source: doc/theses/aaron_moss_PhD/phd/background.tex @ 397848f5

aaron-thesisarm-ehcleanup-dtorsenumforall-pointer-decayjacob/cs343-translationjenkins-sandboxnew-astnew-ast-unique-expr
Last change on this file since 397848f5 was 397848f5, checked in by Aaron Moss <a3moss@…>, 3 years ago

thesis: typos from Gregor

  • Property mode set to 100644
File size: 32.5 KB
Line 
1\chapter{\CFA{}}
2\label{cfa-chap}
3
4\CFA{} adds a number of features to C, some of them providing significant increases to the expressive power of the language, but all designed to maintain the existing procedural programming paradigm of C and to be as orthogonal as possible to each other.
5To provide background for the contributions in subsequent chapters, this chapter provides a summary of the features of \CFA{} at the time this work was conducted.
6
7Glen Ditchfield laid out the core design of \CFA{} in his 1992 PhD thesis, \emph{Contextual Polymorphism} \cite{Ditchfield92}; in that thesis, Ditchfield presents the theoretical underpinnings of the \CFA{} polymorphism model.
8Building on Ditchfield's design for contextual polymorphism as well as KW-C \cite{Buhr94a}, an earlier set of (largely syntactic) extensions to C, Richard Bilson \cite{Bilson03} built the first version of the \CFA{} compiler, \CFACC{}, in the early 2000's.
9This early \CFACC{} provided basic functionality, but incorporated a number of algorithmic choices that have failed to scale as \CFA{} has developed, lacking the runtime performance for practical use; this thesis is substantially concerned with rectifying those deficits.
10
11The \CFA{} project was revived in 2015 with the intention of building a production-ready language and compiler; at the time of this writing, both \CFA{} and \CFACC{} remain under active development.
12As this development has been proceeding concurrently with the work described in this thesis, the state of \CFA{} has been somewhat of a moving target; however, Moss~\etal~\cite{Moss18} provides a reasonable summary of the current design.
13Notable features added during this period include generic types (Chapter~\ref{generic-chap}), constructors and destructors \cite{Schluntz17}, improved support for tuples \cite{Schluntz17}, reference types \cite{Moss18}, first-class concurrent and parallel programming support \cite{Delisle18}, as well as numerous pieces of syntactic sugar and the start of an idiomatic standard library \cite{Moss18}.
14
15The selection of features presented in this chapter are chosen to elucidate the design constraints of the work presented in this thesis.
16In some cases the interactions of multiple features make this design a significantly more complex problem than any individual feature; in other cases a feature that does not by itself add any complexity to expression resolution triggers previously rare edge cases more frequently.
17
18\section{Procedural Paradigm}
19
20It is important to note that \CFA{} is not an object-oriented language.
21This is a deliberate choice intended to maintain the applicability of the programming model and language idioms already possessed by C programmers.
22This choice is in marked contrast to \CC{}, which is a much larger and more complex language, and requires extensive developer re-training to write idiomatic, efficient code in \CC{}'s object-oriented paradigm.
23
24Particularly, \CFA{} has no concept of \emph{subclass}, and thus no need to integrate an inheritance-based form of polymorphism with its parametric and overloading-based polymorphism.
25While \CFA{} does have a system of implicit type conversions derived from C's ``usual arithmetic conversions'' \cite[\S{}6.3.1.8]{C11} and these conversions may be thought of as something like an inheritance hierarchy, the underlying semantics are significantly different and such an analogy is loose at best.
26The graph structure of the \CFA{} type conversions (discussed in Section~\ref{conv-cost-sec}) is also markedly different than an inheritance hierarchy; it has neither a top nor a bottom type, and does not satisfy the lattice properties typical of inheritance hierarchies.
27
28Another aspect of \CFA{}'s procedural paradigm is that it retains C's translation-unit-based encapsulation model, rather than class-based encapsulation such as in \CC{}.
