuple.
     430
     431Ultimately, both of these extensions introduce complexity into the model, with relatively little peceived benefit.
    430432
    431433\section{Casting}
    432434In C, the cast operator is used to explicitly convert between types.
    433 In \CFA, the cast operator has a secondary use, which is type ascription, since it force the expression resolution algorithm to choose the lowest cost conversion to the target type.
     435In \CFA, the cast operator has a secondary use, which is type ascription.
    434436That is, a cast can be used to select the type of an expression when it is ambiguous, as in the call to an overloaded function.
    435437\begin{cfacode}
    … …  
    440442(int)f();  // choose (2)
    441443\end{cfacode}
    442 Since casting is a fundamental operation in \CFA, casts need to be given a meaningful interpretation in the context of tuples.
     444Since casting is a fundamental operation in \CFA, casts should be given a meaningful interpretation in the context of tuples.
    443445Taking a look at standard C provides some guidance with respect to the way casts should work with tuples.
    444446\begin{cfacode}[numbers=left]
    … …  
    446448void g();
    447449
    448 (void)f();  // valid, ignore results
    449 (int)g();   // invalid, void cannot be converted to int
     450(void)f();
     451(int)g();
    450452
    451453struct A { int x; };
    452 (struct A)f();  // invalid
     454(struct A)f();
    453455\end{cfacode}
    454456In C, line 4 is a valid cast, which calls @f@ and discards its result.
    455457On the other hand, line 5 is invalid, because @g@ does not produce a result, so requesting an @int@ to materialize from nothing is nonsensical.
    456 Finally, line 8 is also invalid, because in C casts only provide conversion between scalar types \cite[p.~91]{C11}.
    457 For consistency, this implies that any case wherein the number of components increases as a result of the cast is invalid, while casts that have the same or fewer number of components may be valid.
     458Finally, line 8 is also invalid, because in C casts only provide conversion between scalar types \cite{C11}.
     459For consistency, this implies that any case wherein the number of components increases as a result of the cast is invalid, while casts which have the same or fewer number of components may be valid.
    458460
    459461Formally, a cast to tuple type is valid when $T_n \leq S_m$, where $T_n$ is the number of components in the target type and $S_m$ is the number of components in the source type, and for each $i$ in $[0, n)$, $S_i$ can be cast to $T_i$.
    … …  
    507509\end{cfacode}
    508510Note that due to the implicit tuple conversions, this function is not restricted to the addition of two triples.
    509 A call to this plus operator type checks as long as a total of 6 non-tuple arguments are passed after flattening, and all of the arguments have a common type that can bind to @T@, with a pairwise @?+?@ over @T@.
    510 For example, these expressions also succeed and produce the same value.
    511 \begin{cfacode}
    512 ([x.0, x.1]) + ([x.2, 10, 20, 30]);  // x + ([10, 20, 30])
    513 x.0 + ([x.1, x.2, 10, 20, 30]);      // x + ([10, 20, 30])
     511A call to this plus operator type checks as long as a total of 6 non-tuple arguments are passed after flattening, and all of the arguments have a common type which can bind to @T@, with a pairwise @?+?@ over @T@.
     512For example, these expressions will also succeed and produce the same value.
     513\begin{cfacode}
     514([x.0, x.1]) + ([x.2, 10, 20, 30]);
     515x.0 + ([x.1, x.2, 10, 20, 30]);
    514516\end{cfacode}
    515517This presents a potential problem if structure is important, as these three expressions look like they should have different meanings.
    516 Furthermore, these calls can be made ambiguous by introducing seemingly different functions.
     518Further, these calls can be made ambiguous by adding seemingly different functions.
    517519\begin{cfacode}
    518520forall(otype T | { T ?+?(T, T); })
    … …  
    522524\end{cfacode}
    523525It is also important to note that these calls could be disambiguated if the function return types were different, as they likely would be for a reasonable implementation of @?+?@, since the return type is used in overload resolution.
    524 Still, these semantics are a deficiency of the current argument matching algorithm, and depending on the function, differing return values may not always be appropriate.
    525 These issues could be rectified by applying an appropriate cost to the structuring and flattening conversions, which are currently 0-cost conversions.
     526Still, this is a deficiency of the current argument matching algorithm, and depending on the function, differing return values may not always be appropriate.
     527It's possible that this could be rectified by applying an appropriate cost to the structuring and flattening conversions, which are currently 0-cost conversions.
    526528Care would be needed in this case to ensure that exact matches do not incur such a cost.
    527529\begin{cfacode}
    … …  
    534536\end{cfacode}
    535537
    536 Until this point, it has been assumed that assertion arguments must match the parameter type exactly, modulo polymorphic specialization (i.e., no implicit conversions are applied to assertion arguments).
     538Until this point, it has been assumed that assertion arguments must match the parameter type exactly, modulo polymorphic specialization (i.e. no implicit conversions are applied to assertion arguments).
    537539This decision presents a conflict with the flexibility of tuples.
    538540\subsection{Assertion Inference}
    … …  
    615617In the call to @f@, the second and third argument components are structured into a tuple argument.
    616618
    617 Expressions that may contain side effects are made into \emph{unique expressions} before being expanded by the flattening conversion.
     619Expressions which may contain side effects are made into \emph{unique expressions} before being expanded by the flattening conversion.
    618620Each unique expression is assigned an identifier and is guaranteed to be executed exactly once.
    619621\begin{cfacode}
    … …  
    622624g(h());
    623625\end{cfacode}
    624 Internally, this is converted to pseudo-\CFA
     626Interally, this is converted to
    625627\begin{cfacode}
    626628void g(int, double);
    627629[int, double] h();
    628 lazy [int, double] unq0 = h(); // deferred execution
    629 g(unq0.0, unq0.1);             // execute h() once
    630 \end{cfacode}
    631 That is, the function @h@ is evaluated lazily and its result is stored for subsequent accesses.
     630let unq<0> = f() : g(unq<0>.0, unq<0>.1);  // notation?
     631\end{cfacode}
    632632Ultimately, unique expressions are converted into two variables and an expression.
    633633\begin{cfacode}
    … …  
    638638[int, double] _unq0;
    639639g(
    640   (_unq0_finished_ ? _unq0 : (_unq0 = h(), _unq0_finished_ = 1, _unq0)).0,
    641   (_unq0_finished_ ? _unq0 : (_unq0 = h(), _unq0_finished_ = 1, _unq0)).1,
     640  (_unq0_finished_ ? _unq0 : (_unq0 = f(), _unq0_finished_ = 1, _unq0)).0,
     641  (_unq0_finished_ ? _unq0 : (_unq0 = f(), _unq0_finished_ = 1, _unq0)).1,
    642642);
    643643\end{cfacode}
    … …  
    646646Every subsequent evaluation of the unique expression then results in an access to the stored result of the actual expression.
    647647
    648 Currently, the \CFA translator has a very broad, imprecise definition of impurity (side-effects), where every function call is assumed to be impure.
