source: doc/papers/general/Paper.tex @ fb11446e

aaron-thesisarm-ehcleanup-dtorsdeferred_resndemanglerjacob/cs343-translationjenkins-sandboxnew-astnew-ast-unique-exprnew-envno_listpersistent-indexerresolv-newwith_gc
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2
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28% Names used in the document.
29
30\newcommand{\CFAIcon}{\textsf{C}\raisebox{\depth}{\rotatebox{180}{\textsf{A}}}\xspace} % Cforall symbolic name
31\newcommand{\CFA}{\protect\CFAIcon}             % safe for section/caption
32\newcommand{\CFL}{\textrm{Cforall}\xspace}      % Cforall symbolic name
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40\newcommand{\Csharp}{C\raisebox{-0.7ex}{\Large$^\sharp$}\xspace} % C# symbolic name
41
42%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
43
44\newcommand{\Textbf}[2][red]{{\color{#1}{\textbf{#2}}}}
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46\newcommand{\TODO}[1]{} % TODO elided
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113}{%
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115}% cquote
116
117% CFA programming language, based on ANSI C (with some gcc additions)
118\lstdefinelanguage{CFA}[ANSI]{C}{
119        morekeywords={
120                _Alignas, _Alignof, __alignof, __alignof__, asm, __asm, __asm__, _At, __attribute,
121                __attribute__, auto, _Bool, catch, catchResume, choose, _Complex, __complex, __complex__,
122                __const, __const__, disable, dtype, enable, exception, __extension__, fallthrough, fallthru,
123                finally, forall, ftype, _Generic, _Imaginary, inline, __label__, lvalue, _Noreturn, one_t,
124                otype, restrict, _Static_assert, throw, throwResume, trait, try, ttype, typeof, __typeof,
125                __typeof__, virtual, with, zero_t},
126        morekeywords=[2]{
127                _Atomic, coroutine, is_coroutine, is_monitor, is_thread, monitor, mutex, nomutex, or,
128                resume, suspend, thread, _Thread_local, waitfor, when, yield},
129        moredirectives={defined,include_next}%
130}
131
132\lstset{
133language=CFA,
134columns=fullflexible,
135basicstyle=\linespread{0.9}\sf,                                                 % reduce line spacing and use sanserif font
136stringstyle=\tt,                                                                                % use typewriter font
137tabsize=5,                                                                                              % N space tabbing
138xleftmargin=\parindentlnth,                                                             % indent code to paragraph indentation
139%mathescape=true,                                                                               % LaTeX math escape in CFA code $...$
140escapechar=\$,                                                                                  % LaTeX escape in CFA code
141keepspaces=true,                                                                                %
142showstringspaces=false,                                                                 % do not show spaces with cup
143showlines=true,                                                                                 % show blank lines at end of code
144aboveskip=4pt,                                                                                  % spacing above/below code block
145belowskip=3pt,
146% replace/adjust listing characters that look bad in sanserif
147literate={-}{\makebox[1ex][c]{\raisebox{0.4ex}{\rule{0.8ex}{0.1ex}}}}1 {^}{\raisebox{0.6ex}{$\scriptscriptstyle\land\,$}}1
148        {~}{\raisebox{0.3ex}{$\scriptstyle\sim\,$}}1 {@}{\small{@}}1 % {`}{\ttfamily\upshape\hspace*{-0.1ex}`}1
149        {<-}{$\leftarrow$}2 {=>}{$\Rightarrow$}2 {->}{\makebox[1ex][c]{\raisebox{0.4ex}{\rule{0.8ex}{0.075ex}}}\kern-0.2ex\textgreater}2,
150moredelim=**[is][\color{red}]{`}{`},
151}% lstset
152
153% inline code @...@
154\lstMakeShortInline@%
155
156\lstnewenvironment{cfa}[1][]
157{\lstset{#1}}
158{}
159\lstnewenvironment{C++}[1][]                            % use C++ style
160{\lstset{language=C++,moredelim=**[is][\protect\color{red}]{`}{`},#1}\lstset{#1}}
161{}
162
163
164\title{\protect\CFA : Adding Modern Programming Language Features to C}
165
166\author{Aaron Moss, Robert Schluntz, Peter Buhr}
167% \email{a3moss@uwaterloo.ca}
168% \email{rschlunt@uwaterloo.ca}
169% \email{pabuhr@uwaterloo.ca}
170% \affiliation{%
171%       \institution{University of Waterloo}
172%       \department{David R. Cheriton School of Computer Science}
173%       \streetaddress{Davis Centre, University of Waterloo}
174%       \city{Waterloo}
175%       \state{ON}
176%       \postcode{N2L 3G1}
177%       \country{Canada}
178% }
179
180%\terms{generic, tuple, variadic, types}
181%\keywords{generic types, tuple types, variadic types, polymorphic functions, C, Cforall}
182
183\begin{document}
184\maketitle
185
186
187\begin{abstract}
188The C programming language is a foundational technology for modern computing with millions of lines of code implementing everything from commercial operating-systems to hobby projects.
189This installation base and the programmers producing it represent a massive software-engineering investment spanning decades and likely to continue for decades more.
190Nevertheless, C, first standardized over thirty years ago, lacks many features that make programming in more modern languages safer and more productive.
191
192The goal of the \CFA project is to create an extension of C that provides modern safety and productivity features while still ensuring strong backwards compatibility with C and its programmers.
193Prior projects have attempted similar goals but failed to honour C programming-style; for instance, adding object-oriented or functional programming with garbage collection is a non-starter for many C developers.
194Specifically, \CFA is designed to have an orthogonal feature-set based closely on the C programming paradigm, so that \CFA features can be added \emph{incrementally} to existing C code-bases, and C programmers can learn \CFA extensions on an as-needed basis, preserving investment in existing code and programmers.
195This paper presents a quick tour of \CFA features showing how their design avoids shortcomings of similar features in C and other C-like languages.
196Finally, experimental results are presented to validate several of the new features.
197\end{abstract}
198
199
200\section{Introduction}
201
202The C programming language is a foundational technology for modern computing with millions of lines of code implementing everything from commercial operating-systems to hobby projects.
203This installation base and the programmers producing it represent a massive software-engineering investment spanning decades and likely to continue for decades more.
204The TIOBE~\cite{TIOBE} ranks the top 5 most \emph{popular} programming languages as: Java 15\%, \Textbf{C 12\%}, \Textbf{\CC 5.5\%}, Python 5\%, \Csharp 4.5\% = 42\%, where the next 50 languages are less than 4\% each with a long tail.
205The top 3 rankings over the past 30 years are:
206\begin{center}
207\setlength{\tabcolsep}{10pt}
208\lstDeleteShortInline@%
209\begin{tabular}{@{}rccccccc@{}}
210                & 2018  & 2013  & 2008  & 2003  & 1998  & 1993  & 1988  \\ \hline
211Java    & 1             & 2             & 1             & 1             & 18    & -             & -             \\
212\Textbf{C}& \Textbf{2} & \Textbf{1} & \Textbf{2} & \Textbf{2} & \Textbf{1} & \Textbf{1} & \Textbf{1} \\
213\CC             & 3             & 4             & 3             & 3             & 2             & 2             & 5             \\
214\end{tabular}
215\lstMakeShortInline@%
216\end{center}
217Love it or hate it, C is extremely popular, highly used, and one of the few systems languages.
218In many cases, \CC is often used solely as a better C.
219Nonetheless, C, first standardized over thirty years ago, lacks many features that make programming in more modern languages safer and more productive.
220
221\CFA (pronounced ``C-for-all'', and written \CFA or Cforall) is an evolutionary extension of the C programming language that aims to add modern language features to C while maintaining both source compatibility with C and a familiar programming model for programmers.
222The four key design goals for \CFA~\cite{Bilson03} are:
223(1) The behaviour of standard C code must remain the same when translated by a \CFA compiler as when translated by a C compiler;
224(2) Standard C code must be as fast and as small when translated by a \CFA compiler as when translated by a C compiler;
225(3) \CFA code must be at least as portable as standard C code;
226(4) Extensions introduced by \CFA must be translated in the most efficient way possible.
227These goals ensure existing C code-bases can be converted to \CFA incrementally with minimal effort, and C programmers can productively generate \CFA code without training beyond the features being used.
228\CC is used similarly, but has the disadvantages of multiple legacy design-choices that cannot be updated and active divergence of the language model from C, requiring significant effort and training to incrementally add \CC to a C-based project.
229
230\CFA is currently implemented as a source-to-source translator from \CFA to the gcc-dialect of C~\cite{GCCExtensions}, allowing it to leverage the portability and code optimizations provided by gcc, meeting goals (1)--(3).
231Ultimately, a compiler is necessary for advanced features and optimal performance.
232All of the features discussed in this paper are working, unless a feature states it is a future feature for completion.
233
234Finally, it is impossible to describe a programming language without usages before definitions.
235Therefore, syntax and semantics appear before explanations;
236hence, patience is necessary until details are presented.
237
238
239\section{Polymorphic Functions}
240
241\CFA introduces both ad-hoc and parametric polymorphism to C, with a design originally formalized by Ditchfield~\cite{Ditchfield92}, and first implemented by Bilson~\cite{Bilson03}.
242Shortcomings are identified in existing approaches to generic and variadic data types in C-like languages and how these shortcomings are avoided in \CFA.
