Changeset d5a52cc for doc/user/user.tex


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Timestamp:
Nov 28, 2017, 3:04:37 PM (6 years ago)
Author:
Rob Schluntz <rschlunt@…>
Branches:
ADT, aaron-thesis, arm-eh, ast-experimental, cleanup-dtors, deferred_resn, demangler, enum, forall-pointer-decay, jacob/cs343-translation, jenkins-sandbox, master, new-ast, new-ast-unique-expr, new-env, no_list, persistent-indexer, pthread-emulation, qualifiedEnum, resolv-new, with_gc
Children:
4e7a4e6
Parents:
1393df07 (diff), 8a0a64d9 (diff)
Note: this is a merge changeset, the changes displayed below correspond to the merge itself.
Use the (diff) links above to see all the changes relative to each parent.
Message:

Merge branch 'master' of plg.uwaterloo.ca:/u/cforall/software/cfa/cfa-cc

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1 edited

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  • doc/user/user.tex

    r1393df07 rd5a52cc  
    1111%% Created On       : Wed Apr  6 14:53:29 2016
    1212%% Last Modified By : Peter A. Buhr
    13 %% Last Modified On : Sun Aug  6 10:24:21 2017
    14 %% Update Count     : 3036
     13%% Last Modified On : Mon Nov 27 18:09:59 2017
     14%% Update Count     : 3143
    1515%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    1616
    … …  
    5959\CFAStyle                                                                                               % use default CFA format-style
    6060\lstnewenvironment{C++}[1][]                            % use C++ style
    61 {\lstset{language=C++,moredelim=**[is][\protect\color{red}]{®}{®}#1}}
     61{\lstset{language=C++,moredelim=**[is][\protect\color{red}]{®}{®},#1}}
    6262{}
    6363
    … …  
    777777  case 7:
    778778        ...
    779         ®break®                                         §\C{// redundant explicit end of switch}§
     779        ®break®                                         §\C{// explicit end of switch (redundant)}§
    780780  default:
    781781        j = 3;
    … …  
    806806                ®int k = 0;®                    §\C{// allowed at different nesting levels}§
    807807                ...
     808          ®case 2:®                                     §\C{// disallow case in nested statements}§
    808809        }
    809810  ...
    … …  
    962963\end{cfa}
    963964
     965The components in the "with" clause
     966
     967  with a, b, c { ... }
     968
     969serve 2 purposes: each component provides a type and object. The type must be a
     970structure type. Enumerations are already opened, and I think a union is opened
     971to some extent, too. (Or is that just unnamed unions?) The object is the target
     972that the naked structure-fields apply to. The components are open in "parallel"
     973at the scope of the "with" clause/statement, so opening "a" does not affect
     974opening "b", etc. This semantic is different from Pascal, which nests the
     975openings.
     976
     977Having said the above, it seems reasonable to allow a "with" component to be an
     978expression. The type is the static expression-type and the object is the result
     979of the expression. Again, the type must be an aggregate. Expressions require
     980parenthesis around the components.
     981
     982  with( a, b, c ) { ... }
     983
     984Does this now make sense?
     985
     986Having written more CFA code, it is becoming clear to me that I *really* want
     987the "with" to be implemented because I hate having to type all those object
     988names for fields. It's a great way to drive people away from the language.
     989
    964990
    965991\section{Exception Handling}
    … …  
    9771003try {
    9781004        f(...);
    979 } catch( E e : §boolean-predicate§ ) {                  §\C[8cm]{// termination handler}§
     1005} catch( E e ; §boolean-predicate§ ) {                  §\C[8cm]{// termination handler}§
    9801006        // recover and continue
    981 } catchResume( E e : §boolean-predicate§ ) {    §\C{// resumption handler}\CRT§
     1007} catchResume( E e ; §boolean-predicate§ ) {    §\C{// resumption handler}\CRT§
    9821008        // repair and return
    9831009} finally {
    … …  
    18721898\end{cfa}
    18731899This syntax allows a prototype declaration to be created by cutting and pasting source text from the routine definition header (or vice versa).
    1874 It is possible to declare multiple routine-prototypes in a single declaration, but the entire type specification is distributed across \emph{all} routine names in the declaration list (see~\VRef{s:Declarations}), \eg:
    1875 \begin{quote2}
    1876 \begin{tabular}{@{}l@{\hspace{3em}}l@{}}
    1877 \multicolumn{1}{c@{\hspace{3em}}}{\textbf{\CFA}}        & \multicolumn{1}{c}{\textbf{C}}        \\
    1878 \begin{cfa}
    1879 [ int ] f( int ), g;
    1880 \end{cfa}
    1881 &
    1882 \begin{cfa}
    1883 int f( int ), g( int );
    1884 \end{cfa}
    1885 \end{tabular}
    1886 \end{quote2}
     1900Like C, it is possible to declare multiple routine-prototypes in a single declaration, where the return type is distributed across \emph{all} routine names in the declaration list (see~\VRef{s:Declarations}), \eg:
     1901\begin{cfa}
     1902C :             const double bar1(), bar2( int ), bar3( double );
     1903§\CFA§: [const double] foo(), foo( int ), foo( double ) { return 3.0; }
     1904\end{cfa}
     1905\CFA allows the last routine in the list to define its body.