29As such, any language feature that requires code to be exposed in header files (\eg{} \CC{} templates) also eliminates encapsulation in \CFA{}.
30Given this constraint, \CFA{} is carefully designed to allow separate compilation for its added language features under the existing C usage patterns.
31
32\section{Name Overloading} \label{overloading-sec}
33
34In C, no more than one variable or function in the same scope may share the same name\footnote{Technically, C has multiple separated namespaces, one holding \lstinline{struct}, \lstinline{union}, and \lstinline{enum} tags, one holding labels, one holding \lstinline{typedef} names, variable, function, and enumerator identifiers, and one for each \lstinline{struct} and \lstinline{union} type holding the field names \cite[\S{}6.2.3]{C11}.}, and variable or function declarations in inner scopes with the same name as a declaration in an outer scope hide the outer declaration.
35This restriction makes finding the proper declaration to match to a variable expression or function application a simple matter of lexically-scoped name lookup, which can be easily and efficiently implemented.
36\CFA{}, on the other hand, allows overloading of variable and function names so long as the overloaded declarations do not have the same type, avoiding the multiplication of variable and function names for different types common in the C standard library, as in the following example:
37
38\begin{cfa}
39#include <limits.h>
40
41const int max = INT_MAX;  $\C[1.75in]{// (1)}$
42const double max = DBL_MAX;  $\C[1.75in]{// (2)}$
43int max(int a, int b) { return a < b ? b : a; }  $\C[1.75in]{// (3)}$
44double max(double a, double b) { return a < b ? b : a; }  $\C[1.75in]{// (4)}$
45
46max( 7, -max );  $\C[3.75in]{// uses (3) and (1), by matching int from 7}$
47max( max, 3.14 );  $\C[3.75in]{// uses (4) and (2), by matching double from 3.14}$
48max( max, -max );  $\C[3.75in]{// ERROR, ambiguous}$
49int m = max( max, -max );  $\C[3.75in]{// uses (3) and (1) twice, by matching return type}$
50\end{cfa}
51
52The final expression in the preceding example includes a feature of \CFA{} name overloading not shared by \CC{}, the ability to disambiguate expressions based on their return type. This provides greater flexibility and power than the parameter-based overload selection of \CC{}, though at the cost of greater complexity in the resolution algorithm.
53
54While the wisdom of giving both the maximum value of a type and the function to take the maximum of two values the same name in the example above is debatable, \eg{} some developers may prefer !MAX! for the former, the guiding philosophy of \CFA{} is ``describe, don't prescribe'' --- we prefer to trust programmers with powerful tools, and there is no technical reason to restrict overloading between variables and functions.
55However, the expressivity of \CFA{}'s name overloading does have the consequence that simple table lookup is insufficient to match identifiers to declarations, and a type-matching algorithm must be part of expression resolution.
56
57\subsection{Operator Overloading}
58
59C does allow name overloading in one context: operator overloading.
60From the perspective of the type system, there is nothing special about operators as opposed to other functions, nor is it desirable to restrict the clear and readable syntax of operators to only the built-in types.
61For these reasons, \CFA{}, like \CC{} and many other programming languages, allows overloading of operators by writing specially-named functions where !?! stands in for the operands.
62This syntax is more natural than the operator overloading syntax of \CC{}, which requires ``dummy'' parameters to disambiguate overloads of similarly-named pre- and postfix operators\footnote{This example uses \CFA{}'s reference types, described in Section~\ref{type-features-sec}}:
63
64\begin{cfa}
65struct counter { int x; };
66
67counter& `++?`(counter& c) { ++c.x; return c; }  $\C[2in]{// pre-increment}$
68counter `?++`(counter& c) {  $\C[2in]{// post-increment}$
69        counter tmp = c; ++c; return tmp;
70}
71bool `?<?`(const counter& a, const counter& b) {  $\C[2in]{// comparison}$
72        return a.x < b.x;
73}
74\end{cfa}
75
76Together, \CFA{}'s backward-compatibility with C and the inclusion of this operator overloading feature imply that \CFA{} must select among function overloads using a method compatible with C's ``usual arithmetic conversions'' \cite[\S{}6.3.1.8]{C11}, so as to present user programmers with only a single set of overloading rules.