243Specifically, the solution is both reusable and type-checked, as well as conforming to the design goals of \CFA with ergonomic use of existing C abstractions.
244The new constructs are empirically compared with C and \CC approaches via performance experiments in Section~\ref{sec:eval}.
245
246
247\subsection{Name Overloading}
248\label{s:NameOverloading}
249
250\begin{quote}
251There are only two hard things in Computer Science: cache invalidation and \emph{naming things} -- Phil Karlton
252\end{quote}
253C already has a limited form of ad-hoc polymorphism in the form of its basic arithmetic operators, which apply to a variety of different types using identical syntax.
254\CFA extends the built-in operator overloading by allowing users to define overloads for any function, not just operators, and even any variable;
255Section~\ref{sec:libraries} includes a number of examples of how this overloading simplifies \CFA programming relative to C.
256Code generation for these overloaded functions and variables is implemented by the usual approach of mangling the identifier names to include a representation of their type, while \CFA decides which overload to apply based on the same ``usual arithmetic conversions'' used in C to disambiguate operator overloads.
257As an example:
258
259\begin{cfa}
260int max = 2147483647;                                           $\C[3.75in]{// (1)}$
261double max = 1.7976931348623157E+308;   $\C{// (2)}$
262int max( int a, int b ) { return a < b ? b : a; }  $\C{// (3)}$
263double max( double a, double b ) { return a < b ? b : a; }  $\C{// (4)}\CRT$
264max( 7, -max );                                                         $\C{// uses (3) and (1), by matching int from constant 7}$
265max( max, 3.14 );                                                       $\C{// uses (4) and (2), by matching double from constant 3.14}$
266max( max, -max );                                                       $\C{// ERROR: ambiguous}$
267int m = max( max, -max );                                       $\C{// uses (3) and (1) twice, by matching return type}$
268\end{cfa}
269\CFA maximizes the ability to reuse names to aggressively address the naming problem.
270In some cases, hundreds of names can be reduced to tens, resulting in a significant cognitive reduction.
271In the above, the name @max@ has a consistent meaning, and a programmer only needs to remember the single concept: maximum.
272To prevent significant ambiguities, \CFA uses the return type in selecting overloads, \eg in the assignment to @m@, the compiler use @m@'s type to unambiguously select the most appropriate call to function @max@ (as does Ada).
273As is shown later, there are a number of situations where \CFA takes advantage of available type information to disambiguate, where other programming languages generate ambiguities.
274
275\Celeven added @_Generic@ expressions, which is used in preprocessor macros to provide a form of ad-hoc polymorphism;
276however, this polymorphism is both functionally and ergonomically inferior to \CFA name overloading.
277The macro wrapping the generic expression imposes some limitations;
278\eg, it cannot implement the example above, because the variables @max@ are ambiguous with the functions @max@.
279Ergonomic limitations of @_Generic@ include the necessity to put a fixed list of supported types in a single place and manually dispatch to appropriate overloads, as well as possible namespace pollution from the dispatch functions, which must all have distinct names.
280For backwards compatibility, \CFA supports @_Generic@ expressions, but it is an unnecessary mechanism. \TODO{actually implement that}
281
282% http://fanf.livejournal.com/144696.html
283% http://www.robertgamble.net/2012/01/c11-generic-selections.html
284% https://abissell.com/2014/01/16/c11s-_generic-keyword-macro-applications-and-performance-impacts/
285
286
287\subsection{\texorpdfstring{\LstKeywordStyle{forall} Functions}{forall Functions}}
288\label{sec:poly-fns}
289
290The signature feature of \CFA is parametric-polymorphic functions~\cite{forceone:impl,Cormack90,Duggan96} with functions generalized using a @forall@ clause (giving the language its name):
291\begin{cfa}
292`forall( otype T )` T identity( T val ) { return val; }
293int forty_two = identity( 42 );                         $\C{// T is bound to int, forty\_two == 42}$
294\end{cfa}
295This @identity@ function can be applied to any complete \newterm{object type} (or @otype@).
296The type variable @T@ is transformed into a set of additional implicit parameters encoding sufficient information about @T@ to create and return a variable of that type.
297The \CFA implementation passes the size and alignment of the type represented by an @otype@ parameter, as well as an assignment operator, constructor, copy constructor and destructor.
298If this extra information is not needed, \eg for a pointer, the type parameter can be declared as a \newterm{data type} (or @dtype@).
299
300In \CFA, the polymorphic runtime-cost is spread over each polymorphic call, because more arguments are passed to polymorphic functions;
301the experiments in Section~\ref{sec:eval} show this overhead is similar to \CC virtual-function calls.
302A design advantage is that, unlike \CC template-functions, \CFA polymorphic-functions are compatible with C \emph{separate compilation}, preventing compilation and code bloat.
303
304Since bare polymorphic-types provide a restricted set of available operations, \CFA provides a \newterm{type assertion}~\cite[pp.~37-44]{Alphard} mechanism to provide further type information, where type assertions may be variable or function declarations that depend on a polymorphic type-variable.
305For example, the function @twice@ can be defined using the \CFA syntax for operator overloading:
306\begin{cfa}
307forall( otype T `| { T ?+?(T, T); }` ) T twice( T x ) { return x `+` x; }       $\C{// ? denotes operands}$
308int val = twice( twice( 3.7 ) );
309\end{cfa}
310which works for any type @T@ with a matching addition operator.
311The polymorphism is achieved by creating a wrapper function for calling @+@ with @T@ bound to @double@, then passing this function to the first call of @twice@.
312There is now the option of using the same @twice@ and converting the result to @int@ on assignment, or creating another @twice@ with type parameter @T@ bound to @int@ because \CFA uses the return type~\cite{Cormack81,Baker82,Ada} in its type analysis.
313The first approach has a late conversion from @double@ to @int@ on the final assignment, while the second has an eager conversion to @int@.
314\CFA minimizes the number of conversions and their potential to lose information, so it selects the first approach, which corresponds with C-programmer intuition.
315
316Crucial to the design of a new programming language are the libraries to access thousands of external software features.
317Like \CC, \CFA inherits a massive compatible library-base, where other programming languages must rewrite or provide fragile inter-language communication with C.
318A simple example is leveraging the existing type-unsafe (@void *@) C @bsearch@ to binary search a sorted float array:
319\begin{cfa}
320void * bsearch( const void * key, const void * base, size_t nmemb, size_t size,
321                                         int (* compar)( const void *, const void * ));
322int comp( const void * t1, const void * t2 ) {
323         return *(double *)t1 < *(double *)t2 ? -1 : *(double *)t2 < *(double *)t1 ? 1 : 0;
324}
325double key = 5.0, vals[10] = { /* 10 sorted float values */ };
326double * val = (double *)bsearch( &key, vals, 10, sizeof(vals[0]), comp );      $\C{// search sorted array}$
327\end{cfa}
328which can be augmented simply with a generalized, type-safe, \CFA-overloaded wrappers:
329\begin{cfa}
330forall( otype T | { int ?<?( T, T ); } ) T * bsearch( T key, const T * arr, size_t size ) {
331        int comp( const void * t1, const void * t2 ) { /* as above with double changed to T */ }
332        return (T *)bsearch( &key, arr, size, sizeof(T), comp );
333}
334forall( otype T | { int ?<?( T, T ); } ) unsigned int bsearch( T key, const T * arr, size_t size ) {
335        T * result = bsearch( key, arr, size ); $\C{// call first version}$
336        return result ? result - arr : size;    $\C{// pointer subtraction includes sizeof(T)}$
337}
338double * val = bsearch( 5.0, vals, 10 );        $\C{// selection based on return type}$
339int posn = bsearch( 5.0, vals, 10 );
340\end{cfa}
341The nested function @comp@ provides the hidden interface from typed \CFA to untyped (@void *@) C, plus the cast of the result.
342Providing a hidden @comp@ function in \CC is awkward as lambdas do not use C calling-conventions and template declarations cannot appear at block scope.
343As well, an alternate kind of return is made available: position versus pointer to found element.
344\CC's type-system cannot disambiguate between the two versions of @bsearch@ because it does not use the return type in overload resolution, nor can \CC separately compile a template @bsearch@.
345
346\CFA has replacement libraries condensing hundreds of existing C functions into tens of \CFA overloaded functions, all without rewriting the actual computations (see Section~\ref{sec:libraries}).
347For example, it is possible to write a type-safe \CFA wrapper @malloc@ based on the C @malloc@:
348\begin{cfa}
349forall( dtype T | sized(T) ) T * malloc( void ) { return (T *)malloc( sizeof(T) ); }
350int * ip = malloc();                                            $\C{// select type and size from left-hand side}$
351double * dp = malloc();
352struct S {...} * sp = malloc();
353\end{cfa}
354where the return type supplies the type/size of the allocation, which is impossible in most type systems.
355
356Call-site inferencing and nested functions provide a localized form of inheritance.
357For example, the \CFA @qsort@ only sorts in ascending order using @<@.
358However, it is trivial to locally change this behaviour:
359\begin{cfa}
360forall( otype T | { int ?<?( T, T ); } ) void qsort( const T * arr, size_t size ) { /* use C qsort */ }
361{
362        int ?<?( double x, double y ) { return x `>` y; }       $\C{// locally override behaviour}$
363        qsort( vals, size );                                    $\C{// descending sort}$
364}
365\end{cfa}
366Within the block, the nested version of @?<?@ performs @?>?@ and this local version overrides the built-in @?<?@ so it is passed to @qsort@.