     1906
    18871907Declaration qualifiers can only appear at the start of a \CFA routine declaration,\footref{StorageClassSpecifier} \eg:
    18881908\begin{cfa}
    … …  
    22352255\label{s:MRV_Functions}
    22362256
    2237 In standard C, functions can return at most one value.
     2257In C and most programming languages, functions return at most one value;
     2258however, many operations have multiple outcomes, some exceptional (see~\VRef{s:ExceptionHandling}).
    22382259To emulate functions with multiple return values, \emph{\Index{aggregation}} and/or \emph{\Index{aliasing}} is used.
    2239 In the former situation, the function designer creates a record type that combines all of the return values into a single type.
    2240 For example, consider a function returning the most frequently occurring letter in a string, and its frequency.
    2241 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.
    2242 \begin{cfa}
    2243 struct mf_ret {
    2244         int freq;
    2245         char ch;
    2246 };
    2247 
    2248 struct mf_ret most_frequent(const char * str) {
    2249         char freqs [26] = { 0 };
    2250         struct mf_ret ret = { 0, 'a' };
    2251         for (int i = 0; str[i] != '\0'; ++i) {
    2252                 if (isalpha(str[i])) {        // only count letters
    2253                         int ch = tolower(str[i]);   // convert to lower case
    2254                         int idx = ch-'a';
    2255                         if (++freqs[idx] > ret.freq) {  // update on new max
    2256                           ret.freq = freqs[idx];
    2257                           ret.ch = ch;
    2258                         }
    2259                 }
    2260         }
    2261         return ret;
    2262 }
    2263 
    2264 const char * str = "hello world";
    2265 struct mf_ret ret = most_frequent(str);
    2266 printf("%s -- %d %c\n", str, ret.freq, ret.ch);
    2267 \end{cfa}
    2268 Of note, the designer must come up with a name for the return type and for each of its fields.
    2269 Unnecessary naming is a common programming language issue, introducing verbosity and a complication of the user's mental model.
    2270 That is, adding another named type creates another association in the programmer's mind that needs to be kept track of when reading and writing code.
    2271 As such, this technique is effective when used sparingly, but can quickly get out of hand if many functions need to return different combinations of types.
    2272 
    2273 In the latter approach, the designer simulates multiple return values by passing the additional return values as pointer parameters.
    2274 The pointer parameters are assigned inside of the routine body to emulate a return.
    2275 Using the same example,
    2276 \begin{cfa}
    2277 int most_frequent(const char * str, char * ret_ch) {
    2278         char freqs [26] = { 0 };
    2279         int ret_freq = 0;
    2280         for (int i = 0; str[i] != '\0'; ++i) {
    2281                 if (isalpha(str[i])) {        // only count letters
    2282                         int ch = tolower(str[i]);   // convert to lower case
    2283                         int idx = ch-'a';
    2284                         if (++freqs[idx] > ret_freq) {  // update on new max
    2285                           ret_freq = freqs[idx];
    2286                           *ret_ch = ch;   // assign to out parameter
    2287                         }
    2288                 }
    2289         }
    2290         return ret_freq;  // only one value returned directly
    2291 }
    2292 
    2293 const char * str = "hello world";
    2294 char ch;                            // pre-allocate return value
    2295 int freq = most_frequent(str, &ch); // pass return value as out parameter
    2296 printf("%s -- %d %c\n", str, freq, ch);
    2297 \end{cfa}
    2298 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.
    2299 Also, 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.
    2300 Furthermore, 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.
    2301 On a related note, C does not provide a standard mechanism to state that a parameter is going to be used as an additional return value, which makes the job of ensuring that a value is returned more difficult for the compiler.
    2302 Interestingly, there is a subtle bug in the previous example, in that ©ret_ch© is never assigned for a string that does not contain any letters, which can lead to undefined behaviour.
    2303 In this particular case, it turns out that the frequency return value also doubles as an error code, where a frequency of 0 means the character return value should be ignored.
     2260
     2261In the former approach, a record type is created combining all of the return values.
     2262For example, consider C's \Indexc{div} function, which returns the quotient and remainder for a division of an integer value.
     2263\begin{cfa}
     2264typedef struct { int quot, rem; } div_t;        §\C[7cm]{// from include stdlib.h}§
     2265div_t div( int num, int den );
     2266div_t qr = div( 13, 5 );                                        §\C{// return quotient/remainder aggregate}§
     2267printf( "%d %d\n", qr.quot, qr.rem );           §\C{// print quotient/remainder}§
     2268\end{cfa}
     2269This approach requires a name for the return type and fields, where \Index{naming} is a common programming-language issue.
     2270That is, naming creates an association that must be managed when reading and writing code.
     2271While effective when used sparingly, this approach does not scale when functions need to return multiple combinations of types.
     2272
     2273In the latter approach, additional return values are passed as pointer parameters.
     2274A pointer parameter is assigned inside the routine to emulate a return.