77
78\subsection{Special Literal Types}
79
80Literal !0! is also used polymorphically in C; it may be either integer zero or the null value of any pointer type.
81\CFA{} provides a special type for the !0! literal, !zero_t!, so that users can define a zero value for their own types without being forced to create a conversion from an integer or pointer type; \CFA{} also includes implicit conversions from !zero_t! to the !int! and pointer type constructors\footnote{See Section~\ref{type-features-sec}} from !zero_t! for backward compatibility.
82
83According to the C standard \cite[\S{}6.8.4.1]{C11}, !0! is the only false value; any value that compares equal to zero is false, while any value that does not is true.
84By this rule, Boolean contexts such as !if ( x )! can always be equivalently rewritten as \lstinline{if ( (x) != 0 )}.
85\CFACC{} applies this rewriting in all Boolean contexts, so any type !T! can be made ``truthy'' (that is, given a Boolean interpretation) in \CFA{} by defining an operator overload \lstinline{int ?!=?(T, zero_t)}.
86\CC{} takes a different approach to user-defined truthy types, allowing definition of an implicit conversion operator to !bool!; prior to the introduction of the !explicit! keyword for conversion operators in \CCeleven{} this approach also allowed undesired implicit conversions to all other arithmetic types, a shortcoming not shared by the \CFA{} design.
87
88\CFA{} also includes a special type for !1!, !one_t!; like !zero_t!, !one_t! has built-in implicit conversions to the various integral types so that !1! maintains its expected semantics in legacy code.
89The addition of !one_t! allows generic algorithms to handle the unit value uniformly for types where it is meaningful; a simple example of this is that polymorphic functions\footnote{discussed in Section~\ref{poly-func-sec}} in the \CFA{} prelude define !++x! and !x++! in terms of !x += 1!, allowing users to idiomatically define all forms of increment for a type !T! by defining the single function !T& ?+=?(T&, one_t)!; analogous overloads for the decrement operators are also present, and programmers can override any of these functions for a particular type if desired.
90
91\CFA{} previously allowed !0! and !1! to be the names of polymorphic variables, with separate overloads for !int 0!, !int 1!, and !forall(dtype T) T* 0!.
92While designing \CFA{} generic types (see Chapter~\ref{generic-chap}), it was discovered that the parametric polymorphic zero variable is not generalizable to other types; though all null pointers have the same in-memory representation, the same cannot be said of the zero values of arbitrary types.
93As such, variables that could represent !0! and !1! were phased out in favour of functions that could generate those values for a given type as appropriate.
94
95\section{Polymorphic Functions} \label{poly-func-sec}
96
97The most significant type-system feature \CFA{} adds is parametric-polymorphic functions.
98Such functions are written using a !forall! clause (which gives the language its name):
99
100\begin{cfa}
101`forall(otype T)`
102T identity(T x) { return x; }
103\end{cfa}
104
105The !identity! function above can be applied to any complete object type (or ``!otype!'').
106The type variable !T! is transformed into a set of additional implicit parameters to !identity!, which encode sufficient information about !T! to create and return a variable of that type.
107\CFA{} passes the size and alignment of the type represented by an !otype! parameter, as well as a default constructor, copy constructor, assignment operator, and destructor.
108Types which do not have one or more of these operators visible cannot be bound to !otype! parameters, but may be bound to un-constrained !dtype! (``data type'') type variables.
109In this design, the runtime cost of polymorphism is spread over each polymorphic call, due to passing more arguments to polymorphic functions; the experiments in Chapter~\ref{generic-chap} indicate that this overhead is comparable to that of \CC{} virtual function calls.