367Hence, programmers can easily form local environments, adding and modifying appropriate functions, to maximize reuse of other existing functions and types.
368
369
370\subsection{Traits}
371
372\CFA provides \newterm{traits} to name a group of type assertions, where the trait name allows specifying the same set of assertions in multiple locations, preventing repetition mistakes at each function declaration:
373\begin{cfa}
374trait `summable`( otype T ) {
375        void ?{}( T *, zero_t );                                $\C{// constructor from 0 literal}$
376        T ?+?( T, T );                                                  $\C{// assortment of additions}$
377        T ?+=?( T *, T );
378        T ++?( T * );
379        T ?++( T * ); };
380
381forall( otype T `| summable( T )` ) T sum( T a[$\,$], size_t size ) {  // use trait
382        `T` total = { `0` };                                    $\C{// instantiate T from 0 by calling its constructor}$
383        for ( unsigned int i = 0; i < size; i += 1 ) total `+=` a[i]; $\C{// select appropriate +}$
384        return total; }
385\end{cfa}
386
387In fact, the set of @summable@ trait operators is incomplete, as it is missing assignment for type @T@, but @otype@ is syntactic sugar for the following implicit trait:
388\begin{cfa}
389trait otype( dtype T | sized(T) ) {  // sized is a pseudo-trait for types with known size and alignment
390        void ?{}( T & );                                                $\C{// default constructor}$
391        void ?{}( T &, T );                                             $\C{// copy constructor}$
392        void ?=?( T &, T );                                             $\C{// assignment operator}$
393        void ^?{}( T & ); };                                    $\C{// destructor}$
394\end{cfa}
395Given the information provided for an @otype@, variables of polymorphic type can be treated as if they were a complete type: stack-allocatable, default or copy-initialized, assigned, and deleted.
396
397In summation, the \CFA type-system uses \newterm{nominal typing} for concrete types, matching with the C type-system, and \newterm{structural typing} for polymorphic types.
398Hence, trait names play no part in type equivalence;
399the names are simply macros for a list of polymorphic assertions, which are expanded at usage sites.
400Nevertheless, trait names form a logical subtype-hierarchy with @dtype@ at the top, where traits often contain overlapping assertions, \eg operator @+@.
401Traits are used like interfaces in Java or abstract base-classes in \CC, but without the nominal inheritance-relationships.
402Instead, each polymorphic function (or generic type) defines the structural type needed for its execution (polymorphic type-key), and this key is fulfilled at each call site from the lexical environment, which is similar to Go~\cite{Go} interfaces.
403Hence, new lexical scopes and nested functions are used extensively to create local subtypes, as in the @qsort@ example, without having to manage a nominal-inheritance hierarchy.
404(Nominal inheritance can be approximated with traits using marker variables or functions, as is done in Go.)
405
406% Nominal inheritance can be simulated with traits using marker variables or functions:
407% \begin{cfa}
408% trait nominal(otype T) {
409%     T is_nominal;
410% };
411% int is_nominal;                                                               $\C{// int now satisfies the nominal trait}$
412% \end{cfa}
413%
414% Traits, however, are significantly more powerful than nominal-inheritance interfaces; most notably, traits may be used to declare a relationship \emph{among} multiple types, a property that may be difficult or impossible to represent in nominal-inheritance type systems:
415% \begin{cfa}
416% trait pointer_like(otype Ptr, otype El) {
417%     lvalue El *?(Ptr);                                                $\C{// Ptr can be dereferenced into a modifiable value of type El}$
418% }
419% struct list {
420%     int value;
421%     list * next;                                                              $\C{// may omit "struct" on type names as in \CC}$
422% };
423% typedef list * list_iterator;
424%
425% lvalue int *?( list_iterator it ) { return it->value; }
426% \end{cfa}
427% In the 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. Given 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 an @int@, while @(*it).value = 42;@ interprets @*it@ as a @list@).
428% While 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.
429
430
431\section{Generic Types}
432
433A significant shortcoming of standard C is the lack of reusable type-safe abstractions for generic data structures and algorithms.
434Broadly speaking, there are three approaches to implement abstract data-structures in C.
435One approach is to write bespoke data-structures for each context in which they are needed.
436While this approach is flexible and supports integration with the C type-checker and tooling, it is also tedious and error-prone, especially for more complex data structures.
437A second approach is to use @void *@--based polymorphism, \eg the C standard-library functions @bsearch@ and @qsort@, which allow reuse of code with common functionality.
438However, basing all polymorphism on @void *@ eliminates the type-checker's ability to ensure that argument types are properly matched, often requiring a number of extra function parameters, pointer indirection, and dynamic allocation that is not otherwise needed.
439A third approach to generic code is to use preprocessor macros, which does allow the generated code to be both generic and type-checked, but errors may be difficult to interpret.
440Furthermore, writing and using preprocessor macros is unnatural and inflexible.
441
442\CC, Java, and other languages use \newterm{generic types} to produce type-safe abstract data-types.
443\CFA also implements generic types that integrate efficiently and naturally with the existing polymorphic functions, while retaining backwards compatibility with C and providing separate compilation.
444However, for known concrete parameters, the generic-type definition can be inlined, like \CC templates.
445
446A generic type can be declared by placing a @forall@ specifier on a @struct@ or @union@ declaration, and instantiated using a parenthesized list of types after the type name:
447\begin{cfa}
448forall( otype R, otype S ) struct pair {
449        R first;
450        S second;
451};
452forall( otype T ) T value( pair( const char *, T ) p ) { return p.second; } $\C{// dynamic}$
453forall( dtype F, otype T ) T value( pair( F *, T * ) p ) { return *p.second; } $\C{// dtype-static (concrete)}$
454
455pair( const char *, int ) p = { "magic", 42 }; $\C{// concrete}$
456int i = value( p );
457pair( void *, int * ) q = { 0, &p.second }; $\C{// concrete}$
458i = value( q );
459double d = 1.0;
460pair( double *, double * ) r = { &d, &d }; $\C{// concrete}$
461d = value( r );
462\end{cfa}
463
464\CFA classifies generic types as either \newterm{concrete} or \newterm{dynamic}.
465Concrete types have a fixed memory layout regardless of type parameters, while dynamic types vary in memory layout depending on their type parameters.
466A \newterm{dtype-static} type has polymorphic parameters but is still concrete.
467Polymorphic pointers are an example of dtype-static types, \eg @forall(dtype T) T *@ is a polymorphic type, but for any @T@, @T *@  is a fixed-sized pointer, and therefore, can be represented by a @void *@ in code generation.
468
469\CFA generic types also allow checked argument-constraints.
470For example, the following declaration of a sorted set-type ensures the set key supports equality and relational comparison:
471\begin{cfa}
472forall( otype Key | { _Bool ?==?(Key, Key); _Bool ?<?(Key, Key); } ) struct sorted_set;
473\end{cfa}
474
475
476\subsection{Concrete Generic-Types}
477
478The \CFA translator template-expands concrete generic-types into new structure types, affording maximal inlining.
479To enable inter-operation among equivalent instantiations of a generic type, the translator saves the set of instantiations currently in scope and reuses the generated structure declarations where appropriate.
480A function declaration that accepts or returns a concrete generic-type produces a declaration for the instantiated structure in the same scope, which all callers may reuse.
481For example, the concrete instantiation for @pair( const char *, int )@ is:
482\begin{cfa}
483struct _pair_conc0 {
484        const char * first;
485        int second;
486};
487\end{cfa}
488
489A concrete generic-type with dtype-static parameters is also expanded to a structure type, but this type is used for all matching instantiations.
490In the above example, the @pair( F *, T * )@ parameter to @value@ is such a type; its expansion is below and it is used as the type of the variables @q@ and @r@ as well, with casts for member access where appropriate:
491\begin{cfa}
492struct _pair_conc1 {
493        void * first;
494        void * second;
495};
496\end{cfa}
497
498
499\subsection{Dynamic Generic-Types}
500
501Though \CFA implements concrete generic-types efficiently, it also has a fully general system for dynamic generic types.
502As mentioned in Section~\ref{sec:poly-fns}, @otype@ function parameters (in fact all @sized@ polymorphic parameters) come with implicit size and alignment parameters provided by the caller.
503Dynamic generic-types also have an \newterm{offset array} containing structure-member offsets.
504A dynamic generic-@union@ needs no such offset array, as all members are at offset 0, but size and alignment are still necessary.
505Access to members of a dynamic structure is provided at runtime via base-displacement addressing with the structure pointer and the member offset (similar to the @offsetof@ macro), moving a compile-time offset calculation to runtime.
506
507The offset arrays are statically generated where possible.
508If a dynamic generic-type is declared to be passed or returned by value from a polymorphic function, the translator can safely assume the generic type is complete (\ie has a known layout) at any call-site, and the offset array is passed from the caller;
509if the generic type is concrete at the call site, the elements of this offset array can even be statically generated using the C @offsetof@ macro.
510As an example, the body of the second @value@ function is implemented like this:
511\begin{cfa}
512_assign_T(_retval, p + _offsetof_pair[1]); $\C{// return *p.second}$
513\end{cfa}
514@_assign_T@ is passed in as an implicit parameter from @otype T@, and takes two @T*@ (@void*@ in the generated code), a destination and a source; @_retval@ is the pointer to a caller-allocated buffer for the return value, the usual \CFA method to handle dynamically-sized return types.