     2275For example, consider C's \Indexc{modf} function, which returns the integral and fractional part of a floating-point value.
     2276\begin{cfa}
     2277double modf( double x, double * i );            §\C{// from include math.h}§
     2278double intp, frac = modf( 13.5, &intp );        §\C{// return integral and fractional components}§
     2279printf( "%g %g\n", intp, frac );                        §\C{// print integral/fractional components}§
     2280\end{cfa}
     2281This approach requires allocating storage for the return values, which complicates the call site with a sequence of variable declarations leading to the call.
     2282Also, while a disciplined use of ©const© can give clues about whether a pointer parameter is used as an \Index{out parameter}, it is not obvious from the routine signature whether the callee expects such a parameter to be initialized before the call.
     2283Furthermore, while many C routines that accept pointers are safe for a ©NULL© argument, there are many C routines that are not null-safe.
     2284Finally, C does not provide a mechanism to state that a parameter is going to be used as an additional return value, which makes the job of ensuring that a value is returned more difficult for the compiler.
    23042285Still, not every routine with multiple return values should be required to return an error code, and error codes are easily ignored, so this is not a satisfying solution.
    23052286As with the previous approach, this technique can simulate multiple return values, but in practice it is verbose and error prone.
    23062287
    2307 In \CFA, functions can be declared to return multiple values with an extension to the function declaration syntax.
     2288\CFA allows functions to return multiple values by extending the function declaration syntax.
    23082289Multiple return values are declared as a comma-separated list of types in square brackets in the same location that the return type appears in standard C function declarations.
     2290\begin{cfa}
     2291[ char, int, double ] f( ... );
     2292\end{cfa}
    23092293The ability to return multiple values from a function requires a new syntax for the return statement.
    23102294For consistency, the return statement in \CFA accepts a comma-separated list of expressions in square brackets.
    2311 The 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.
     2295\begin{cfa}
     2296return [ c, i, d ];
     2297\end{cfa}
     2298The expression resolution ensures the correct form is used depending on the values being returned and the return type of the current function.
    23122299A multiple-returning function with return type ©T© can return any expression that is implicitly convertible to ©T©.
    2313 Using the running example, the ©most_frequent© function can be written using multiple return values as such,
    2314 \begin{cfa}
    2315 [int, char] most_frequent(const char * str) {
    2316         char freqs [26] = { 0 };
    2317         int ret_freq = 0;
    2318         char ret_ch = 'a';  // arbitrary default value for consistent results
    2319         for (int i = 0; str[i] != '\0'; ++i) {
    2320                 if (isalpha(str[i])) {        // only count letters
    2321                         int ch = tolower(str[i]);   // convert to lower case
    2322                         int idx = ch-'a';
    2323                         if (++freqs[idx] > ret_freq) {  // update on new max
    2324                           ret_freq = freqs[idx];
    2325                           ret_ch = ch;
    2326                         }
    2327                 }
    2328         }
    2329         return [ret_freq, ret_ch];
    2330 }
    2331 \end{cfa}
    2332 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.
    2333 
    2334 The addition of multiple-return-value functions necessitates a syntax for accepting multiple values at the call-site.
     2300
     2301A common use of a function's output is input to another function.
     2302\CFA allows this case, without any new syntax;
     2303a multiple-returning function can be used in any of the contexts where an expression is allowed.
     2304When a function call is passed as an argument to another call, the best match of actual arguments to formal parameters is evaluated given all possible expression interpretations in the current scope.
     2305\begin{cfa}
     2306void g( int, int );                                                     §\C{// 1}§
     2307void g( double, double );                                       §\C{// 2}§
     2308g( div( 13, 5 ) );                                                      §\C{// select 1}§
     2309g( modf( 13.5 ) );                                                      §\C{// select 2}§
     2310\end{cfa}
     2311In this case, there are two overloaded ©g© routines.
     2312Both calls to ©g© expect two arguments that are matched by the two return values from ©div© and ©modf©. respectively, which are fed directly to the first and second parameters of ©g©.
     2313As well, both calls to ©g© have exact type matches for the two different versions of ©g©, so these exact matches are chosen.
     2314When type matches are not exact, conversions are used to find a best match.
     2315
     2316The previous examples can be rewritten passing the multiple returned-values directly to the ©printf© function call.
     2317\begin{cfa}
     2318[ int, int ] div( int x, int y );                       §\C{// from include stdlib}§
     2319printf( "%d %d\n", div( 13, 5 ) );                      §\C{// print quotient/remainder}§
     2320
     2321[ double, double ] modf( double x );            §\C{// from include math}§
     2322printf( "%g %g\n", modf( 13.5 ) );                      §\C{// print integral/fractional components}§
     2323\end{cfa}
     2324This 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.
     2325
     2326Finally, the addition of multiple-return-value functions necessitates a syntax for retaining the multiple values at the call-site versus their temporary existence during a call.
    23352327The simplest mechanism for retaining a return value in C is variable assignment.
    2336 By assigning the return value into a variable, its value can be retrieved later at any point in the program.
     2328By assigning the multiple return-values into multiple variables, the values can be retrieved later.