    649 This notion could be made more precise for certain intrinsic, auto-generated, and built-in functions, and could analyze function bodies, when they are available, to recursively detect impurity, to eliminate some unique expressions.
    650 It is possible that lazy evaluation could be exposed to the user through a lazy keyword with little additional effort.
     648Currently, the \CFA translator has a very broad, imprecise definition of impurity, where any function call is assumed to be impure.
     649This notion could be made more precise for certain intrinsic, autogenerated, and builtin functions, and could analyze function bodies when they are available to recursively detect impurity, to eliminate some unique expressions.
     650It's possible that unique expressions could be exposed to the user through a language feature, but it's not immediately obvious that there is a benefit to doing so.
    651651
    652652Tuple member expressions are recursively expanded into a list of member access expressions.
    … …  
    655655x.[0, 1.[0, 2]];
    656656\end{cfacode}
    657 which becomes
     657Which becomes
    658658\begin{cfacode}
    659659[x.0, [x.1.0, x.1.2]];
    660660\end{cfacode}
    661 Tuple-member expressions also take advantage of unique expressions in the case of possible impurity.
     661Tuple member expressions also take advantage of unique expressions in the case of possible impurity.
    662662
    663663Finally, the various kinds of tuple assignment, constructors, and destructors generate GNU C statement expressions.
    … …  
    711711});
    712712\end{cfacode}
    713 A variable is generated to store the value produced by a statement expression, since its fields may need to be constructed with a non-trivial constructor and it may need to be referred to multiple time, e.g., in a unique expression.
     713A variable is generated to store the value produced by a statement expression, since its fields may need to be constructed with a non-trivial constructor and it may need to be referred to multiple time, e.g. in a unique expression.
    714714$N$ LHS variables are generated and constructed using the address of the tuple components, and a single RHS variable is generated to store the value of the RHS without any loss of precision.
    715715A nested statement expression is generated that performs the individual assignments and constructs the return value using the results of the individual assignments.
    … …  
    785785The use of statement expressions allows the translator to arbitrarily generate additional temporary variables as needed, but binds the implementation to a non-standard extension of the C language.
    786786There are other places where the \CFA translator makes use of GNU C extensions, such as its use of nested functions, so this is not a new restriction.
     787
     788\section{Variadic Functions}
     789% TODO: should this maybe be its own chapter?
     790C provides variadic functions through the manipulation of @va_list@ objects.
     791A variadic function is one which contains at least one parameter, followed by @...@ as the last token in the parameter list.
     792In particular, some form of \emph{argument descriptor} is needed to inform the function of the number of arguments and their types.
     793Two common argument descriptors are format strings or and counter parameters.
     794It's important to note that both of these mechanisms are inherently redundant, because they require the user to specify information that the compiler knows explicitly.
     795This required repetition is error prone, because it's easy for the user to add or remove arguments without updating the argument descriptor.
     796In addition, C requires the programmer to hard code all of the possible expected types.
     797As a result, it is cumbersome to write a function that is open to extension.
     798For example, a simple function which sums $N$ @int@s,
     799\begin{cfacode}
     800int sum(int N, ...) {
     801  va_list args;
     802  va_start(args, N);
     803  int ret = 0;
     804  while(N) {
     805    ret += va_arg(args, int);  // have to specify type
     806    N--;
     807  }
     808  va_end(args);
     809  return ret;
     810}
     811sum(3, 10, 20, 30);  // need to keep counter in sync
     812\end{cfacode}
     813The @va_list@ type is a special C data type that abstracts variadic argument manipulation.
     814The @va_start@ macro initializes a @va_list@, given the last named parameter.
     815Each use of the @va_arg@ macro allows access to the next variadic argument, given a type.
     816Since the function signature does not provide any information on what types can be passed to a variadic function, the compiler does not perform any error checks on a variadic call.
     817As such, it is possible to pass any value to the @sum@ function, including pointers, floating-point numbers, and structures.
     818In the case where the provided type is not compatible with the argument's actual type after default argument promotions, or if too many arguments are accessed, the behaviour is undefined \cite{C11}.
     819Furthermore, there is no way to perform the necessary error checks in the @sum@ function at run-time, since type information is not carried into the function body.
     820Since they rely on programmer convention rather than compile-time checks, variadic functions are generally unsafe.
     821
     822In practice, compilers can provide warnings to help mitigate some of the problems.
     823For example, GCC provides the @format@ attribute to specify that a function uses a format string, which allows the compiler to perform some checks related to the standard format specifiers.
     824Unfortunately, this does not permit extensions to the format string syntax, so a programmer cannot extend the attribute to warn for mismatches with custom types.
     825
     826Needless to say, C's variadic functions are a deficient language feature.
     827Two options were examined to provide better, type-safe variadic functions in \CFA.
     828\subsection{Whole Tuple Matching}
     829Option 1 is to change the argument matching algorithm, so that type parameters can match whole tuples, rather than just their components.
     830This option could be implemented with two phases of argument matching when a function contains type parameters and the argument list contains tuple arguments.
     831If flattening and structuring fail to produce a match, a second attempt at matching the function and argument combination is made where tuple arguments are not expanded and structure must match exactly, modulo non-tuple implicit conversions.
     832For example:
     833\begin{cfacode}
     834  forall(otype T, otype U | { T g(U); })
     835  void f(T, U);
     836
     837  [int, int] g([int, int, int, int]);
     838
     839  f([1, 2], [3, 4, 5, 6]);
     840\end{cfacode}
     841With flattening and structuring, the call is first transformed into @f(1, 2, 3, 4, 5, 6)@.
     842Since the first argument of type @T@ does not have a tuple type, unification decides that @T=int@ and @1@ is matched as the first parameter.
     843Likewise, @U@ does not have a tuple type, so @U=int@ and @2@ is accepted as the second parameter.
     844There are now no remaining formal parameters, but there are remaining arguments and the function is not variadic, so the match fails.
     845
     846With the addition of an exact matching attempt, @T=[int,int]@ and @U=[int,int,int,int]@ and so the arguments type check.