110% \TODO{rerun experiments, possibly look at vtable variant}
111
112One benefit of this design is that it allows polymorphic functions to be separately compiled.
113The forward declaration !forall(otype T) T identity(T);! uniquely defines a single callable function, which may be implemented in a different file.
114The fact that there is only one implementation of each polymorphic function also reduces compile times relative to the template-expansion approach taken by \CC{}, as well as reducing binary sizes and runtime pressure on instruction cache by re-using a single version of each function.
115
116\subsection{Type Assertions}
117
118Since bare polymorphic types do not provide a great range of available operations, \CFA{} provides a \emph{type assertion} mechanism to provide further information about a type\footnote{This example introduces a convention used throughout this thesis of disambiguating overloaded names with subscripts; the subscripts do not appear in \CFA{} source code.}:
119
120\begin{cfa}
121forall(otype T `| { T twice(T); }`)
122T four_times(T x) { return twice( twice(x) ); }
123double twice$\(_1\)$(double d) { return d * 2.0; }
124
125double ans = four_times( 10.5 )$\C[2.75in]{// T bound to double, ans == 42.0}$
126\end{cfa}
127
128These type assertions may be either variable or function declarations that depend on a polymorphic type variable.
129!four_times! may only be called with an argument for which there exists a function named !twice! that can take that argument and return another value of the same type; a pointer to the appropriate function is passed as an additional implicit parameter of the call to !four_times!.
130
131Monomorphic specializations of polymorphic functions can themselves be used to satisfy type assertions.
132For instance, !twice! could have been defined like this:
133
134\begin{cfa}
135forall(otype S | { S ?+?(S, S); })
136S twice$\(_2\)$(S x) { return x + x; }
137\end{cfa}
138
139Specializing this polymorphic function with !S = double! produces a monomorphic function which can  be used to satisfy the type assertion on !four_times!.
140\CFACC{} accomplishes this by creating a wrapper function calling !twice!$_2$ with !S! bound to !double!, then providing this wrapper function to !four_times!\footnote{\lstinline{twice}$_2$ could also have had a type parameter named \lstinline{T}; \CFA{} specifies renaming of the type parameters, which would avoid the name conflict with the type variable \lstinline{T} of \lstinline{four_times}}.
141However, !twice!$_2$ also works for any type !S! that has an addition operator defined for it.
142
143Finding appropriate functions to satisfy type assertions is essentially a recursive case of expression resolution, as it takes a name (that of the type assertion) and attempts to match it to a suitable declaration in the current scope.
144If a polymorphic function can be used to satisfy one of its own type assertions, this recursion may not terminate, as it is possible that that function is examined as a candidate for its own assertion unboundedly repeatedly.
145To avoid such infinite loops, \CFACC{} imposes a fixed limit on the possible depth of recursion, similar to that employed by most \CC{} compilers for template expansion; this restriction means that there are some otherwise semantically well-typed expressions that cannot be resolved by \CFACC{}.
146
147% \subsection{Deleted Declarations}
148
149% Particular type combinations can be exempted from matching a given polymorphic function through use of a \emph{deleted function declaration}:
150
151% \begin{cfa}
152% int somefn(char) = void;
153% \end{cfa}
154
155% This feature is based on a \CCeleven{} feature typically used to make a type non-copyable by deleting its copy constructor and assignment operator\footnote{In previous versions of \CC{}, a type could be made non-copyable by declaring a private copy constructor and assignment operator, but not defining either. This idiom is well-known, but depends on some rather subtle and \CC{}-specific rules about private members and implicitly-generated functions; the deleted function form is both clearer and less verbose.} or forbidding some interpretations of a polymorphic function by specifically deleting the forbidden overloads\footnote{Specific polymorphic function overloads can also be forbidden in previous \CC{} versions through use of template metaprogramming techniques, though this advanced usage is beyond the skills of many programmers. A similar effect can be produced on an ad-hoc basis at the appropriate call sites through use of casts to determine the function type. In both cases, the deleted-function form is clearer and more concise.}.