    23372329As such, \CFA allows assigning multiple values from a function into multiple variables, using a square-bracketed list of lvalue expressions on the left side.
    23382330\begin{cfa}
    2339 const char * str = "hello world";
    2340 int freq;
    2341 char ch;
    2342 [freq, ch] = most_frequent(str);  // assign into multiple variables
    2343 printf("%s -- %d %c\n", str, freq, ch);
    2344 \end{cfa}
    2345 It is also common to use a function's output as the input to another function.
    2346 \CFA also allows this case, without any new syntax.
    2347 When a function call is passed as an argument to another call, the expression resolver attempts to find the best match of actual arguments to formal parameters given all of the possible expression interpretations in the current scope \cite{Bilson03}.
    2348 For example,
    2349 \begin{cfa}
    2350 void process(int);       // (1)
    2351 void process(char);      // (2)
    2352 void process(int, char); // (3)
    2353 void process(char, int); // (4)
    2354 
    2355 process(most_frequent("hello world"));  // selects (3)
    2356 \end{cfa}
    2357 In this case, there is only one option for a function named ©most_frequent© that takes a string as input.
    2358 This function returns two values, one ©int© and one ©char©.
    2359 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.
    2360 This 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.
    2361 
    2362 \section{Tuple Expressions}
     2331int quot, rem;
     2332[ quot, rem ] = div( 13, 5 );                           §\C{// assign multiple variables}§
     2333printf( "%d %d\n", quot, rem );                         §\C{// print quotient/remainder}\CRT§
     2334\end{cfa}
     2335Here, the multiple return-values are matched in much the same way as passing multiple return-values to multiple parameters in a call.
     2336
     2337
     2338\subsection{Expressions}
     2339
    23632340Multiple-return-value functions provide \CFA with a new syntax for expressing a combination of expressions in the return statement and a combination of types in a function signature.
    2364 These notions can be generalized to provide \CFA with \emph{tuple expressions} and \emph{tuple types}.
     2341These notions are generalized to provide \CFA with \newterm{tuple expression}s and \newterm{tuple type}s.
    23652342A tuple expression is an expression producing a fixed-size, ordered list of values of heterogeneous types.
    2366 The type of a tuple expression is the tuple of the subexpression types, or a \emph{tuple type}.
     2343The type of a tuple expression is the tuple of the subexpression types, or a tuple type.
     2344
    23672345In \CFA, a tuple expression is denoted by a comma-separated list of expressions enclosed in square brackets.
    23682346For example, the expression ©[5, 'x', 10.5]© has type ©[int, char, double]©.
    … …  
    23712349The order of evaluation of the components in a tuple expression is unspecified, to allow a compiler the greatest flexibility for program optimization.
    23722350It is, however, guaranteed that each component of a tuple expression is evaluated for side-effects, even if the result is not used.
    2373 Multiple-return-value functions can equivalently be called \emph{tuple-returning functions}.
    2374 
    2375 \subsection{Tuple Variables}
    2376 The call-site of the ©most_frequent© routine has a notable blemish, in that it required the preallocation of return variables in a manner similar to the aliasing example, since it is impossible to declare multiple variables of different types in the same declaration in standard C.
    2377 In \CFA, it is possible to overcome this restriction by declaring a \emph{tuple variable}.
    2378 \begin{cfa}[emph=ret, emphstyle=\color{red}]
    2379 const char * str = "hello world";
    2380 [int, char] ret = most_frequent(str);  // initialize tuple variable
    2381 printf("%s -- %d %c\n", str, ret);
    2382 \end{cfa}
    2383 It is now possible to accept multiple values into a single piece of storage, in much the same way that it was previously possible to pass multiple values from one function call to another.
    2384 These variables can be used in any of the contexts where a tuple expression is allowed, such as in the ©printf© function call.
    2385 As in the ©process© example, the components of the tuple value are passed as separate parameters to ©printf©, allowing very simple printing of tuple expressions.
    2386 One way to access the individual components is with a simple assignment, as in previous examples.
    2387 \begin{cfa}
    2388 int freq;
    2389 char ch;
    2390 [freq, ch] = ret;
    2391 \end{cfa}
    2392 
    2393 \begin{sloppypar}
     2351Multiple-return-value functions can equivalently be called \newterm{tuple-returning functions}.
     2352
     2353
     2354\subsection{Variables}
     2355
     2356The previous call of ©div© still requires the preallocation of multiple return-variables in a manner similar to the aliasing example.
     2357In \CFA, it is possible to overcome this restriction by declaring a \newterm{tuple variable}.
     2358\begin{cfa}
     2359[int, int] ®qr® = div( 13, 5 );                 §\C{// initialize tuple variable}§
     2360printf( "%d %d\n", ®qr® );                              §\C{// print quotient/remainder}§
     2361\end{cfa}
     2362It is now possible to match the multiple return-values to a single variable, in much the same way as \Index{aggregation}.
     2363As well, the components of the tuple value are passed as separate parameters to ©printf©, allowing direct printing of tuple variables.