     847Likewise, when inferring assertion @g@, an exact match is found.
     848
     849This approach is strict with respect to argument structure by nature, which makes it syntactically awkward to use in ways that the existing tuple design is not.
     850For example, consider a @new@ function which allocates memory using @malloc@ and constructs the result, using arbitrary arguments.
     851\begin{cfacode}
     852struct Array;
     853void ?{}(Array *, int, int, int);
     854
     855forall(dtype T, otype Params | sized(T) | { void ?{}(T *, Params); })
     856T * new(Params p) {
     857  return malloc(){ p };
     858}
     859Array(int) * x = new([1, 2, 3]);
     860\end{cfacode}
     861The call to @new@ is not particularly appealing, since it requires the use of square brackets at the call-site, which is not required in any other function call.
     862This shifts the burden from the compiler to the programmer, which is almost always wrong, and creates an odd inconsistency within the language.
     863Similarly, in order to pass 0 variadic arguments, an explicit empty tuple must be passed into the argument list, otherwise the exact matching rule would not have an argument to bind against.
     864
     865It should be otherwise noted that the addition of an exact matching rule only affects the outcome for polymorphic type binding when tuples are involved.
     866For non-tuple arguments, exact matching and flattening \& structuring are equivalent.
     867For tuple arguments to a function without polymorphic formal parameters, flattening and structuring work whenever an exact match would have worked, since the tuple is flattened and implicitly restructured to its original structure.
     868Thus there is nothing to be gained from permitting the exact matching rule to take effect when a function does not contain polymorphism and none of the arguments are tuples.
     869
     870Overall, this option takes a step in the right direction, but is contrary to the flexibility of the existing tuple design.
     871
     872\subsection{A New Typeclass}
     873A second option is the addition of another kind of type parameter, @ttype@.
     874Matching against a @ttype@ parameter consumes all remaining argument components and packages them into a tuple, binding to the resulting tuple of types.
     875In a given parameter list, there should be at most one @ttype@ parameter that must occur last, otherwise the call can never resolve, given the previous rule.
     876% TODO: a similar rule exists in C/C++ for "..."
     877This idea essentially matches normal variadic semantics, with a strong feeling of similarity to \CCeleven variadic templates.
     878As such, @ttype@ variables will also be referred to as argument packs.
     879This is the option that has been added to \CFA.
     880
     881Like variadic templates, the main way to manipulate @ttype@ polymorphic functions is through recursion.
     882Since nothing is known about a parameter pack by default, assertion parameters are key to doing anything meaningful.
     883Unlike variadic templates, @ttype@ polymorphic functions can be separately compiled.
     884
     885For example, a simple translation of the C sum function using @ttype@ would look like
     886\begin{cfacode}
     887int sum(){ return 0; }        // (0)
     888forall(ttype Params | { int sum(Params); })
     889int sum(int x, Params rest) { // (1)
     890  return x+sum(rest);
     891}
     892sum(10, 20, 30);
     893\end{cfacode}
     894Since (0) does not accept any arguments, it is not a valid candidate function for the call @sum(10, 20, 30)@.
     895In order to call (1), @10@ is matched with @x@, and the argument resolution moves on to the argument pack @rest@, which consumes the remainder of the argument list and @Params@ is bound to @[20, 30]@.
     896In order to finish the resolution of @sum@, an assertion parameter which matches @int sum(int, int)@ is required.
     897Like in the previous iteration, (0) is not a valid candiate, so (1) is examined with @Params@ bound to @[int]@, requiring the assertion @int sum(int)@.
     898Next, (0) fails, and to satisfy (1) @Params@ is bound to @[]@, requiring an assertion @int sum()@.
     899Finally, (0) matches and (1) fails, which terminates the recursion.
     900Effectively, this traces as @sum(10, 20, 30)@ $\rightarrow$ @10+sum(20, 30)@ $\rightarrow$ @10+(20+sum(30))@ $\rightarrow$ @10+(20+(30+sum()))@ $\rightarrow$ @10+(20+(30+0))@.
     901
     902A point of note is that this version does not require any form of argument descriptor, since the \CFA type system keeps track of all of these details.
     903It might be reasonable to take the @sum@ function a step further to enforce a minimum number of arguments, which could be done simply
     904\begin{cfacode}
     905int sum(int x, int y){
     906  return x+y;
     907}
     908forall(ttype Params | { int sum(int, Params); })
     909int sum(int x, int y, Params rest) {
     910  return sum(x+y, rest);
     911}
     912sum(10, 20, 30);
     913\end{cfacode}
     914
     915One more iteration permits the summation of any summable type, as long as all arguments are the same type.
     916\begin{cfacode}
     917trait summable(otype T) {
     918  T ?+?(T, T);
     919};
     920forall(otype R | summable(R))
     921R sum(R x, R y){
     922  return x+y;
     923}
     924forall(otype R, ttype Params
     925  | summable(R)
     926  | { R sum(R, Params); })
     927R sum(R x, R y, Params rest) {
     928  return sum(x+y, rest);
     929}
     930sum(3, 10, 20, 30);
     931\end{cfacode}
     932Unlike C, it is not necessary to hard code the expected type.
     933This is naturally open to extension, in that any user-defined type with a @?+?@ operator is automatically able to be used with the @sum@ function.
     934That is to say, the programmer who writes @sum@ does not need full program knowledge of every possible data type, unlike what is necessary to write an equivalent function using the standard C mechanisms.
     935
     936Going one last step, it is possible to achieve full generality in \CFA, allowing the summation of arbitrary lists of summable types.
     937\begin{cfacode}
     938trait summable(otype T1, otype T2, otype R) {
     939  R ?+?(T1, T2);
     940};
     941forall(otype T1, otype T2, otype R | summable(T1, T2, R))
     942R sum(T1 x, T2 y) {
     943  return x+y;
     944}
     945forall(otype T1, otype T2, otype T3, ttype Params, otype R
     946  | summable(T1, T2, T3)
     947  | { R sum(T3, Params); })
     948R sum(T1 x, T2 y, Params rest ) {
     949  return sum(x+y, rest);
     950}
     951sum(3, 10.5, 20, 30.3);
     952\end{cfacode}
     953The \CFA translator requires adding explicit @double ?+?(int, double)@ and @double ?+?(double, int)@ functions for this call to work, since implicit conversions are not supported for assertions.
     954
     955C variadic syntax and @ttype@ polymorphism probably should not be mixed, since it is not clear where to draw the line to decide which arguments belong where.
     956Furthermore, it might be desirable to disallow polymorphic functions to use C variadic syntax to encourage a Cforall style.
     957Aside from calling C variadic functions, it is not obvious that there is anything that can be done with C variadics that could not also be done with @ttype@ parameters.
     958
     959Variadic templates in \CC require an ellipsis token to express that a parameter is a parameter pack and to expand a parameter pack.
     960\CFA does not need an ellipsis in either case, since the type class @ttype@ is only used for variadics.