156% Deleted function declarations are implemented in \CFACC{} by adding them to the symbol table as usual, but with a flag set that indicates that the function is deleted.
157% If this deleted declaration is selected as the unique minimal-cost interpretation of an expression than an error is produced.
158
159\subsection{Traits}
160
161\CFA{} provides \emph{traits} as a means to name a group of type assertions, as in the example below\footnote{This example uses \CFA{}'s reference types and constructors, described in Section~\ref{type-features-sec}.}:
162
163\begin{cfa}
164`trait has_magnitude(otype T)` {
165        bool ?<?(T, T)$\C[4in]{// comparison operator}$
166        T -?(T)$\C[4in]{// negation operator}$
167        void ?{}(T&, zero_t)$\C[4in]{// constructor from 0}$
168};
169
170forall(otype M | `has_magnitude(M)`)
171M abs(M m) { return m < (M){0} ? -m : m; }
172
173forall(otype M | `has_magnitude(M)`)
174M max_magnitude(M a, M b) { return abs(a) < abs(b) ? b : a; }
175\end{cfa}
176
177Semantically, traits are simply a named list of type assertions, but they may be used for many of the same purposes that interfaces in Java or abstract base classes in \CC{} are used for.
178Unlike Java interfaces or \CC{} base classes, \CFA{} types do not explicitly state any inheritance relationship to traits they satisfy; this can be considered a form of structural inheritance, similar to interface implementation in Go, as opposed to the nominal inheritance model of Java and \CC{}.
179Nominal inheritance can be simulated in \CFA{} using marker variables in traits:
180
181\begin{cfa}
182trait nominal(otype T) {
183        `T is_nominal;`
184};
185
186int is_nominal;  $\C{// int now satisfies nominal}$
187{
188        char is_nominal;  $\C{// char only satisfies nominal in this scope}$
189}
190\end{cfa}
191
192Traits, however, are significantly more powerful than nominal-inheritance interfaces; firstly, due to the scoping rules of the declarations that satisfy a trait's type assertions, a type may not satisfy a trait everywhere that that type is declared, as with !char! and the !nominal! trait above.
193Secondly, because \CFA{} is not object-oriented and types do not have a closed set of methods, existing C library types can be extended to implement a trait simply by writing the requisite functions\footnote{\CC{} only allows partial extension of C types, because constructors, destructors, conversions, and the assignment, indexing, and function-call operators may only be defined in a class; \CFA{} does not have any of these restrictions.}.
194Finally, traits may be used to declare a relationship among multiple types, a property that may be difficult or impossible to represent in nominal-inheritance type systems\footnote{This example uses \CFA{}'s reference types, described in Section~\ref{type-features-sec}.}:
195
196\begin{cfa}
197trait pointer_like(`otype Ptr, otype El`) {
198        El& *?(Ptr)$\C{// Ptr can be dereferenced to El}$
199};
200
201struct list {
202        int value;
203        list* next; $\C{// may omit struct on type names}$
204};
205
206typedef list* list_iterator;
207
208int& *?(list_iterator it) {
209        return it->value;
210}
211\end{cfa}
212
213In this example above, !(list_iterator, int)! satisfies !pointer_like! by the user-defined dereference function, and !(list_iterator, list)! also satisfies !pointer_like! by the built-in dereference operator for pointers.
214Given a declaration !list_iterator it!, !*it! can be either an !int! or a !list!, with the meaning disambiguated by context (\eg{} !int x = *it;! interprets !*it! as !int!, while !(*it).value = 42;! interprets !*it! as !list!).
215While a nominal-inheritance system with associated types could model one of those two relationships by making !El! an associated type of !Ptr! in the !pointer_like! implementation, few such systems could model both relationships simultaneously.
216
217The flexibility of \CFA{}'s implicit trait-satisfaction mechanism provides programmers with a great deal of power, but also blocks some optimization approaches for expression resolution.