     2364One way to access the individual components of a tuple variable is with assignment.
     2365\begin{cfa}
     2366[ quot, rem ] = qr;                                             §\C{// assign multiple variables}§
     2367\end{cfa}
     2368
    23942369In addition to variables of tuple type, it is also possible to have pointers to tuples, and arrays of tuples.
    23952370Tuple types can be composed of any types, except for array types, since array assignment is disallowed, which makes tuple assignment difficult when a tuple contains an array.
    23962371\begin{cfa}
    2397 [double, int] di;
    2398 [double, int] * pdi
    2399 [double, int] adi[10];
     2372[ double, int ] di;
     2373[ double, int ] * pdi
     2374[ double, int ] adi[10];
    24002375\end{cfa}
    24012376This examples declares a variable of type ©[double, int]©, a variable of type pointer to ©[double, int]©, and an array of ten ©[double, int]©.
    2402 \end{sloppypar}
    2403 
    2404 \subsection{Tuple Indexing}
    2405 
    2406 At times, it is desirable to access a single component of a tuple-valued expression without creating unnecessary temporary variables to assign to.
    2407 Given a tuple-valued expression ©e© and a compile-time constant integer $i$ where $0 \leq i < n$, where $n$ is the number of components in ©e©, ©e.i© accesses the $i$\textsuperscript{th} component of ©e©.
    2408 For example,
     2377
     2378
     2379\subsection{Indexing}
     2380
     2381It is also possible to access a single component of a tuple-valued expression without creating temporary variables.
     2382Given a tuple-valued expression $e$np and a compile-time constant integer $i$ where $0 \leq i < n$, where $n$ is the number of components in $e$, $e.i$ accesses the $i^{\:th}$ component of $e$, \eg:
    24092383\begin{cfa}
    24102384[int, double] x;
    … …  
    24172391p->0 = 5;                                                               §\C{// access int component of tuple pointed-to by p}§
    24182392g( x.1, x.0 );                                                  §\C{// rearrange x to pass to g}§
    2419 double z = [x, f()].0.1;                                §\C{// access second component of first component of tuple expression}§
    2420 \end{cfa}
    2421 As seen above, tuple-index expressions can occur on any tuple-typed expression, including tuple-returning functions, square-bracketed tuple expressions, and other tuple-index expressions, provided the retrieved component is also a tuple.
     2393double z = [ x, f() ].0.1;                              §\C{// access second component of first component of tuple expression}§
     2394\end{cfa}
     2395Tuple-index expressions can occur on any tuple-typed expression, including tuple-returning functions, square-bracketed tuple expressions, and other tuple-index expressions, provided the retrieved component is also a tuple.
    24222396This feature was proposed for \KWC but never implemented \cite[p.~45]{Till89}.
    24232397
     2398
    24242399\subsection{Flattening and Structuring}
     2400
    24252401As evident in previous examples, tuples in \CFA do not have a rigid structure.
    24262402In function call contexts, tuples support implicit flattening and restructuring conversions.
    … …  
    24652441There is only a single definition of ©f©, and 3 arguments with only single interpretations.
    24662442First, the argument alternative list ©[5, 10.2], 4© is flattened to produce the argument list ©5, 10.2, 4©.
    2467 Next, the parameter matching algorithm begins, with $P = $©int© and $A = $©int©, which unifies exactly.
    2468 Moving to the next parameter and argument, $P = $©[double, int]© and $A = $©double©.
    2469 This time, the parameter is a tuple type, so the algorithm applies recursively with $P' = $©double© and $A = $©double©, which unifies exactly.
    2470 Then $P' = $©int© and $A = $©double©, which again unifies exactly.
     2443Next, the parameter matching algorithm begins, with $P =$© int© and $A =$© int©, which unifies exactly.
     2444Moving to the next parameter and argument, $P =$© [double, int]© and $A =$© double©.
     2445This time, the parameter is a tuple type, so the algorithm applies recursively with $P' =$© double© and $A =$© double©, which unifies exactly.
     2446Then $P' =$© int© and $A =$© double©, which again unifies exactly.
    24712447At this point, the end of $P'$ has been reached, so the arguments ©10.2, 4© are structured into the tuple expression ©[10.2, 4]©.
    24722448Finally, the end of the parameter list $P$ has also been reached, so the final expression is ©f(5, [10.2, 4])©.
    24732449
    2474 \section{Tuple Assignment}
     2450
     2451\subsection{Assignment}
    24752452\label{s:TupleAssignment}
    2476 An assignment where the left side of the assignment operator has a tuple type is called tuple assignment.
    2477 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.
     2453
     2454An assignment where the left side of the assignment operator has a tuple type is called \newterm{tuple assignment}.
     2455There are two kinds of tuple assignment depending on whether the right side of the assignment operator has a non-tuple or tuple type, called \newterm[mass assignment]{mass} and \newterm[multiple assignment]{multiple} assignment, respectively.