     961An alternative design could have used an ellipsis combined with an existing type class.
     962This approach was not taken because the largest benefit of the ellipsis token in \CC is the ability to expand a parameter pack within an expression, e.g. in fold expressions, which requires compile-time knowledge of the structure of the parameter pack, which is not available in \CFA.
     963\begin{cppcode}
     964template<typename... Args>
     965void f(Args &... args) {
     966  g(&args...);  // expand to addresses of pack elements
     967}
     968\end{cppcode}
     969As such, the addition of an ellipsis token would be purely an aesthetic change in \CFA today.
     970
     971It is possible to write a type-safe variadic print routine, which can replace @printf@
     972\begin{cfacode}
     973struct S { int x, y; };
     974forall(otype T, ttype Params |
     975  { void print(T); void print(Params); })
     976void print(T arg, Params rest) {
     977  print(arg);
     978  print(rest);
     979}
     980void print(char * x) { printf("%s", x); }
     981void print(int x) { printf("%d", x);  }
     982void print(S s) { print("{ ", s.x, ",", s.y, " }"); }
     983print("s = ", (S){ 1, 2 }, "\n");
     984\end{cfacode}
     985This example routine showcases a variadic-template-like decomposition of the provided argument list.
     986The individual @print@ routines allow printing a single element of a type.
     987The polymorphic @print@ allows printing any list of types, as long as each individual type has a @print@ function.
     988The individual print functions can be used to build up more complicated @print@ routines, such as for @S@, which is something that cannot be done with @printf@ in C.
     989
     990It is also possible to use @ttype@ polymorphism to provide arbitrary argument forwarding functions.
     991For example, it is possible to write @new@ as a library function.
     992Example 2: new (i.e. type-safe malloc + constructors)
     993\begin{cfacode}
     994struct Array;
     995void ?{}(Array *, int, int, int);
     996
     997forall(dtype T, ttype Params | sized(T) | { void ?{}(T *, Params); })
     998T * new(Params p) {
     999  return malloc(){ p }; // construct result of malloc
     1000}
     1001Array * x = new(1, 2, 3);
     1002\end{cfacode}
     1003The @new@ function provides the combination of type-safe @malloc@ with a constructor call, so that it becomes impossible to forget to construct dynamically allocated objects.
     1004This provides the type-safety of @new@ in \CC, without the need to specify the allocated type, thanks to return-type inference.
     1005
     1006In the call to @new@, @Array@ is selected to match @T@, and @Params@ is expanded to match @[int, int, int, int]@. To satisfy the assertions, a constructor with an interface compatible with @void ?{}(Array *, int, int, int)@ must exist in the current scope.
     1007
     1008\subsection{Implementation}
     1009
     1010The definition of @new@
     1011\begin{cfacode}
     1012forall(dtype T | sized(T)) T * malloc();
     1013
     1014forall(dtype T, ttype Params | sized(T) | { void ?{}(T *, Params); })
     1015T * new(Params p) {
     1016  return malloc(){ p }; // construct result of malloc
     1017}
     1018\end{cfacode}
     1019Generates the following
     1020\begin{cfacode}
     1021void *malloc(long unsigned int _sizeof_T, long unsigned int _alignof_T);
     1022
     1023void *new(
     1024  void (*_adapter_)(void (*)(), void *, void *),
     1025  long unsigned int _sizeof_T,
     1026  long unsigned int _alignof_T,
     1027  long unsigned int _sizeof_Params,
     1028  long unsigned int _alignof_Params,
     1029  void (* _ctor_T)(void *, void *),
     1030  void *p
     1031){
     1032  void *_retval_new;
     1033  void *_tmp_cp_ret0;
     1034  void *_tmp_ctor_expr0;
     1035  _retval_new=
     1036    (_adapter_(_ctor_T,
     1037      (_tmp_ctor_expr0=(_tmp_cp_ret0=malloc(_sizeof_2tT, _alignof_2tT),
     1038        _tmp_cp_ret0)),
     1039      p),
     1040    _tmp_ctor_expr0); // ?{}
     1041  *(void **)&_tmp_cp_ret0; // ^?{}
     1042  return _retval_new;
     1043}
     1044\end{cfacode}
     1045The constructor for @T@ is called indirectly through the adapter function on the result of @malloc@ and the parameter pack.
     1046The variable that was allocated and constructed is then returned from @new@.
     1047
     1048A call to @new@
     1049\begin{cfacode}
     1050struct S { int x, y; };
     1051void ?{}(S *, int, int);
     1052
     1053S * s = new(3, 4);
     1054\end{cfacode}
     1055Generates the following
     1056\begin{cfacode}
     1057struct _tuple2_ {  // _tuple2_(T0, T1)
     1058  void *field_0;
     1059  void *field_1;
     1060};
     1061struct _conc__tuple2_0 {  // _tuple2_(int, int)
     1062  int field_0;
     1063  int field_1;
     1064};
     1065struct _conc__tuple2_0 _tmp_cp1;  // tuple argument to new
     1066struct S *_tmp_cp_ret1;           // return value from new
     1067void _thunk0(  // ?{}(S *, [int, int])
     1068  struct S *_p0,
     1069  struct _conc__tuple2_0 _p1
     1070){
     1071  _ctor_S(_p0, _p1.field_0, _p1.field_1);  // restructure tuple parameter
     1072}
     1073void _adapter(void (*_adaptee)(), void *_p0, void *_p1){
     1074  // apply adaptee to arguments after casting to actual types
     1075  ((void (*)(struct S *, struct _conc__tuple2_0))_adaptee)(
     1076    _p0,
     1077    *(struct _conc__tuple2_0 *)_p1
     1078  );
     1079}
     1080struct S *s = (struct S *)(_tmp_cp_ret1=
     1081  new(
     1082    _adapter,
     1083    sizeof(struct S),
     1084    __alignof__(struct S),
     1085    sizeof(struct _conc__tuple2_0),
     1086    __alignof__(struct _conc__tuple2_0),
     1087    (void (*)(void *, void *))&_thunk0,
     1088    (({ // copy construct tuple argument to new
     1089      int *__multassign_L0 = (int *)&_tmp_cp1.field_0;
     1090      int *__multassign_L1 = (int *)&_tmp_cp1.field_1;
     1091      int __multassign_R0 = 3;
     1092      int __multassign_R1 = 4;
     1093      ((*__multassign_L0=__multassign_R0 /* ?{} */) ,
     1094       (*__multassign_L1=__multassign_R1 /* ?{} */));
     1095    }), &_tmp_cp1)
     1096  ), _tmp_cp_ret1);
     1097*(struct S **)&_tmp_cp_ret1; // ^?{}  // destroy return value from new
     1098({  // destroy argument temporary
     1099  int *__massassign_L0 = (int *)&_tmp_cp1.field_0;
     1100  int *__massassign_L1 = (int *)&_tmp_cp1.field_1;
     1101  ((*__massassign_L0 /* ^?{} */) , (*__massassign_L1 /* ^?{} */));
     1102});
     1103\end{cfacode}
     1104Of note, @_thunk0@ is generated to translate calls to @?{}(S *, [int, int])@ into calls to @?{}(S *, int, int)@.
     1105The call to @new@ constructs a tuple argument using the supplied arguments.
     1106
     1107The @print@ function
     1108\begin{cfacode}
     1109forall(otype T, ttype Params |
     1110  { void print(T); void print(Params); })
     1111void print(T arg, Params rest) {
     1112  print(arg);
     1113  print(rest);
     1114}
     1115\end{cfacode}
     1116Generates
     1117\begin{cfacode}
     1118void print_variadic(
     1119  void (*_adapterF_7tParams__P)(void (*)(), void *),
     1120  void (*_adapterF_2tT__P)(void (*)(), void *),
     1121  void (*_adapterF_P2tT2tT__MP)(void (*)(), void *, void *),
     1122  void (*_adapterF2tT_P2tT2tT_P_MP)(void (*)(), void *, void *, void *),
     1123  long unsigned int _sizeof_T,
     1124  long unsigned int _alignof_T,
     1125  long unsigned int _sizeof_Params,
     1126  long unsigned int _alignof_Params,
     1127  void *(*_assign_TT)(void *, void *),