218The ability of types to begin or cease to satisfy traits when declarations go into or out of scope makes caching of trait satisfaction judgments difficult, and the ability of traits to take multiple type parameters can lead to a combinatorial explosion of work in any attempt to pre-compute trait satisfaction relationships.
219
220\section{Implicit Conversions} \label{implicit-conv-sec}
221
222In addition to the multiple interpretations of an expression produced by name overloading and polymorphic functions, \CFA{} must support all of the implicit conversions present in C for backward compatibility, producing further candidate interpretations for expressions.
223As mentioned above, C does not have an inheritance hierarchy of types, but the C standard's rules for the ``usual arithmetic conversions'' \cite[\S{}6.3.1.8]{C11} define which of the built-in types are implicitly convertible to which other types, as well as which implicit conversions to apply for mixed arguments to binary operators.
224\CFA{} adds rules to the usual arithmetic conversions defining the cost of binding a polymorphic type variable in a function call; such bindings are cheaper than any \emph{unsafe} (narrowing) conversion, \eg{} !int! to !char!, but more expensive than any \emph{safe} (widening) conversion, \eg{} !int! to !double!.
225One contribution of this thesis, discussed in Section~\ref{conv-cost-sec}, is a number of refinements to this cost model to more efficiently resolve polymorphic function calls.
226
227The expression resolution problem, which is the focus of Chapter~\ref{resolution-chap}, is to find the unique minimal-cost interpretation of each expression in the program, where all identifiers must be matched to a declaration, and implicit conversions or polymorphic bindings of the result of an expression may increase the cost of the expression.
228While semantically valid \CFA{} code always has such a unique minimal-cost interpretation, \CFACC{} must also be able to detect and report as errors expressions that have either no interpretation or multiple ambiguous minimal-cost interpretations.
229Note that which subexpression interpretation is minimal-cost may require contextual information to disambiguate.
230For instance, in the example in Section~\ref{overloading-sec}, !max(max, -max)! cannot be unambiguously resolved, but !int m = max(max, -max)! has a single minimal-cost resolution.
231While the interpretation !int m = (int)max((double)max, -(double)max)! is also a valid interpretation, it is not minimal-cost due to the unsafe cast from the !double! result of !max! to !int!\footnote{The two \lstinline{double} casts function as type ascriptions selecting \lstinline{double max} rather than casts from \lstinline{int max} to \lstinline{double}, and as such are zero-cost. The \lstinline{int} to \lstinline{double} conversion could be forced if desired with two casts: \lstinline{(double)(int)max}}.
232These contextual effects make the expression resolution problem for \CFA{} both theoretically and practically difficult, but the observation driving the work in Chapter~\ref{resolution-chap} is that of the many top-level expressions in a given program, most are straightforward and idiomatic so that programmers writing and maintaining the code can easily understand them; it follows that effective heuristics for common cases can bring down compiler runtime enough that a small proportion of harder-to-resolve expressions does not inordinately increase overall compiler runtime or memory usage.
233
234\section{Type Features} \label{type-features-sec}
235
236The name overloading and polymorphism features of \CFA{} have the greatest effect on language design and compiler runtime, but there are a number of other features in the type system that have a smaller effect but are useful for code examples.
237These features are described here.
238
239\subsection{Reference Types}
240
241One of the key ergonomic improvements in \CFA{} is reference types, designed and implemented by Robert Schluntz \cite{Schluntz17}.
242Given some type !T!, a !T&! (``reference to !T!'') is essentially an automatically dereferenced pointer.
243These types allow seamless pass-by-reference for function parameters, without the extraneous dereferencing syntax present in C; they also allow easy aliasing of nested values with a similarly convenient syntax.
244The addition of reference types also eliminated two syntactic special-cases present in previous versions of \CFA{}.