    24782456\begin{cfa}
    24792457int x;
    24802458double y;
    24812459[int, double] z;
    2482 [y, x] = 3.14;  // mass assignment
    2483 [x, y] = z;     // multiple assignment
    2484 z = 10;         // mass assignment
    2485 z = [x, y];     // multiple assignment
     2460[y, x] = 3.14;                                                  §\C{// mass assignment}§
     2461[x, y] = z;                                                         §\C{// multiple assignment}§
     2462z = 10;                                                         §\C{// mass assignment}§
     2463z = [x, y];                                                             §\C{// multiple assignment}§
    24862464\end{cfa}
    24872465Let $L_i$ for $i$ in $[0, n)$ represent each component of the flattened left side, $R_i$ represent each component of the flattened right side of a multiple assignment, and $R$ represent the right side of a mass assignment.
    … …  
    24902468For example, the following is invalid because the number of components on the left does not match the number of components on the right.
    24912469\begin{cfa}
    2492 [int, int] x, y, z;
    2493 [x, y] = z;   // multiple assignment, invalid 4 != 2
     2470[ int, int ] x, y, z;
     2471[ x, y ] = z;                                              §\C{// multiple assignment, invalid 4 != 2}§
    24942472\end{cfa}
    24952473Multiple assignment assigns $R_i$ to $L_i$ for each $i$.
    … …  
    25072485\begin{cfa}
    25082486int x = 10, y = 20;
    2509 [x, y] = [y, x];
     2487[ x, y ] = [ y, x ];
    25102488\end{cfa}
    25112489After executing this code, ©x© has the value ©20© and ©y© has the value ©10©.
    … …  
    25262504        int a, b;
    25272505        double c, d;
    2528         [void] f([int, int]);
    2529         f([c, a] = [b, d] = 1.5);  // assignments in parameter list
     2506        [ void ] f( [ int, int ] );
     2507        f( [ c, a ] = [ b, d ] = 1.5 );  // assignments in parameter list
    25302508\end{cfa}
    25312509The tuple expression begins with a mass assignment of ©1.5© into ©[b, d]©, which assigns ©1.5© into ©b©, which is truncated to ©1©, and ©1.5© into ©d©, producing the tuple ©[1, 1.5]© as a result.
    … …  
    25332511Finally, the tuple ©[1, 1]© is used as an expression in the call to ©f©.
    25342512
    2535 \subsection{Tuple Construction}
     2513
     2514\subsection{Construction}
     2515
    25362516Tuple construction and destruction follow the same rules and semantics as tuple assignment, except that in the case where there is no right side, the default constructor or destructor is called on each component of the tuple.
    25372517As constructors and destructors did not exist in previous versions of \CFA or in \KWC, this is a primary contribution of this thesis to the design of tuples.
    … …  
    25692549The 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.
    25702550
    2571 \section{Member-Access Tuple Expression}
     2551
     2552\subsection{Member-Access Expression}
    25722553\label{s:MemberAccessTuple}
    2573 It is possible to access multiple fields from a single expression using a \emph{Member-Access Tuple Expression}.
     2554
     2555Tuples may be used to select multiple fields of a record by field name.
    25742556The result is a single tuple-valued expression whose type is the tuple of the types of the members.
    25752557For example,
    25762558\begin{cfa}
    2577 struct S { int x; double y; char * z; } s;
     2559struct S { char x; int y; double z; } s;
    25782560s.[x, y, z];
    25792561\end{cfa}
    2580 Here, the type of ©s.[x, y, z]© is ©[int, double, char *]©.
    2581 A member tuple expression has the form ©a.[x, y, z];© where ©a© is an expression with type ©T©, where ©T© supports member access expressions, and ©x, y, z© are all members of ©T© with types ©T$_x$©, ©T$_y$©, and ©T$_z$© respectively.
    2582 Then the type of ©a.[x, y, z]© is ©[T_x, T_y, T_z]©.
    2583 
    2584 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 (\eg, rearrange components, drop components, duplicate components, etc.).
    2585 \begin{cfa}
    2586 [int, int, long, double] x;
    2587 void f(double, long);
    2588 
    2589 f(x.[0, 3]);          // f(x.0, x.3)
    2590 x.[0, 1] = x.[1, 0];  // [x.0, x.1] = [x.1, x.0]
    2591 [long, int, long] y = x.[2, 0, 2];
    2592 \end{cfa}
    2593 
    2594 It is possible for a member tuple expression to contain other member access expressions.
    2595 For example,
     2562Here, the type of ©s.[ x, y, z ]© is ©[ char, int, double ]©.
     2563A member tuple expression has the form \emph{e}©.[x, y, z];© where \emph{e} is an expression with type ©T©, where ©T© supports member access expressions, and ©x, y, z© are all members of ©T© with types ©T$_x$©, ©T$_y$©, and ©T$_z$© respectively.
     2564Then the type of \emph{e}©.[x, y, z]© is ©[T$_x$, T$_y$, T$_z$]©.
     2565
     2566A member-access tuple may be used anywhere a tuple can be used, \eg:
     2567\begin{cfa}
     2568s.[ y, z, x ] = [ 3, 3.2, 'x' ];                §\C{// equivalent to s.x = 'x', s.y = 3, s.z = 3.2}§
     2569f( s.[ y, z ] );                                                §\C{// equivalent to f( s.y, s.z )}§
     2570\end{cfa}
     2571Note, the fields appearing in a record-field tuple may be specified in any order;
     2572also, it is unnecessary to specify all the fields of a struct in a multiple record-field tuple.