     1128  void (*_ctor_T)(void *),
     1129  void (*_ctor_TT)(void *, void *),
     1130  void (*_dtor_T)(void *),
     1131  void (*print_T)(void *),
     1132  void (*print_Params)(void *),
     1133  void *arg,
     1134  void *rest
     1135){
     1136  void *_tmp_cp0 = __builtin_alloca(_sizeof_T);
     1137  _adapterF_2tT__P(  // print(arg)
     1138    ((void (*)())print_T),
     1139    (_adapterF_P2tT2tT__MP( // copy construct argument
     1140      ((void (*)())_ctor_TT),
     1141      _tmp_cp0,
     1142      arg
     1143    ), _tmp_cp0)
     1144  );
     1145  _dtor_T(_tmp_cp0);  // destroy argument temporary
     1146  _adapterF_7tParams__P(  // print(rest)
     1147    ((void (*)())print_Params),
     1148    rest
     1149  );
     1150}
     1151\end{cfacode}
     1152The @print_T@ routine is called indirectly through an adapter function with a copy constructed argument, followed by an indirect call to @print_Params@.
     1153
     1154A call to print
     1155\begin{cfacode}
     1156void print(const char * x) { printf("%s", x); }
     1157void print(int x) { printf("%d", x);  }
     1158
     1159print("x = ", 123, ".\n");
     1160\end{cfacode}
     1161Generates the following
     1162\begin{cfacode}
     1163void print_string(const char *x){
     1164  int _tmp_cp_ret0;
     1165  (_tmp_cp_ret0=printf("%s", x)) , _tmp_cp_ret0;
     1166  *(int *)&_tmp_cp_ret0; // ^?{}
     1167}
     1168void print_int(int x){
     1169  int _tmp_cp_ret1;
     1170  (_tmp_cp_ret1=printf("%d", x)) , _tmp_cp_ret1;
     1171  *(int *)&_tmp_cp_ret1; // ^?{}
     1172}
     1173
     1174struct _tuple2_ {  // _tuple2_(T0, T1)
     1175  void *field_0;
     1176  void *field_1;
     1177};
     1178struct _conc__tuple2_0 {  // _tuple2_(int, const char *)
     1179  int field_0;
     1180  const char *field_1;
     1181};
     1182struct _conc__tuple2_0 _tmp_cp6;  // _tuple2_(int, const char *)
     1183const char *_thunk0(const char **_p0, const char *_p1){
     1184        // const char * ?=?(const char **, const char *)
     1185  return *_p0=_p1;
     1186}
     1187void _thunk1(const char **_p0){ // void ?{}(const char **)
     1188  *_p0; // ?{}
     1189}
     1190void _thunk2(const char **_p0, const char *_p1){
     1191        // void ?{}(const char **, const char *)
     1192  *_p0=_p1; // ?{}
     1193}
     1194void _thunk3(const char **_p0){ // void ^?{}(const char **)
     1195  *_p0; // ^?{}
     1196}
     1197void _thunk4(struct _conc__tuple2_0 _p0){ // void print([int, const char *])
     1198  struct _tuple1_ { // _tuple1_(T0)
     1199    void *field_0;
     1200  };
     1201  struct _conc__tuple1_1 { // _tuple1_(const char *)
     1202    const char *field_0;
     1203  };
     1204  void _thunk5(struct _conc__tuple1_1 _pp0){ // void print([const char *])
     1205    print_string(_pp0.field_0);  // print(rest.0)
     1206  }
     1207  void _adapter_i_pii_(void (*_adaptee)(), void *_ret, void *_p0, void *_p1){
     1208    *(int *)_ret=((int (*)(int *, int))_adaptee)(_p0, *(int *)_p1);
     1209  }
     1210  void _adapter_pii_(void (*_adaptee)(), void *_p0, void *_p1){
     1211    ((void (*)(int *, int ))_adaptee)(_p0, *(int *)_p1);
     1212  }
     1213  void _adapter_i_(void (*_adaptee)(), void *_p0){
     1214    ((void (*)(int))_adaptee)(*(int *)_p0);
     1215  }
     1216  void _adapter_tuple1_5_(void (*_adaptee)(), void *_p0){
     1217    ((void (*)(struct _conc__tuple1_1 ))_adaptee)(*(struct _conc__tuple1_1 *)_p0);
     1218  }
     1219  print_variadic(
     1220    _adapter_tuple1_5,
     1221    _adapter_i_,
     1222    _adapter_pii_,
     1223    _adapter_i_pii_,
     1224    sizeof(int),
     1225    __alignof__(int),
     1226    sizeof(struct _conc__tuple1_1),
     1227    __alignof__(struct _conc__tuple1_1),
     1228    (void *(*)(void *, void *))_assign_i,     // int ?=?(int *, int)
     1229    (void (*)(void *))_ctor_i,                // void ?{}(int *)
     1230    (void (*)(void *, void *))_ctor_ii,       // void ?{}(int *, int)
     1231    (void (*)(void *))_dtor_ii,               // void ^?{}(int *)
     1232    (void (*)(void *))print_int,              // void print(int)
     1233    (void (*)(void *))&_thunk5,               // void print([const char *])
     1234    &_p0.field_0,                             // rest.0
     1235    &(struct _conc__tuple1_1 ){ _p0.field_1 } // [rest.1]
     1236  );
     1237}
     1238struct _tuple1_ {  // _tuple1_(T0)
     1239  void *field_0;
     1240};
     1241struct _conc__tuple1_6 {  // _tuple_1(const char *)
     1242  const char *field_0;
     1243};
     1244const char *_temp0;
     1245_temp0="x = ";
     1246void _adapter_pstring_pstring_string(
     1247  void (*_adaptee)(),
     1248  void *_ret,
     1249  void *_p0,
     1250  void *_p1
     1251){
     1252  *(const char **)_ret=
     1253    ((const char *(*)(const char **, const char *))_adaptee)(
     1254      _p0,
     1255      *(const char **)_p1
     1256    );
     1257}
     1258void _adapter_pstring_string(void (*_adaptee)(), void *_p0, void *_p1){
     1259  ((void (*)(const char **, const char *))_adaptee)(_p0, *(const char **)_p1);
     1260}
     1261void _adapter_string_(void (*_adaptee)(), void *_p0){
     1262  ((void (*)(const char *))_adaptee)(*(const char **)_p0);
     1263}
     1264void _adapter_tuple2_0_(void (*_adaptee)(), void *_p0){
     1265  ((void (*)(struct _conc__tuple2_0 ))_adaptee)(*(struct _conc__tuple2_0 *)_p0);
     1266}
     1267print_variadic(
     1268  _adapter_tuple2_0_,
     1269  _adapter_string_,
     1270  _adapter_pstring_string_,
     1271  _adapter_pstring_pstring_string_,
     1272  sizeof(const char *),
     1273  __alignof__(const char *),
     1274  sizeof(struct _conc__tuple2_0 ),
     1275  __alignof__(struct _conc__tuple2_0 ),
     1276  (void *(*)(void *, void *))&_thunk0, // const char * ?=?(const char **, const char *)
     1277  (void (*)(void *))&_thunk1,          // void ?{}(const char **)
     1278  (void (*)(void *, void *))&_thunk2,  // void ?{}(const char **, const char *)
     1279  (void (*)(void *))&_thunk3,          // void ^?{}(const char **)
     1280  (void (*)(void *))print_string,      // void print(const char *)
     1281  (void (*)(void *))&_thunk4,          // void print([int, const char *])
     1282  &_temp0,                             // "x = "
     1283  (({  // copy construct tuple argument to print
     1284    int *__multassign_L0 = (int *)&_tmp_cp6.field_0;
     1285    const char **__multassign_L1 = (const char **)&_tmp_cp6.field_1;
     1286    int __multassign_R0 = 123;
     1287    const char *__multassign_R1 = ".\n";
     1288    ((*__multassign_L0=__multassign_R0 /* ?{} */),
     1289     (*__multassign_L1=__multassign_R1 /* ?{} */));
     1290  }), &_tmp_cp6)                        // [123, ".\n"]
     1291);
     1292({  // destroy argument temporary
     1293  int *__massassign_L0 = (int *)&_tmp_cp6.field_0;
     1294  const char **__massassign_L1 = (const char **)&_tmp_cp6.field_1;
     1295  ((*__massassign_L0 /* ^?{} */) , (*__massassign_L1 /* ^?{} */));
     1296});
     1297\end{cfacode}
     1298The type @_tuple2_@ is generated to allow passing the @rest@ argument to @print_variadic@.
     1299Thunks 0 through 3 provide wrappers for the @otype@ parameters for @const char *@, while @_thunk4@ translates a call to @print([int, const char *])@ into a call to @print_variadic(int, [const char *])@.
     1300This all builds to a call to @print_variadic@, with the appropriate copy construction of the tuple argument.
     1301
     1302\section{Future Work}