515@_offsetof_pair@ is the offset array passed into @value@; this array is generated at the call site as:
516\begin{cfa}
517size_t _offsetof_pair[] = { offsetof(_pair_conc0, first), offsetof(_pair_conc0, second) }
518\end{cfa}
519
520In some cases the offset arrays cannot be statically generated.
521For instance, modularity is generally provided in C by including an opaque forward-declaration of a structure and associated accessor and mutator functions in a header file, with the actual implementations in a separately-compiled @.c@ file.
522\CFA supports this pattern for generic types, but the caller does not know the actual layout or size of the dynamic generic-type, and only holds it by a pointer.
523The \CFA translator automatically generates \newterm{layout functions} for cases where the size, alignment, and offset array of a generic struct cannot be passed into a function from that function's caller.
524These layout functions take as arguments pointers to size and alignment variables and a caller-allocated array of member offsets, as well as the size and alignment of all @sized@ parameters to the generic structure (un@sized@ parameters are forbidden from being used in a context that affects layout).
525Results of these layout functions are cached so that they are only computed once per type per function. %, as in the example below for @pair@.
526Layout functions also allow generic types to be used in a function definition without reflecting them in the function signature.
527For instance, a function that strips duplicate values from an unsorted @vector(T)@ would likely have a pointer to the vector as its only explicit parameter, but use some sort of @set(T)@ internally to test for duplicate values.
528This function could acquire the layout for @set(T)@ by calling its layout function with the layout of @T@ implicitly passed into the function.
529
530Whether a type is concrete, dtype-static, or dynamic is decided solely on the @forall@'s type parameters.
531This design allows opaque forward declarations of generic types, \eg @forall(otype T)@ @struct Box@ -- like in C, all uses of @Box(T)@ can be separately compiled, and callers from other translation units know the proper calling conventions to use.
532If the definition of a structure type is included in deciding whether a generic type is dynamic or concrete, some further types may be recognized as dtype-static (\eg @forall(otype T)@ @struct unique_ptr { T * p }@ does not depend on @T@ for its layout, but the existence of an @otype@ parameter means that it \emph{could}.), but preserving separate compilation (and the associated C compatibility) in the existing design is judged to be an appropriate trade-off.
533
534
535\subsection{Applications}
536\label{sec:generic-apps}
537
538The reuse of dtype-static structure instantiations enables useful programming patterns at zero runtime cost.
539The most important such pattern is using @forall(dtype T) T *@ as a type-checked replacement for @void *@, \eg creating a lexicographic comparison for pairs of pointers used by @bsearch@ or @qsort@:
540\begin{cfa}
541forall(dtype T) int lexcmp( pair( T *, T * ) * a, pair( T *, T * ) * b, int (* cmp)( T *, T * ) ) {
542        return cmp( a->first, b->first ) ? : cmp( a->second, b->second );
543}
544\end{cfa}
545Since @pair(T *, T * )@ is a concrete type, there are no implicit parameters passed to @lexcmp@, so the generated code is identical to a function written in standard C using @void *@, yet the \CFA version is type-checked to ensure the fields of both pairs and the arguments to the comparison function match in type.
546
547Another useful pattern enabled by reused dtype-static type instantiations is zero-cost \newterm{tag-structures}.
548Sometimes information is only used for type-checking and can be omitted at runtime, \eg:
549\begin{cfa}
550forall(dtype Unit) struct scalar { unsigned long value; };
551struct metres {};
552struct litres {};
553
554forall(dtype U) scalar(U) ?+?( scalar(U) a, scalar(U) b ) {
555        return (scalar(U)){ a.value + b.value };
556}
557scalar(metres) half_marathon = { 21093 };
558scalar(litres) swimming_pool = { 2500000 };
559scalar(metres) marathon = half_marathon + half_marathon;
560scalar(litres) two_pools = swimming_pool + swimming_pool;
561marathon + swimming_pool;                                       $\C{// compilation ERROR}$
562\end{cfa}
563@scalar@ is a dtype-static type, so all uses have a single structure definition, containing @unsigned long@, and can share the same implementations of common functions like @?+?@.
564These implementations may even be separately compiled, unlike \CC template functions.
565However, the \CFA type-checker ensures matching types are used by all calls to @?+?@, preventing nonsensical computations like adding a length to a volume.
566
567
568\section{Tuples}
569\label{sec:tuples}
570
571In many languages, functions can return at most one value;
572however, many operations have multiple outcomes, some exceptional.
573Consider C's @div@ and @remquo@ functions, which return the quotient and remainder for a division of integer and float values, respectively.
574\begin{cfa}
575typedef struct { int quo, rem; } div_t;         $\C{// from include stdlib.h}$
576div_t div( int num, int den );
577double remquo( double num, double den, int * quo );
578div_t qr = div( 13, 5 );                                        $\C{// return quotient/remainder aggregate}$
579int q;
580double r = remquo( 13.5, 5.2, &q );                     $\C{// return remainder, alias quotient}$
581\end{cfa}
582@div@ aggregates the quotient/remainder in a structure, while @remquo@ aliases a parameter to an argument.
583Both approaches are awkward.
584Alternatively, a programming language can directly support returning multiple values, \eg in \CFA:
585\begin{cfa}
586[ int, int ] div( int num, int den );           $\C{// return two integers}$
587[ double, double ] div( double num, double den ); $\C{// return two doubles}$
588int q, r;                                                                       $\C{// overloaded variable names}$
589double q, r;
590[ q, r ] = div( 13, 5 );                                        $\C{// select appropriate div and q, r}$
591[ q, r ] = div( 13.5, 5.2 );                            $\C{// assign into tuple}$
592\end{cfa}
593Clearly, this approach is straightforward to understand and use;
594therefore, why do few programming languages support this obvious feature or provide it awkwardly?
595The answer is that there are complex consequences that cascade through multiple aspects of the language, especially the type-system.
596This section show these consequences and how \CFA handles them.
597
598
599\subsection{Tuple Expressions}
600
601The addition of multiple-return-value functions (MRVF) are \emph{useless} without a syntax for accepting multiple values at the call-site.
602The simplest mechanism for capturing the return values is variable assignment, allowing the values to be retrieved directly.
603As such, \CFA allows assigning multiple values from a function into multiple variables, using a square-bracketed list of lvalue expressions (as above), called a \newterm{tuple}.
604
605However, functions also use \newterm{composition} (nested calls), with the direct consequence that MRVFs must also support composition to be orthogonal with single-returning-value functions (SRVF), \eg:
606\begin{cfa}
607printf( "%d %d\n", div( 13, 5 ) );                      $\C{// return values seperated into arguments}$
608\end{cfa}
609Here, the values returned by @div@ are composed with the call to @printf@ by flattening the tuple into separate arguments.
610However, the \CFA type-system must support significantly more complex composition:
611\begin{cfa}
612[ int, int ] foo$\(_1\)$( int );                        $\C{// overloaded foo functions}$
613[ double ] foo$\(_2\)$( int );
614void bar( int, double, double );
615`bar`( foo( 3 ), foo( 3 ) );
616\end{cfa}
617The type-resolver only has the tuple return-types to resolve the call to @bar@ as the @foo@ parameters are identical, which involves unifying the possible @foo@ functions with @bar@'s parameter list.
618No combination of @foo@s are an exact match with @bar@'s parameters, so the resolver applies C conversions.
619The minimal cost is @bar( foo@$_1$@( 3 ), foo@$_2$@( 3 ) )@, giving (@int@, {\color{ForestGreen}@int@}, @double@) to (@int@, {\color{ForestGreen}@double@}, @double@) with one {\color{ForestGreen}safe} (widening) conversion from @int@ to @double@ versus ({\color{red}@double@}, {\color{ForestGreen}@int@}, {\color{ForestGreen}@int@}) to ({\color{red}@int@}, {\color{ForestGreen}@double@}, {\color{ForestGreen}@double@}) with one {\color{red}unsafe} (narrowing) conversion from @double@ to @int@ and two safe conversions.
620
621
622\subsection{Tuple Variables}
623
624An important observation from function composition is that new variable names are not required to initialize parameters from an MRVF.
625\CFA also allows declaration of tuple variables that can be initialized from an MRVF, since it can be awkward to declare multiple variables of different types, \eg:
626\begin{cfa}
627[ int, int ] qr = div( 13, 5 );                         $\C{// tuple-variable declaration and initialization}$
628[ double, double ] qr = div( 13.5, 5.2 );
629\end{cfa}
630where the tuple variable-name serves the same purpose as the parameter name(s).
631Tuple variables can be composed of any types, except for array types, since array sizes are generally unknown in C.
632
633One way to access the tuple-variable components is with assignment or composition:
634\begin{cfa}
635[ q, r ] = qr;                                                          $\C{// access tuple-variable components}$
636printf( "%d %d\n", qr );
637\end{cfa}
638\CFA also supports \newterm{tuple indexing} to access single components of a tuple expression:
639\begin{cfa}
640[int, int] * p = &qr;                                           $\C{// tuple pointer}$
641int rem = qr`.1`;                                                       $\C{// access remainder}$
642int quo = div( 13, 5 )`.0`;                                     $\C{// access quotient}$
643p`->0` = 5;                                                                     $\C{// change quotient}$
644bar( qr`.1`, qr );                                                      $\C{// pass remainder and quotient/remainder}$
645rem = [div( 13, 5 ), 42]`.0.1`;                         $\C{// access 2nd component of 1st component of tuple expression}$
646\end{cfa}
647
648
649\subsection{Flattening and Restructuring}
650
651In function call contexts, tuples support implicit flattening and restructuring conversions.