     2573
     2574Since tuple-index expressions are a form of member-access expression, it is possible to use tuple-index expressions in conjunction with member-access expressions to restructure a tuple (\eg, rearrange components, drop components, duplicate components, etc.).
     2575\begin{cfa}
     2576[ int, int, long, double ] x;
     2577void f( double, long );
     2578
     2579f( x.[ 0, 3 ] );                                                §\C{// f( x.0, x.3 )}§
     2580x.[ 0, 1 ] = x.[ 1, 0 ];                                §\C{// [ x.0, x.1 ] = [ x.1, x.0 ]}§
     2581[ long, int, long ] y = x.[ 2, 0, 2 ];
     2582\end{cfa}
     2583
     2584It is possible for a member tuple expression to contain other member access expressions, \eg:
    25962585\begin{cfa}
    25972586struct A { double i; int j; };
    25982587struct B { int * k; short l; };
    25992588struct C { int x; A y; B z; } v;
    2600 v.[x, y.[i, j], z.k];
    2601 \end{cfa}
    2602 This expression is equivalent to ©[v.x, [v.y.i, v.y.j], v.z.k]©.
    2603 That is, the aggregate expression is effectively distributed across the tuple, which allows simple and easy access to multiple components in an aggregate, without repetition.
     2589v.[ x, y.[ i, j ], z.k ];
     2590\end{cfa}
     2591This expression is equivalent to ©[ v.x, [ v.y.i, v.y.j ], v.z.k ]©.
     2592That is, the aggregate expression is effectively distributed across the tuple allowing simple and easy access to multiple components in an aggregate without repetition.
    26042593It is guaranteed that the aggregate expression to the left of the ©.© in a member tuple expression is evaluated exactly once.
    2605 As such, it is safe to use member tuple expressions on the result of a side-effecting function.
    2606 \begin{cfa}
    2607 [int, float, double] f();
    2608 [double, float] x = f().[2, 1];
     2594As such, it is safe to use member tuple expressions on the result of a function with side-effects.
     2595\begin{cfa}
     2596[ int, float, double ] f();
     2597[ double, float ] x = f().[ 2, 1 ];             §\C{// f() called once}§
    26092598\end{cfa}
    26102599
    … …  
    26122601Since \CFA permits these tuple-access expressions using structures, unions, and tuples, \emph{member tuple expression} or \emph{field tuple expression} is more appropriate.
    26132602
    2614 It is possible to extend member-access expressions further.
    2615 Currently, 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.
    2616 In the interest of orthogonal design, \CFA could apply some meaning to the remaining combinations as well.
    2617 For example,
    2618 \begin{cfa}
    2619 struct S { int x, y; } s;
    2620 [S, S] z;
    2621 
    2622 s.x;  // access member
    2623 z.0;  // access component
    2624 
    2625 s.1;  // ???
    2626 z.y;  // ???
    2627 \end{cfa}
    2628 One possibility is for ©s.1© to select the second member of ©s©.
    2629 Under this interpretation, it becomes possible to not only access members of a struct by name, but also by position.
    2630 Likewise, it seems natural to open this mechanism to enumerations as well, wherein the left side would be a type, rather than an expression.
    2631 One benefit of this interpretation is familiarity, since it is extremely reminiscent of tuple-index expressions.
    2632 On 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.
    2633 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.
    2634 
    2635 As for ©z.y©, one interpretation is to extend the meaning of member tuple expressions.
    2636 That is, currently the tuple must occur as the member, \ie to the right of the dot.
    2637 Allowing 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.
    2638 In 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©.
    2639 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.
    2640 Perhaps 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.
    2641 The 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.
    2642 
    2643 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.
    2644 \begin{cfa}
    2645 struct { int x, y; };
    2646 [S, S] z;
    2647 z.[x, y];  // ???
    2648 // => [z.0, z.1].[x, y]
    2649 // => [z.0.x, z.0.y, z.1.x, z.1.y]
    2650 // or
    2651 // => [z.x, z.y]
    2652 // => [[z.0, z.1].x, [z.0, z.1].y]
    2653 // => [z.0.x, z.1.x, z.0.y, z.1.y]
    2654 \end{cfa}
    2655 Depending on exactly how the two tuples are combined, different results can be achieved.
    2656 As such, a specific ordering would need to be imposed to make this feature useful.
    2657 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.
    2658 
    2659 A second possibility is for \CFA to have named tuples, as they exist in Swift and D.
    2660 \begin{cfa}
    2661 typedef [int x, int y] Point2D;
    2662 Point2D p1, p2;
    2663 p1.x + p1.y + p2.x + p2.y;
    2664 p1.0 + p1.1 + p2.0 + p2.1;  // equivalent
    2665 \end{cfa}
    2666 In this simpler interpretation, a tuple type carries with it a list of possibly empty identifiers.
    2667 This approach fits naturally with the named return-value feature, and would likely go a long way towards implementing it.