  • src/ControlStruct/LabelGenerator.cc

    rf674479 ra0fc78a  
    2020#include "SynTree/Label.h"
    2121#include "SynTree/Attribute.h"
    22 #include "SynTree/Statement.h"
    2322
    2423namespace ControlStruct {
    … …  
    3231        }
    3332
    34         Label LabelGenerator::newLabel( std::string suffix, Statement * stmt ) {
     33        Label LabelGenerator::newLabel( std::string suffix ) {
    3534                std::ostringstream os;
    3635                os << "__L" << current++ << "__" << suffix;
    37                 if ( stmt && ! stmt->get_labels().empty() ) {
    38                         os << "_" << stmt->get_labels().front() << "__";
    39                 }
    4036                std::string ret = os.str();
    4137                Label l( ret );

  • src/ControlStruct/LabelGenerator.h

    rf674479 ra0fc78a  
    55// file "LICENCE" distributed with Cforall.
    66//
    7 // LabelGenerator.h --
     7// LabelGenerator.h -- 
    88//
    99// Author           : Rodolfo G. Esteves
    … …  
    2424          public:
    2525                static LabelGenerator *getGenerator();
    26                 Label newLabel(std::string suffix, Statement * stmt = nullptr);
     26                Label newLabel(std::string suffix = "");
    2727                void reset() { current = 0; }
    2828                void rewind() { current--; }