652Tuple flattening recursively expands a tuple into the list of its basic components.
653Tuple structuring packages a list of expressions into a value of tuple type, \eg:
654%\lstDeleteShortInline@%
655%\par\smallskip
656%\begin{tabular}{@{}l@{\hspace{1.5\parindent}}||@{\hspace{1.5\parindent}}l@{}}
657\begin{cfa}
658int f( int, int );
659int g( [int, int] );
660int h( int, [int, int] );
661[int, int] x;
662int y;
663f( x );                 $\C{// flatten}$
664g( y, 10 );             $\C{// structure}$
665h( x, y );              $\C{// flatten and structure}$
666\end{cfa}
667%\end{cfa}
668%&
669%\begin{cfa}
670%\end{tabular}
671%\smallskip\par\noindent
672%\lstMakeShortInline@%
673In the call to @f@, @x@ is implicitly flattened so the components of @x@ are passed as the two arguments.
674In the call to @g@, the values @y@ and @10@ are structured into a single argument of type @[int, int]@ to match the parameter type of @g@.
675Finally, in the call to @h@, @x@ is flattened to yield an argument list of length 3, of which the first component of @x@ is passed as the first parameter of @h@, and the second component of @x@ and @y@ are structured into the second argument of type @[int, int]@.
676The flexible structure of tuples permits a simple and expressive function call syntax to work seamlessly with both SRVF and MRVF, and with any number of arguments of arbitrarily complex structure.
677
678
679\subsection{Tuple Assignment}
680
681An assignment where the left side is a tuple type is called \newterm{tuple assignment}.
682There 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 \newterm{multiple} and \newterm{mass assignment}, respectively.
683%\lstDeleteShortInline@%
684%\par\smallskip
685%\begin{tabular}{@{}l@{\hspace{1.5\parindent}}||@{\hspace{1.5\parindent}}l@{}}
686\begin{cfa}
687int x = 10;
688double y = 3.5;
689[int, double] z;
690z = [x, y];                                                                     $\C{// multiple assignment}$
691[x, y] = z;                                                                     $\C{// multiple assignment}$
692z = 10;                                                                         $\C{// mass assignment}$
693[y, x] = 3.14;                                                          $\C{// mass assignment}$
694\end{cfa}
695%\end{cfa}
696%&
697%\begin{cfa}
698%\end{tabular}
699%\smallskip\par\noindent
700%\lstMakeShortInline@%
701Both kinds of tuple assignment have parallel semantics, so that each value on the left and right side is evaluated before any assignments occur.
702As a result, it is possible to swap the values in two variables without explicitly creating any temporary variables or calling a function, \eg, @[x, y] = [y, x]@.
703This semantics means mass assignment differs from C cascading assignment (\eg @a = b = c@) in that conversions are applied in each individual assignment, which prevents data loss from the chain of conversions that can happen during a cascading assignment.
704For example, @[y, x] = 3.14@ performs the assignments @y = 3.14@ and @x = 3.14@, yielding @y == 3.14@ and @x == 3@;
705whereas, C cascading assignment @y = x = 3.14@ performs the assignments @x = 3.14@ and @y = x@, yielding @3@ in @y@ and @x@.
706Finally, tuple assignment is an expression where the result type is the type of the left-hand side of the assignment, just like all other assignment expressions in C.
707This example shows mass, multiple, and cascading assignment used in one expression:
708\begin{cfa}
709void f( [int, int] );
710f( [x, y] = z = 1.5 );                                          $\C{// assignments in parameter list}$
711\end{cfa}
712
713
714\subsection{Member Access}
715
716It is also possible to access multiple fields from a single expression using a \newterm{member-access}.
717The result is a single tuple-valued expression whose type is the tuple of the types of the members, \eg:
718\begin{cfa}
719struct S { int x; double y; char * z; } s;
720s.[x, y, z] = 0;
721\end{cfa}
722Here, the mass assignment sets all members of @s@ to zero.
723Since 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, drop, and duplicate components).
724%\lstDeleteShortInline@%
725%\par\smallskip
726%\begin{tabular}{@{}l@{\hspace{1.5\parindent}}||@{\hspace{1.5\parindent}}l@{}}
727\begin{cfa}
728[int, int, long, double] x;
729void f( double, long );
730x.[0, 1] = x.[1, 0];                                            $\C{// rearrange: [x.0, x.1] = [x.1, x.0]}$
731f( x.[0, 3] );                                                          $\C{// drop: f(x.0, x.3)}$
732[int, int, int] y = x.[2, 0, 2];                        $\C{// duplicate: [y.0, y.1, y.2] = [x.2, x.0.x.2]}$
733\end{cfa}
734%\end{cfa}
735%&
736%\begin{cfa}
737%\end{tabular}
738%\smallskip\par\noindent
739%\lstMakeShortInline@%
740It is also possible for a member access to contain other member accesses, \eg:
741\begin{cfa}
742struct A { double i; int j; };
743struct B { int * k; short l; };
744struct C { int x; A y; B z; } v;
745v.[x, y.[i, j], z.k];                                           $\C{// [v.x, [v.y.i, v.y.j], v.z.k]}$
746\end{cfa}
747
748
749\begin{comment}
750\subsection{Casting}
751
752In C, the cast operator is used to explicitly convert between types.
753In \CFA, the cast operator has a secondary use as type ascription.
754That 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:
755\begin{cfa}
756int f();     // (1)
757double f()// (2)
758
759f();       // ambiguous - (1),(2) both equally viable
760(int)f()// choose (2)
761\end{cfa}
762
763Since casting is a fundamental operation in \CFA, casts should be given a meaningful interpretation in the context of tuples.
764Taking a look at standard C provides some guidance with respect to the way casts should work with tuples:
765\begin{cfa}
766int f();
767void g();
768
769(void)f()// (1)
770(int)g()// (2)
771\end{cfa}
772In C, (1) is a valid cast, which calls @f@ and discards its result.
773On the other hand, (2) is invalid, because @g@ does not produce a result, so requesting an @int@ to materialize from nothing is nonsensical.
774Generalizing these principles, any cast 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.
775
776Formally, 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 $T_i$.
777Excess elements ($S_j$ for all $j$ in $[n, m)$) are evaluated, but their values are discarded so that they are not included in the result expression.
778This approach follows naturally from the way that a cast to @void@ works in C.
779
780For example, in
781\begin{cfa}
782[int, int, int] f();
783[int, [int, int], int] g();
784
785([int, double])f();           $\C{// (1)}$
786([int, int, int])g();         $\C{// (2)}$
787([void, [int, int]])g();      $\C{// (3)}$
788([int, int, int, int])g();    $\C{// (4)}$
789([int, [int, int, int]])g()$\C{// (5)}$
790\end{cfa}
791
792(1) discards the last element of the return value and converts the second element to @double@.
793Since @int@ is effectively a 1-element tuple, (2) discards the second component of the second element of the return value of @g@.
794If @g@ is free of side effects, this expression is equivalent to @[(int)(g().0), (int)(g().1.0), (int)(g().2)]@.
795Since @void@ is effectively a 0-element tuple, (3) discards the first and third return values, which is effectively equivalent to @[(int)(g().1.0), (int)(g().1.1)]@).
796
797Note that a cast is not a function call in \CFA, so flattening and structuring conversions do not occur for cast expressions\footnote{User-defined conversions have been considered, but for compatibility with C and the existing use of casts as type ascription, any future design for such conversions would require more precise matching of types than allowed for function arguments and parameters.}.
798As such, (4) is invalid because the cast target type contains 4 components, while the source type contains only 3.
799Similarly, (5) is invalid because the cast @([int, int, int])(g().1)@ is invalid.
800That is, it is invalid to cast @[int, int]@ to @[int, int, int]@.
801\end{comment}
802
803
804\subsection{Polymorphism}
805
806Tuples also integrate with \CFA polymorphism as a kind of generic type.
807Due to the implicit flattening and structuring conversions involved in argument passing, @otype@ and @dtype@ parameters are restricted to matching only with non-tuple types, \eg:
808\begin{cfa}
809forall(otype T, dtype U) void f( T x, U * y );
810f( [5, "hello"] );
811\end{cfa}
812where @[5, "hello"]@ is flattened, giving argument list @5, "hello"@, and @T@ binds to @int@ and @U@ binds to @const char@.
813Tuples, however, may contain polymorphic components.
814For example, a plus operator can be written to add two triples together.
815\begin{cfa}
816forall(otype T | { T ?+?( T, T ); }) [T, T, T] ?+?( [T, T, T] x, [T, T, T] y ) {
817        return [x.0 + y.0, x.1 + y.1, x.2 + y.2];
818}
819[int, int, int] x;
820int i1, i2, i3;
821[i1, i2, i3] = x + ([10, 20, 30]);
822\end{cfa}
823
824Flattening and restructuring conversions are also applied to tuple types in polymorphic type assertions.