245Considering a call !a += b! to a compound assignment operator, the previous declaration for that operator was !lvalue int ?+=?(int*, int)! -- to mutate the left argument, the built-in operators were special-cased to implicitly take the address of that argument, while the special !lvalue! syntax was used to mark the return type of a function as a mutable reference.
246With references, this declaration can be re-written as !int& ?+=?(int&, int)! -- the reference semantics generalize the implicit address-of on the left argument and allow it to be used in user-declared functions, while also subsuming the (now removed) !lvalue! syntax for function return types.
247
248The C standard makes heavy use of the concept of \emph{lvalue}, an expression with a memory address; its complement, \emph{rvalue} (a non-addressable expression) is not explicitly named in the standard.
249In \CFA{}, the distinction between lvalue and rvalue can be re-framed in terms of reference and non-reference types, with the benefit of being able to express the difference in user code.
250\CFA{} references preserve the existing qualifier-dropping implicit lvalue-to-rvalue conversion from C (\eg{} a !const volatile int&! can be implicitly copied to a bare !int!)
251To make reference types more easily usable in legacy pass-by-value code, \CFA{} also adds an implicit rvalue-to-lvalue conversion, implemented by storing the value in a compiler-generated temporary variable and passing a reference to that temporary.
252To mitigate the ``!const! hell'' problem present in \CC{}, there is also a qualifier-dropping lvalue-to-lvalue conversion implemented by copying into a temporary:
253
254\begin{cfa}
255const int magic = 42;
256
257void inc_print( int& x ) { printf("%d\n", ++x); }
258
259print_inc( magic ); $\C{// legal; implicitly generated code in red below:}$
260
261`int tmp = magic;` $\C{// to safely strip const-qualifier}$
262`print_inc( tmp );` $\C{// tmp is incremented, magic is unchanged}$
263\end{cfa}
264
265Despite the similar syntax, \CFA{} references are significantly more flexible than \CC{} references.
266The primary issue with \CC{} references is that it is impossible to extract the address of the reference variable rather than the address of the referred-to variable.
267This breaks a number of the usual compositional properties of the \CC{} type system, \eg{} a reference cannot be re-bound to another variable, nor is it possible to take a pointer to, array of, or reference to a reference.
268\CFA{} supports all of these use cases without further added syntax.
269The key to this syntax-free feature support is an observation made by the author that the address of a reference is a lvalue.
270In C, the address-of operator !&x! can only be applied to lvalue expressions, and always produces an immutable rvalue; \CFA{} supports reference re-binding by assignment to the address of a reference, and pointers to references by repeating the address-of operator:
271
272\begin{cfa}
273int x = 2, y = 3;
274int& r = x;  $\C{// r aliases x}$
275&r = &y; $\C{// r now aliases y}$
276int** p = &&r; $\C{// p points to r}$
277\end{cfa}
278
279For better compatibility with C, the \CFA{} team has chosen not to differentiate function overloads based on top-level reference types, and as such their contribution to the difficulty of \CFA{} expression resolution is largely restricted to the implementation details of normalization conversions and adapters.
280
281\subsection{Resource Management} \label{ctor-sec}
282
283\CFA{} also supports the RAII (``Resource Acquisition is Initialization'') idiom originated by \CC{}, thanks to the object lifetime work of Robert Schluntz \cite{Schluntz17}.
284This idiom allows a safer and more principled approach to resource management by tying acquisition of a resource to object initialization, with the corresponding resource release executed automatically at object finalization.
285A wide variety of conceptual resources may be conveniently managed by this scheme, including heap memory, file handles, and software locks.
286
287\CFA{}'s implementation of RAII is based on special constructor and destructor operators, available via the !x{ ... }! constructor syntax and !^x{ ... }! destructor syntax.
288Each type has an overridable compiler-generated zero-argument constructor, copy constructor, assignment operator, and destructor, as well as a field-wise constructor for each appropriate prefix of the member fields of !struct! types.
289For !struct! types the default versions of these operators call their equivalents on each field of the !struct!.