  • src/ControlStruct/MLEMutator.cc

    rf674479 ra0fc78a  
    5656                bool labeledBlock = !(cmpndStmt->get_labels().empty());
    5757                if ( labeledBlock ) {
    58                         Label brkLabel = generator->newLabel("blockBreak", cmpndStmt);
     58                        Label brkLabel = generator->newLabel("blockBreak");
    5959                        enclosingControlStructures.push_back( Entry( cmpndStmt, brkLabel ) );
    6060                } // if
    … …  
    8080                // whether brkLabel and contLabel are used with branch statements and will recursively do the same to nested
    8181                // loops
    82                 Label brkLabel = generator->newLabel("loopBreak", loopStmt);
    83                 Label contLabel = generator->newLabel("loopContinue", loopStmt);
     82                Label brkLabel = generator->newLabel("loopBreak");
     83                Label contLabel = generator->newLabel("loopContinue");
    8484                enclosingControlStructures.push_back( Entry( loopStmt, brkLabel, contLabel ) );
    8585                loopStmt->set_body ( loopStmt->get_body()->acceptMutator( *this ) );
    8686
    87                 assert( ! enclosingControlStructures.empty() );
    8887                Entry &e = enclosingControlStructures.back();
    8988                // sanity check that the enclosing loops have been popped correctly
    … …  
    109108                bool labeledBlock = !(ifStmt->get_labels().empty());
    110109                if ( labeledBlock ) {
    111                         Label brkLabel = generator->newLabel("blockBreak", ifStmt);
     110                        Label brkLabel = generator->newLabel("blockBreak");
    112111                        enclosingControlStructures.push_back( Entry( ifStmt, brkLabel ) );
    113112                } // if
    114113
    115114                Parent::mutate( ifStmt );
    116 
     115               
    117116                if ( labeledBlock ) {
    118117                        if ( ! enclosingControlStructures.back().useBreakExit().empty() ) {
    … …  
    127126        Statement *MLEMutator::handleSwitchStmt( SwitchClass *switchStmt ) {
    128127                // generate a label for breaking out of a labeled switch
    129                 Label brkLabel = generator->newLabel("switchBreak", switchStmt);
     128                Label brkLabel = generator->newLabel("switchBreak");
    130129                enclosingControlStructures.push_back( Entry(switchStmt, brkLabel) );
    131130                mutateAll( switchStmt->get_statements(), *this );
    … …  
    159158
    160159                std::list< Entry >::reverse_iterator targetEntry;
    161                 switch ( branchStmt->get_type() ) {
    162                         case BranchStmt::Goto:
    163                                 return branchStmt;
    164                         case BranchStmt::Continue:
    165                         case BranchStmt::Break: {
    166                                 bool isContinue = branchStmt->get_type() == BranchStmt::Continue;
    167                                 // unlabeled break/continue
    168                                 if ( branchStmt->get_target() == "" ) {
    169                                         if ( isContinue ) {
    170                                                 // continue target is outermost loop
    171                                                 targetEntry = std::find_if( enclosingControlStructures.rbegin(), enclosingControlStructures.rend(), [](Entry &e) { return isLoop( e.get_controlStructure() ); } );
    172                                         } else {
    173                                                 // break target is outmost control structure
    174                                                 if ( enclosingControlStructures.empty() ) throw SemanticError( "'break' outside a loop, switch, or labelled block" );
    175                                                 targetEntry = enclosingControlStructures.rbegin();
    176                                         } // if
    177                                 } else {
    178                                         // labeled break/continue - lookup label in table to find attached control structure
    179                                         targetEntry = std::find( enclosingControlStructures.rbegin(), enclosingControlStructures.rend(), (*targetTable)[branchStmt->get_target()] );
    180                                 } // if
    181                                 // ensure that selected target is valid
    182                                 if ( targetEntry == enclosingControlStructures.rend() || (isContinue && ! isLoop( targetEntry->get_controlStructure() ) ) ) {
    183                                         throw SemanticError( toString( (isContinue ? "'continue'" : "'break'"), " target must be an enclosing ", (isContinue ? "loop: " : "control structure: "), originalTarget ) );
    184                                 } // if
    185                                 break;
    186                         }
    187                         default:
    188                                 assert( false );
    189                 } // switch
     160                if ( branchStmt->get_type() == BranchStmt::Goto ) {
     161                        return branchStmt;
     162                } else if ( branchStmt->get_type() == BranchStmt::Continue) {
     163                        // continue target must be a loop
     164                        if ( branchStmt->get_target() == "" ) {
     165                                targetEntry = std::find_if( enclosingControlStructures.rbegin(), enclosingControlStructures.rend(), [](Entry &e) { return isLoop( e.get_controlStructure() ); } );
     166                        } else {
     167                                // labelled continue - lookup label in table ot find attached control structure
     168                                targetEntry = std::find( enclosingControlStructures.rbegin(), enclosingControlStructures.rend(), (*targetTable)[branchStmt->get_target()] );
     169                        } // if
     170                        if ( targetEntry == enclosingControlStructures.rend() || ! isLoop( targetEntry->get_controlStructure() ) ) {
     171                                throw SemanticError( "'continue' target must be an enclosing loop: " + originalTarget );
     172                        } // if
     173                } else if ( branchStmt->get_type() == BranchStmt::Break ) {
     174                        if ( enclosingControlStructures.empty() ) throw SemanticError( "'break' outside a loop, switch, or labelled block" );
     175                        targetEntry = enclosingControlStructures.rbegin();
     176                } else {
     177                        assert( false );
     178                } // if
     179
     180                if ( branchStmt->get_target() != "" && targetTable->find( branchStmt->get_target() ) == targetTable->end() ) {
     181                        throw SemanticError("The label defined in the exit loop statement does not exist: " + originalTarget );  // shouldn't happen (since that's already checked)
     182                } // if
    190183
    191184                // branch error checks, get the appropriate label name and create a goto
    … …  
    204197                } // switch
    205198
    206                 // transform break/continue statements into goto to simplify later handling of branches
    207                 delete branchStmt;
    208                 return new BranchStmt( std::list<Label>(), exitLabel, BranchStmt::Goto );
     199                if ( branchStmt->get_target() == "" && branchStmt->get_type() != BranchStmt::Continue ) {
     200                        // unlabelled break/continue - can keep branch as break/continue for extra semantic information, but add
     201                        // exitLabel as its destination so that label passes can easily determine where the break/continue goes to
     202                        branchStmt->set_target( exitLabel );
     203                        return branchStmt;
     204                } else {
     205                        // labelled break/continue - can't easily emulate this with break and continue, so transform into a goto
     206                        delete branchStmt;
     207                        return new BranchStmt( std::list<Label>(), exitLabel, BranchStmt::Goto );
     208                } // if
    209209        }
    210210