825\begin{cfa}
826int f( [int, double], double );
827forall(otype T, otype U | { T f( T, U, U ); }) void g( T, U );
828g( 5, 10.21 );
829\end{cfa}
830Hence, function parameter and return lists are flattened for the purposes of type unification allowing the example to pass expression resolution.
831This relaxation is possible by extending the thunk scheme described by Bilson~\cite{Bilson03}.
832% Whenever a candidate's parameter structure does not exactly match the formal parameter's structure, a thunk is generated to specialize calls to the actual function:
833% \begin{cfa}
834% int _thunk( int _p0, double _p1, double _p2 ) { return f( [_p0, _p1], _p2 ); }
835% \end{cfa}
836% so the thunk provides flattening and structuring conversions to inferred functions, improving the compatibility of tuples and polymorphism.
837% These thunks are generated locally using gcc nested-functions, rather hositing them to the external scope, so they can easily access local state.
838
839
840\subsection{Variadic Tuples}
841\label{sec:variadic-tuples}
842
843To define variadic functions, \CFA adds a new kind of type parameter, @ttype@ (tuple type).
844Matching against a @ttype@ parameter consumes all remaining argument components and packages them into a tuple, binding to the resulting tuple of types.
845In a given parameter list, there must be at most one @ttype@ parameter that occurs last, which matches normal variadic semantics, with a strong feeling of similarity to \CCeleven variadic templates.
846As such, @ttype@ variables are also called \newterm{argument packs}.
847
848Like variadic templates, the main way to manipulate @ttype@ polymorphic functions is via recursion.
849Since nothing is known about a parameter pack by default, assertion parameters are key to doing anything meaningful.
850Unlike variadic templates, @ttype@ polymorphic functions can be separately compiled.
851For example, a generalized @sum@ function written using @ttype@:
852\begin{cfa}
853int sum$\(_0\)$() { return 0; }
854forall(ttype Params | { int sum( Params ); } ) int sum$\(_1\)$( int x, Params rest ) {
855        return x + sum( rest );
856}
857sum( 10, 20, 30 );
858\end{cfa}
859Since @sum@\(_0\) does not accept any arguments, it is not a valid candidate function for the call @sum(10, 20, 30)@.
860In order to call @sum@\(_1\), @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]@.
861The process continues until @Params@ is bound to @[]@, requiring an assertion @int sum()@, which matches @sum@\(_0\) and terminates the recursion.
862Effectively, this algorithm 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))@.
863
864It is reasonable to take the @sum@ function a step further to enforce a minimum number of arguments:
865\begin{cfa}
866int sum( int x, int y ) { return x + y; }
867forall(ttype Params | { int sum( int, Params ); } ) int sum( int x, int y, Params rest ) {
868        return sum( x + y, rest );
869}
870\end{cfa}
871One more step permits the summation of any summable type with all arguments of the same type:
872\begin{cfa}
873trait summable(otype T) {
874        T ?+?( T, T );
875};
876forall(otype R | summable( R ) ) R sum( R x, R y ) {
877        return x + y;
878}
879forall(otype R, ttype Params | summable(R) | { R sum(R, Params); } ) R sum(R x, R y, Params rest) {
880        return sum( x + y, rest );
881}
882\end{cfa}
883Unlike C variadic functions, it is unnecessary to hard code the number and expected types.
884Furthermore, this code is extendable for any user-defined type with a @?+?@ operator.
885Summing arbitrary heterogeneous lists is possible with similar code by adding the appropriate type variables and addition operators.
886
887It is also possible to write a type-safe variadic print function to replace @printf@:
888\begin{cfa}
889struct S { int x, y; };
890forall(otype T, ttype Params | { void print(T); void print(Params); }) void print(T arg, Params rest) {
891        print(arg);  print(rest);
892}
893void print( const char * x ) { printf( "%s", x ); }
894void print( int x ) { printf( "%d", x ); }
895void print( S s ) { print( "{ ", s.x, ",", s.y, " }" ); }
896print( "s = ", (S){ 1, 2 }, "\n" );
897\end{cfa}
898This example showcases a variadic-template-like decomposition of the provided argument list.
899The individual @print@ functions allow printing a single element of a type.
900The polymorphic @print@ allows printing any list of types, where as each individual type has a @print@ function.
901The individual print functions can be used to build up more complicated @print@ functions, such as @S@, which cannot be done with @printf@ in C.
902This mechanism is used to seamlessly print tuples in the \CFA I/O library (see Section~\ref{s:IOLibrary}).
903
904Finally, it is possible to use @ttype@ polymorphism to provide arbitrary argument forwarding functions.
905For example, it is possible to write @new@ as a library function:
906\begin{cfa}
907forall( otype R, otype S ) void ?{}( pair(R, S) *, R, S );
908forall( dtype T, ttype Params | sized(T) | { void ?{}( T *, Params ); } ) T * new( Params p ) {
909        return ((T *)malloc()){ p };                    $\C{// construct into result of malloc}$
910}
911pair( int, char ) * x = new( 42, '!' );
912\end{cfa}
913The @new@ function provides the combination of type-safe @malloc@ with a \CFA constructor call, making it impossible to forget constructing dynamically allocated objects.
914This function provides the type-safety of @new@ in \CC, without the need to specify the allocated type again, thanks to return-type inference.
915
916
917\subsection{Implementation}
918
919Tuples are implemented in the \CFA translator via a transformation into \newterm{generic types}.
920For each $N$, the first time an $N$-tuple is seen in a scope a generic type with $N$ type parameters is generated, \eg:
921\begin{cfa}
922[int, int] f() {
923        [double, double] x;
924        [int, double, int] y;
925}
926\end{cfa}
927is transformed into:
928\begin{cfa}
929forall(dtype T0, dtype T1 | sized(T0) | sized(T1)) struct _tuple2 {
930        T0 field_0;                                                             $\C{// generated before the first 2-tuple}$
931        T1 field_1;
932};
933_tuple2(int, int) f() {
934        _tuple2(double, double) x;
935        forall(dtype T0, dtype T1, dtype T2 | sized(T0) | sized(T1) | sized(T2)) struct _tuple3 {
936                T0 field_0;                                                     $\C{// generated before the first 3-tuple}$
937                T1 field_1;
938                T2 field_2;
939        };
940        _tuple3(int, double, int) y;
941}
942\end{cfa}
943\begin{sloppypar}
944Tuple expressions are then simply converted directly into compound literals, \eg @[5, 'x', 1.24]@ becomes @(_tuple3(int, char, double)){ 5, 'x', 1.24 }@.
945\end{sloppypar}
946
947\begin{comment}
948Since tuples are essentially structures, tuple indexing expressions are just field accesses:
949\begin{cfa}
950void f(int, [double, char]);
951[int, double] x;
952
953x.0+x.1;
954printf("%d %g\n", x);
955f(x, 'z');
956\end{cfa}
957Is transformed into:
958\begin{cfa}
959void f(int, _tuple2(double, char));
960_tuple2(int, double) x;
961
962x.field_0+x.field_1;
963printf("%d %g\n", x.field_0, x.field_1);
964f(x.field_0, (_tuple2){ x.field_1, 'z' });
965\end{cfa}
966Note that due to flattening, @x@ used in the argument position is converted into the list of its fields.
967In the call to @f@, the second and third argument components are structured into a tuple argument.
968Similarly, tuple member expressions are recursively expanded into a list of member access expressions.
969
970Expressions that may contain side effects are made into \newterm{unique expressions} before being expanded by the flattening conversion.
971Each unique expression is assigned an identifier and is guaranteed to be executed exactly once:
972\begin{cfa}
973void g(int, double);
974[int, double] h();
975g(h());
976\end{cfa}
977Internally, this expression is converted to two variables and an expression:
978\begin{cfa}
979void g(int, double);
980[int, double] h();
981
982_Bool _unq0_finished_ = 0;
983[int, double] _unq0;
984g(
985        (_unq0_finished_ ? _unq0 : (_unq0 = f(), _unq0_finished_ = 1, _unq0)).0,
986        (_unq0_finished_ ? _unq0 : (_unq0 = f(), _unq0_finished_ = 1, _unq0)).1,
987);
988\end{cfa}
989Since argument evaluation order is not specified by the C programming language, this scheme is built to work regardless of evaluation order.
990The first time a unique expression is executed, the actual expression is evaluated and the accompanying boolean is set to true.
991Every subsequent evaluation of the unique expression then results in an access to the stored result of the actual expression.
992Tuple member expressions also take advantage of unique expressions in the case of possible impurity.
993
994Currently, the \CFA translator has a very broad, imprecise definition of impurity, where any function call is assumed to be impure.
995This notion could be made more precise for certain intrinsic, auto-generated, and builtin functions, and could analyze function bodies when they are available to recursively detect impurity, to eliminate some unique expressions.
996
997The various kinds of tuple assignment, constructors, and destructors generate GNU C statement expressions.
998A 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, \eg in a unique expression.
999The 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.
1000However, there are other places where the \CFA translator makes use of GNU C extensions, such as its use of nested functions, so this restriction is not new.
1001\end{comment}
1002
1003
1004\section{Control Structures}
1005
1006\CFA identifies inconsistent, problematic, and missing control structures in C, and extends, modifies, and adds control structures to increase functionality and safety.