290The main implication of these object lifetime functions for expression resolution is that they are all included as implicit type assertions for !otype! type variables, with a secondary effect being an increase in code size due to the compiler-generated operators.
291Due to these implicit type assertions, assertion resolution is pervasive in \CFA{} polymorphic functions, even those without explicit type assertions.
292Implicitly-generated code is shown in red in the following example:
293
294\begin{cfa}
295struct kv {
296        int key;
297        char* value;
298};
299
300`void ?{} (kv& this) {` $\C[3in]{// default constructor}$
301`       this.key{};` $\C[3in]{// call recursively on members}$
302`       this.value{};
303}
304
305void ?{} (kv& this, int key) {` $\C[3in]{// partial field constructor}$
306`       this.key{ key };
307        this.value{};` $\C[3in]{// default-construct missing fields}$
308`}
309
310void ?{} (kv& this, int key, char* value) {` $\C[3in]{// complete field constructor}$
311`       this.key{ key };
312        this.value{ value };
313}
314
315void ?{} (kv& this, kv that) {` $\C[3in]{// copy constructor}$
316`       this.key{ that.key };
317        this.value{ that.value };
318}
319
320kv ?=? (kv& this, kv that) {` $\C[3in]{// assignment operator}$
321`       this.key = that.key;
322        this.value = that.value;
323}
324
325void ^?{} (kv& this) {` $\C[3in]{// destructor}$
326`       ^this.key{};
327        ^this.value{};
328}`
329
330forall(otype T `| { void ?{}(T&); void ?{}(T&, T); T ?=?(T&, T); void ^?{}(T&); }`)
331void foo(T);
332\end{cfa}
333
334\subsection{Tuple Types}
335
336\CFA{} adds \emph{tuple types} to C, a syntactic facility for referring to lists of values anonymously or with a single identifier.
337An identifier may name a tuple, a function may return one, and a tuple may be implicitly \emph{destructured} into its component values.
338The implementation of tuples in \CFACC{}'s code generation is based on the generic types introduced in Chapter~\ref{generic-chap}, with one compiler-generated generic type for each tuple arity.
339This reuse allows tuples to take advantage of the same runtime optimizations available to generic types, while reducing code bloat.
340An extended presentation of the tuple features of \CFA{} can be found in \cite{Moss18}, but the following example demonstrates the basic features:
341
342\begin{cfa}
343[char, char] x$\(_1\)$ = ['!', '?']; $\C{// tuple type and expression syntax}$
344int x$\(_2\)$ = 2;
345
346forall(otype T)
347[T, T] swap$\(_1\)$( T a, T b ) {
348        return [b, a]; $\C{// one-line swap syntax}$
349}
350
351x = swap( x ); $\C{// destructure x\(_1\) into two elements}$
352$\C{// cannot use x\(_2\), not enough arguments}$
353
354void swap$\(_2\)$( int, char, char );
355
356swap( x, x ); $\C{// swap\(_2\)( x\(_2\), x\(_1\) )}$
357$\C{// not swap\(_1\)( x\(_2\), x\(_2\) ) due to polymorphism cost}$
358\end{cfa}
359
360Tuple destructuring breaks the one-to-one relationship between identifiers and values.
361Hence, some argument-parameter matching strategies for expression resolution are precluded, as well as cheap interpretation filters based on comparing number of parameters and arguments.
362As an example, in the call to !swap( x, x )! above, the second !x! can be resolved starting at the second or third parameter of !swap!$_2$, depending which interpretation of !x! is chosen for the first argument.
363
364\section{Conclusion}
365
366\CFA{} adds a significant number of features to standard C, increasing the expressivity and re-usability of \CFA{} code while maintaining backwards compatibility for both code and larger language paradigms.
367This flexibility does incur significant added compilation costs, however, the mitigation of which are the primary concern of this thesis.
Note: See TracBrowser for help on using the repository browser.