    2668 
    2669 Ultimately, the first two extensions introduce complexity into the model, with relatively little perceived benefit, and so were dropped from consideration.
    2670 Named tuples are a potentially useful addition to the language, provided they can be parsed with a reasonable syntax.
    2671 
    2672 
    2673 \section{Casting}
     2603
     2604\subsection{Casting}
     2605
    26742606In C, the cast operator is used to explicitly convert between types.
    26752607In \CFA, the cast operator has a secondary use, which is type ascription, since it forces the expression resolution algorithm to choose the lowest cost conversion to the target type.
    … …  
    27252657That is, it is invalid to cast ©[int, int]© to ©[int, int, int]©.
    27262658
    2727 \section{Polymorphism}
     2659
     2660\subsection{Polymorphism}
     2661
    27282662Due to the implicit flattening and structuring conversions involved in argument passing, ©otype© and ©dtype© parameters are restricted to matching only with non-tuple types.
    27292663The integration of polymorphism, type assertions, and monomorphic specialization of tuple-assertions are a primary contribution of this thesis to the design of tuples.
    … …  
    27782712Until this point, it has been assumed that assertion arguments must match the parameter type exactly, modulo polymorphic specialization (\ie, no implicit conversions are applied to assertion arguments).
    27792713This decision presents a conflict with the flexibility of tuples.
    2780 \subsection{Assertion Inference}
     2714
     2715
     2716\subsubsection{Assertion Inference}
     2717
    27812718\begin{cfa}
    27822719int f([int, double], double);
    … …  
    29022839Unfortunately, C's syntax for subscripts precluded treating them as tuples.
    29032840The C subscript list has the form ©[i][j]...© and not ©[i, j, ...]©.
    2904 Therefore, there is no syntactic way for a routine returning multiple values to specify the different subscript values, \eg ©f[g()]© always means a single subscript value because there is only one set of brackets.
     2841Therefore, there is no syntactic way for a routine returning multiple values to specify the different subscript values, \eg ©f[ g() ]© always means a single subscript value because there is only one set of brackets.
    29052842Fixing this requires a major change to C because the syntactic form ©M[i, j, k]© already has a particular meaning: ©i, j, k© is a comma expression.
    29062843\end{rationale}
    … …  
    29512888
    29522889
    2953 \section{Mass Assignment}
     2890\subsection{Mass Assignment}
    29542891
    29552892\CFA permits assignment to several variables at once using mass assignment~\cite{CLU}.
    … …  
    29912928
    29922929
    2993 \section{Multiple Assignment}
     2930\subsection{Multiple Assignment}
    29942931
    29952932\CFA also supports the assignment of several values at once, known as multiple assignment~\cite{CLU,Galletly96}.
    … …  
    30322969
    30332970
    3034 \section{Cascade Assignment}
     2971\subsection{Cascade Assignment}
    30352972
    30362973As in C, \CFA mass and multiple assignments can be cascaded, producing cascade assignment.
    … …  
    30482985\end{cfa}
    30492986As in C, the rightmost assignment is performed first, \ie assignment parses right to left.
    3050 
    3051 
    3052 \section{Field Tuples}
    3053 
    3054 Tuples may be used to select multiple fields of a record by field name.
    3055 Its general form is:
    3056 \begin{cfa}
    3057 §\emph{expr}§ . [ §\emph{fieldlist}§ ]
    3058 §\emph{expr}§ -> [ §\emph{fieldlist}§ ]
    3059 \end{cfa}
    3060 \emph{expr} is any expression yielding a value of type record, \eg ©struct©, ©union©.
    3061 Each element of \emph{ fieldlist} is an element of the record specified by \emph{expr}.
    3062 A record-field tuple may be used anywhere a tuple can be used. An example of the use of a record-field tuple is
    3063 the following:
    3064 \begin{cfa}
    3065 struct s {
    3066         int f1, f2;
    3067         char f3;
    3068         double f4;
    3069 } v;
    3070 v.[ f3, f1, f2 ] = ['x', 11, 17 ];      §\C{// equivalent to v.f3 = 'x', v.f1 = 11, v.f2 = 17}§
    3071 f( v.[ f3, f1, f2 ] );                          §\C{// equivalent to f( v.f3, v.f1, v.f2 )}§
    3072 \end{cfa}
    3073 Note, the fields appearing in a record-field tuple may be specified in any order;
    3074 also, it is unnecessary to specify all the fields of a struct in a multiple record-field tuple.
    3075 
    3076 If a field of a ©struct© is itself another ©struct©, multiple fields of this subrecord can be specified using a nested record-field tuple, as in the following example:
    3077 \begin{cfa}
    3078 struct inner {
    3079         int f2, f3;
    3080 };
    3081 struct outer {
    3082         int f1;
    3083         struct inner i;
    3084         double f4;
    3085 } o;
    3086 
    3087 o.[ f1, i.[ f2, f3 ], f4 ] = [ 11, 12, 13, 3.14159 ];
    3088 \end{cfa}
    30892987
    30902988
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