1007
1008
1009\subsection{\texorpdfstring{\LstKeywordStyle{if} Statement}{if Statement}}
1010
1011The @if@ expression allows declarations, similar to @for@ declaration expression:
1012\begin{cfa}
1013if ( int x = f() ) ...                                          $\C{// x != 0}$
1014if ( int x = f(), y = g() ) ...                         $\C{// x != 0 \&\& y != 0}$
1015if ( int x = f(), y = g(); `x < y` ) ...        $\C{// relational expression}$
1016\end{cfa}
1017Unless a relational expression is specified, each variable is compared not equal to 0, which is the standard semantics for the @if@ expression, and the results are combined using the logical @&&@ operator.\footnote{\CC only provides a single declaration always compared not equal to 0.}
1018The scope of the declaration(s) is local to the @if@ statement but exist within both the ``then'' and ``else'' clauses.
1019
1020
1021\subsection{\texorpdfstring{\LstKeywordStyle{switch} Statement}{switch Statement}}
1022
1023There are a number of deficiencies with the C @switch@ statements: enumerating @case@ lists, placement of @case@ clauses, scope of the switch body, and fall through between case clauses.
1024
1025C has no shorthand for specifying a list of case values, whether the list is non-contiguous or contiguous\footnote{C provides this mechanism via fall through.}.
1026\CFA provides a shorthand for a non-contiguous list:
1027\begin{cquote}
1028\lstDeleteShortInline@%
1029\begin{tabular}{@{}l@{\hspace{\parindentlnth}}l@{}}
1030\multicolumn{1}{c@{\hspace{\parindentlnth}}}{\textbf{\CFA}}     & \multicolumn{1}{c}{\textbf{C}}        \\
1031\begin{cfa}
1032case 2, 10, 34, 42:
1033\end{cfa}
1034&
1035\begin{cfa}
1036case 2: case 10: case 34: case 42:
1037\end{cfa}
1038\end{tabular}
1039\lstMakeShortInline@%
1040\end{cquote}
1041for a contiguous list:\footnote{gcc has the same mechanism but awkward syntax, \lstinline@2 ...42@, because a space is required after a number, otherwise the period is a decimal point.}
1042\begin{cquote}
1043\lstDeleteShortInline@%
1044\begin{tabular}{@{}l@{\hspace{\parindentlnth}}l@{}}
1045\multicolumn{1}{c@{\hspace{\parindentlnth}}}{\textbf{\CFA}}     & \multicolumn{1}{c}{\textbf{C}}        \\
1046\begin{cfa}
1047case 2~42:
1048\end{cfa}
1049&
1050\begin{cfa}
1051case 2: case 3: ... case 41: case 42:
1052\end{cfa}
1053\end{tabular}
1054\lstMakeShortInline@%
1055\end{cquote}
1056and a combination:
1057\begin{cfa}
1058case -12~-4, -1~5, 14~21, 34~42:
1059\end{cfa}
1060
1061C allows placement of @case@ clauses \emph{within} statements nested in the @switch@ body (see Duff's device~\cite{Duff83});
1062\begin{cfa}
1063switch ( i ) {
1064  case 0:
1065        for ( int i = 0; i < 10; i += 1 ) {
1066                ...
1067  `case 1:`             // no initialization of loop index
1068                ...
1069        }
1070}
1071\end{cfa}
1072\CFA precludes this form of transfer into a control structure because it causes undefined behaviour, especially with respect to missed initialization, and provides very limited functionality.
1073
1074C allows placement of declaration within the @switch@ body and unreachable code at the start, resulting in undefined behaviour:
1075\begin{cfa}
1076switch ( x ) {
1077        `int y = 1;`                                                    $\C{// unreachable initialization}$
1078        `x = 7;`                                                                $\C{// unreachable code without label/branch}$
1079  case 0:
1080        ...
1081        `int z = 0;`                                                    $\C{// unreachable initialization, cannot appear after case}$
1082        z = 2;
1083  case 1:
1084        `x = z;`                                                                $\C{// without fall through, z is undefined}$
1085}
1086\end{cfa}
1087\CFA allows the declaration of local variables, \eg @y@, at the start of the @switch@ with scope across the entire @switch@ body, \ie all @case@ clauses.
1088\CFA disallows the declaration of local variable, \eg @z@, directly within the @switch@ body, because a declaration cannot occur immediately after a @case@ since a label can only be attached to a statement, and the use of @z@ is undefined in @case 1@ as neither storage allocation nor initialization may have occurred.
1089
1090C @switch@ provides multiple entry points into the statement body, but once an entry point is selected, control continues across \emph{all} @case@ clauses until the end of the @switch@ body, called \newterm{fall through};
1091@case@ clauses are made disjoint by the @break@ statement.
1092While the ability to fall through \emph{is} a useful form of control flow, it does not match well with programmer intuition, resulting in many errors from missing @break@ statements.
1093For backwards compatibility, \CFA provides a \emph{new} control structure, @choose@, which mimics @switch@, but reverses the meaning of fall through (see Figure~\ref{f:ChooseSwitchStatements}).
1094
1095Collectively, these enhancements reduce programmer burden and increase readability and safety.
1096
1097\begin{figure}
1098\centering
1099\lstDeleteShortInline@%
1100\begin{tabular}{@{}l@{\hspace{2\parindentlnth}}l@{}}
1101\multicolumn{1}{c@{\hspace{2\parindentlnth}}}{\textbf{\CFA}}    & \multicolumn{1}{c}{\textbf{C}}        \\
1102\begin{cfa}
1103`choose` ( day ) {
1104  case Mon~Thu:  // program
1105
1106  case Fri:  // program
1107        wallet += pay;
1108        `fallthrough;`
1109  case Sat:  // party
1110        wallet -= party;
1111
1112  case Sun:  // rest
1113
1114  default:  // error
1115}
1116\end{cfa}
1117&
1118\begin{cfa}
1119switch ( day ) {
1120  case Mon: case Tue: case Wed: case Thu:  // program
1121        `break;`
1122  case Fri:  // program
1123        wallet += pay;
1124
1125  case Sat:  // party
1126        wallet -= party;
1127        `break;`
1128  case Sun:  // rest
1129        `break;`
1130  default:  // error
1131}
1132\end{cfa}
1133\end{tabular}
1134\lstMakeShortInline@%
1135\caption{\lstinline|choose| versus \lstinline|switch| Statements}
1136\label{f:ChooseSwitchStatements}
1137\end{figure}
1138
1139\begin{comment}
1140Forgotten @break@ statements at the end of @switch@ cases are a persistent sort of programmer error in C, and the @break@ statements themselves introduce visual clutter and an un-C-like keyword-based block delimiter.
1141\CFA addresses this error by introducing a @choose@ statement, which works identically to a @switch@ except that its default end-of-case behaviour is to break rather than to fall through for all non-empty cases.
1142Since empty cases like @case 7:@ in @case 7: case 11:@ still have fall-through semantics and explicit @break@ is still allowed at the end of a @choose@ case, many idiomatic uses of @switch@ in standard C can be converted to @choose@ statements by simply changing the keyword.
1143Where fall-through is desired for a non-empty case, it can be specified with the new @fallthrough@ statement, making @choose@ equivalently powerful to @switch@, but more concise in the common case where most non-empty cases end with a @break@ statement, as in the example below:
1144
1145\begin{cfa}
1146choose( i ) {
1147        case 2:
1148                printf("even ");
1149                fallthrough;
1150        case 3: case 5: case 7:
1151                printf("small prime\n");
1152        case 4,6,8,9:
1153                printf("small composite\n");
1154        case 13~19:
1155                printf("teen\n");
1156        default:
1157                printf("something else\n");
1158}
1159\end{cfa}
1160\end{comment}
1161
1162
1163\subsection{\texorpdfstring{Labelled \LstKeywordStyle{continue} / \LstKeywordStyle{break}}{Labelled continue / break}}
1164
1165While C provides @continue@ and @break@ statements for altering control flow, both are restricted to one level of nesting for a particular control structure.
1166Unfortunately, this restriction forces programmers to use @goto@ to achieve the equivalent control-flow for more than one level of nesting.
1167To prevent having to switch to the @goto@, \CFA extends the @continue@ and @break@ with a target label to support static multi-level exit~\cite{Buhr85}, as in Java.
1168For both @continue@ and @break@, the target label must be directly associated with a @for@, @while@ or @do@ statement;
1169for @break@, the target label can also be associated with a @switch@, @if@ or compound (@{}@) statement.
1170Figure~\ref{f:MultiLevelExit} shows @continue@ and @break@ indicating the specific control structure, and the corresponding C program using only @goto@ and labels.
1171The innermost loop has 7 exit points, which cause continuation or termination of one or more of the 7 nested control-structures.
1172
1173\begin{figure}
1174\lstDeleteShortInline@%
1175\begin{tabular}{@{\hspace{\parindentlnth}}l@{\hspace{\parindentlnth}}l@{\hspace{\parindentlnth}}l@{}}
1176\multicolumn{1}{@{\hspace{\parindentlnth}}c@{\hspace{\parindentlnth}}}{\textbf{\CFA}}   & \multicolumn{1}{@{\hspace{\parindentlnth}}c}{\textbf{C}}      \\
1177\begin{cfa}
1178`LC:` {
1179        ... $declarations$ ...