  • src/ResolvExpr/AlternativeFinder.cc

    rf674479 ra0fc78a  
    211211        }
    212212
    213         void AlternativeFinder::addAnonConversions( const Alternative & alt ) {
    214                 // adds anonymous member interpretations whenever an aggregate value type is seen.
    215                 Expression * expr = alt.expr->clone();
    216                 std::unique_ptr< Expression > manager( expr ); // RAII for expr
    217                 alt.env.apply( expr->get_result() );
    218                 if ( StructInstType *structInst = dynamic_cast< StructInstType* >( expr->get_result() ) ) {
    219                         NameExpr nameExpr( "" );
    220                         addAggMembers( structInst, expr, alt.cost+Cost( 0, 0, 1 ), alt.env, &nameExpr );
    221                 } else if ( UnionInstType *unionInst = dynamic_cast< UnionInstType* >( expr->get_result() ) ) {
    222                         NameExpr nameExpr( "" );
    223                         addAggMembers( unionInst, expr, alt.cost+Cost( 0, 0, 1 ), alt.env, &nameExpr );
    224                 } // if
    225         }
     213        // std::unordered_map< Expression *, UniqueExpr * > ;
    226214
    227215        template< typename StructOrUnionType >
    … …  
    232220                std::list< Declaration* > members;
    233221                aggInst->lookup( name, members );
    234 
    235222                for ( std::list< Declaration* >::const_iterator i = members.begin(); i != members.end(); ++i ) {
    236223                        if ( DeclarationWithType *dwt = dynamic_cast< DeclarationWithType* >( *i ) ) {
    237224                                alternatives.push_back( Alternative( new MemberExpr( dwt, expr->clone() ), env, newCost ) );
    238225                                renameTypes( alternatives.back().expr );
    239                                 addAnonConversions( alternatives.back() ); // add anonymous member interpretations whenever an aggregate value type is seen as a member expression.
    240226                        } else {
    241227                                assert( false );
    … …  
    744730                if ( candidates.empty() && ! errors.isEmpty() ) { throw errors; }
    745731
    746                 // compute conversionsion costs
    747732                for ( AltList::iterator withFunc = candidates.begin(); withFunc != candidates.end(); ++withFunc ) {
    748733                        Cost cvtCost = computeConversionCost( *withFunc, indexer );
    … …  
    766751                        } // if
    767752                } // for
    768                 // function may return struct or union value, in which case we need to add alternatives for implicit conversions to each of the anonymous members
    769                 for ( const Alternative & alt : alternatives ) {
    770                         addAnonConversions( alt );
    771                 }
    772 
    773753                candidates.clear();
    774754                candidates.splice( candidates.end(), alternatives );
    … …  
    905885                        )
    906886                        renameTypes( alternatives.back().expr );
    907                         addAnonConversions( alternatives.back() ); // add anonymous member interpretations whenever an aggregate value type is seen as a name expression.
     887                        if ( StructInstType *structInst = dynamic_cast< StructInstType* >( (*i)->get_type() ) ) {
     888                                NameExpr nameExpr( "" );
     889                                addAggMembers( structInst, &newExpr, Cost( 0, 0, 1 ), env, &nameExpr );
     890                        } else if ( UnionInstType *unionInst = dynamic_cast< UnionInstType* >( (*i)->get_type() ) ) {
     891                                NameExpr nameExpr( "" );
     892                                addAggMembers( unionInst, &newExpr, Cost( 0, 0, 1 ), env, &nameExpr );
     893                        } // if
    908894                } // for
    909895        }

  • src/ResolvExpr/AlternativeFinder.h

    rf674479 ra0fc78a  
    7878                void findSubExprs( InputIterator begin, InputIterator end, OutputIterator out );
    7979
    80                 /// Adds alternatives for anonymous members
    81                 void addAnonConversions( const Alternative & alt );
    8280                /// Adds alternatives for member expressions, given the aggregate, conversion cost for that aggregate, and name of the member
    8381                template< typename StructOrUnionType > void addAggMembers( StructOrUnionType *aggInst, Expression *expr, const Cost &newCost, const TypeEnvironment & env, Expression * member );

  • src/SymTab/Autogen.cc

    rf674479 ra0fc78a  
    498498                makeUnionFieldsAssignment( srcParam, dstParam, back_inserter( funcDecl->get_statements()->get_kids() ) );
    499499                if ( returnVal ) {
    500                         funcDecl->get_statements()->get_kids().push_back( new ReturnStmt( noLabels, new VariableExpr( srcParam ) ) );
     500                        if ( isDynamicLayout ) makeUnionFieldsAssignment( srcParam, returnVal, back_inserter( funcDecl->get_statements()->get_kids() ) );
     501                        else funcDecl->get_statements()->get_kids().push_back( new ReturnStmt( noLabels, new VariableExpr( srcParam ) ) );
    501502                }
    502503        }

  • src/SymTab/Validate.cc

    rf674479 ra0fc78a  
    208208        };
    209209
    210         class ArrayLength : public Visitor {
    211         public:
    212                 /// for array types without an explicit length, compute the length and store it so that it
    213                 /// is known to the rest of the phases. For example,
    214                 ///   int x[] = { 1, 2, 3 };
    215                 ///   int y[][2] = { { 1, 2, 3 }, { 1, 2, 3 } };
    216                 /// here x and y are known at compile-time to have length 3, so change this into
    217                 ///   int x[3] = { 1, 2, 3 };
    218                 ///   int y[3][2] = { { 1, 2, 3 }, { 1, 2, 3 } };
    219                 static void computeLength( std::list< Declaration * > & translationUnit );
    220 
    221                 virtual void visit( ObjectDecl * objDecl );
    222         };
    223 
    224210        class CompoundLiteral final : public GenPoly::DeclMutator {
    225211                Type::StorageClasses storageClasses;
    … …  
    249235                acceptAll( translationUnit, pass3 );
    250236                VerifyCtorDtorAssign::verify( translationUnit );
    251                 ArrayLength::computeLength( translationUnit );
    252237        }
    253238
    … …  
    884869                }
    885870        }
    886 
    887         void ArrayLength::computeLength( std::list< Declaration * > & translationUnit ) {
    888                 ArrayLength len;
    889                 acceptAll( translationUnit, len );
    890         }
    891 
    892         void ArrayLength::visit( ObjectDecl * objDecl ) {
    893                 if ( ArrayType * at = dynamic_cast< ArrayType * >( objDecl->get_type() ) ) {
    894                         if ( at->get_dimension() != nullptr ) return;
    895                         if ( ListInit * init = dynamic_cast< ListInit * >( objDecl->get_init() ) ) {
    896                                 at->set_dimension( new ConstantExpr( Constant::from_ulong( init->get_initializers().size() ) ) );
    897                         }
    898                 }
    899         }
    900871} // namespace SymTab
    901872

  • src/SynTree/Expression.cc

    rf674479 ra0fc78a  
    339339                        return TypeSubstitution( aggInst->get_baseParameters()->begin(), aggInst->get_baseParameters()->end(), aggInst->get_parameters().begin() );
    340340                } else {
    341                         assertf( false, "makeSub expects struct or union type for aggregate, but got: %s", toString( t ).c_str() );
     341                        assertf( false, "makeSub expects struct or union type for aggregate" );
    342342                }
    343343        }
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