1180        `LS:` switch ( ... ) {
1181          case 3:
1182                `LIF:` if ( ... ) {
1183                        `LF:` for ( ... ) {
1184                                `LW:` while ( ... ) {
1185                                        ... break `LC`; ...
1186                                        ... break `LS`; ...
1187                                        ... break `LIF`; ...
1188                                        ... continue `LF;` ...
1189                                        ... break `LF`; ...
1190                                        ... continue `LW`; ...
1191                                        ... break `LW`; ...
1192                                } // while
1193                        } // for
1194                } else {
1195                        ... break `LIF`; ...
1196                } // if
1197        } // switch
1198} // compound
1199\end{cfa}
1200&
1201\begin{cfa}
1202{
1203        ... $declarations$ ...
1204        switch ( ... ) {
1205          case 3:
1206                if ( ... ) {
1207                        for ( ... ) {
1208                                while ( ... ) {
1209                                        ... goto `LC`; ...
1210                                        ... goto `LS`; ...
1211                                        ... goto `LIF`; ...
1212                                        ... goto `LFC`; ...
1213                                        ... goto `LFB`; ...
1214                                        ... goto `LWC`; ...
1215                                        ... goto `LWB`; ...
1216                                  `LWC`: ; } `LWB:` ;
1217                          `LFC:` ; } `LFB:` ;
1218                } else {
1219                        ... goto `LIF`; ...
1220                } `LIF:` ;
1221        } `LS:` ;
1222} `LC:` ;
1223\end{cfa}
1224&
1225\begin{cfa}
1226
1227
1228
1229
1230
1231
1232
1233// terminate compound
1234// terminate switch
1235// terminate if
1236// continue loop
1237// terminate loop
1238// continue loop
1239// terminate loop
1240
1241
1242
1243// terminate if
1244
1245
1246
1247\end{cfa}
1248\end{tabular}
1249\lstMakeShortInline@%
1250\caption{Multi-level Exit}
1251\label{f:MultiLevelExit}
1252\end{figure}
1253
1254With respect to safety, both labelled @continue@ and @break@ are a @goto@ restricted in the following ways:
1255\begin{itemize}
1256\item
1257They cannot create a loop, which means only the looping constructs cause looping.
1258This restriction means all situations resulting in repeated execution are clearly delineated.
1259\item
1260They cannot branch into a control structure.
1261This restriction prevents missing declarations and/or initializations at the start of a control structure resulting in undefined behaviour.
1262\end{itemize}
1263The advantage of the labelled @continue@/@break@ is allowing static multi-level exits without having to use the @goto@ statement, and tying control flow to the target control structure rather than an arbitrary point in a program.
1264Furthermore, the location of the label at the \emph{beginning} of the target control structure informs the reader (eye candy) that complex control-flow is occurring in the body of the control structure.
1265With @goto@, the label is at the end of the control structure, which fails to convey this important clue early enough to the reader.
1266Finally, using an explicit target for the transfer instead of an implicit target allows new constructs to be added or removed without affecting existing constructs.
1267Otherwise, the implicit targets of the current @continue@ and @break@, \ie the closest enclosing loop or @switch@, change as certain constructs are added or removed.
1268
1269
1270\subsection{Exception Handling}
1271
1272The following framework for \CFA exception handling is in place, excluding some run-time type-information and dynamic casts.
1273\CFA provides two forms of exception handling: \newterm{fix-up} and \newterm{recovery} (see Figure~\ref{f:CFAExceptionHandling})~\cite{Buhr92b,Buhr00a}.
1274Both mechanisms provide dynamic call to a handler using dynamic name-lookup, where fix-up has dynamic return and recovery has static return from the handler.
1275\CFA restricts exception types to those defined by aggregate type @exception@.
1276The form of the raise dictates the set of handlers examined during propagation: \newterm{resumption propagation} (@resume@) only examines resumption handlers (@catchResume@); \newterm{terminating propagation} (@throw@) only examines termination handlers (@catch@).
1277If @resume@ or @throw@ have no exception type, it is a reresume/rethrow, meaning the currently exception continues propagation.
1278If there is no current exception, the reresume/rethrow results in a runtime error.
1279
1280\begin{figure}
1281\begin{cquote}
1282\lstDeleteShortInline@%
1283\begin{tabular}{@{}l@{\hspace{2\parindentlnth}}l@{}}
1284\multicolumn{1}{c@{\hspace{2\parindentlnth}}}{\textbf{Resumption}}      & \multicolumn{1}{c}{\textbf{Termination}}      \\
1285\begin{cfa}
1286`exception R { int fix; };`
1287void f() {
1288        R r;
1289        ... `resume( r );` ...
1290        ... r.fix // control does return here after handler
1291}
1292`try` {
1293        ... f(); ...
1294} `catchResume( R r )` {
1295        ... r.fix = ...; // return correction to raise
1296} // dynamic return to _Resume
1297\end{cfa}
1298&
1299\begin{cfa}
1300`exception T {};`
1301void f() {
1302
1303        ... `throw( T{} );` ...
1304        // control does NOT return here after handler
1305}
1306`try` {
1307        ... f(); ...
1308} `catch( T t )` {
1309        ... // recover and continue
1310} // static return to next statement
1311\end{cfa}
1312\end{tabular}
1313\lstMakeShortInline@%
1314\end{cquote}
1315\caption{\CFA Exception Handling}
1316\label{f:CFAExceptionHandling}
1317\end{figure}
1318
1319The set of exception types in a list of catch clause may include both a resumption and termination handler:
1320\begin{cfa}
1321try {
1322        ... resume( `R{}` ); ...
1323} catchResume( `R` r ) { ... throw( R{} ); ... } $\C{\color{red}// H1}$
1324   catch( `R` r ) { ... }                                       $\C{\color{red}// H2}$
1325
1326\end{cfa}
1327The resumption propagation raises @R@ and the stack is not unwound;
1328the exception is caught by the @catchResume@ clause and handler H1 is invoked.
1329The termination propagation in handler H1 raises @R@ and the stack is unwound;
1330the exception is caught by the @catch@ clause and handler H2 is invoked.
1331The termination handler is available because the resumption propagation did not unwind the stack.
1332
1333An additional feature is conditional matching in a catch clause:
1334\begin{cfa}
1335try {
1336        ... write( `datafile`, ... ); ...               $\C{// may throw IOError}$
1337        ... write( `logfile`, ... ); ...
1338} catch ( IOError err; `err.file == datafile` ) { ... } $\C{// handle datafile error}$
1339   catch ( IOError err; `err.file == logfile` ) { ... } $\C{// handle logfile error}$
1340   catch ( IOError err ) { ... }                        $\C{// handler error from other files}$
1341\end{cfa}
1342where the throw inserts the failing file-handle in the I/O exception.
1343Conditional catch cannot be trivially mimicked by other mechanisms because once an exception is caught, handler clauses in that @try@ statement are no longer eligible..
1344
1345The resumption raise can specify an alternate stack on which to raise an exception, called a \newterm{nonlocal raise}:
1346\begin{cfa}
1347resume( $\emph{exception-type}$, $\emph{alternate-stack}$ )
1348resume( $\emph{alternate-stack}$ )
1349\end{cfa}
1350These overloads of @resume@ raise the specified exception or the currently propagating exception (reresume) at another \CFA coroutine or task~\cite{Delisle18}.\footnote{\CFA coroutine and concurrency features are discussed in a separately submitted paper.}
1351Nonlocal raise is restricted to resumption to provide the exception handler the greatest flexibility because processing the exception does not unwind its stack, allowing it to continue after the handle returns.
1352
1353To facilitate nonlocal exception, \CFA provides dynamic enabling and disabling of nonlocal exception-propagation.
1354The constructs for controlling propagation of nonlocal exceptions are the @enable@ and the @disable@ blocks:
1355\begin{cquote}
1356\lstDeleteShortInline@%
1357\begin{tabular}{@{}l@{\hspace{2\parindentlnth}}l@{}}
1358\begin{cfa}
1359enable $\emph{exception-type-list}$ {
1360        // allow non-local resumption
1361}
1362\end{cfa}
1363&
1364\begin{cfa}
1365disable $\emph{exception-type-list}$ {
1366        // disallow non-local resumption
1367}
1368\end{cfa}
1369\end{tabular}
1370\lstMakeShortInline@%
1371\end{cquote}
1372The arguments for @enable@/@disable@ specify the exception types allowed to be propagated or postponed, respectively.
1373Specifying no exception type is shorthand for specifying all exception types.
1374Both @enable@ and @disable@ blocks can be nested, turning propagation on/off on entry, and on exit, the specified exception types are restored to their prior state.
1375Coroutines and tasks start with non-local exceptions disabled, allowing handlers to be put in place, before non-local exceptions are explicitly enabled.
1376\begin{cfa}
1377void main( mytask & c ) {                                       $\C{// thread starts here}$
1378        // non-local exceptions disabled
1379        try {                                                                   $\C{// establish handles for non-local exceptions}$
1380                enable {                                                        $\C{// allow non-local exception delivery}$
1381                        // task body
1382                }
1383        // appropriate catchResume/catch handlers
1384        }
1385}
1386\end{cfa}
1387
1388Finally, \CFA provides a Java like  @finally@ clause after the catch clauses:
1389\begin{cfa}
1390try {
1391        ... f(); ...
1392// catchResume or catch clauses
1393} `finally` {
1394        // house keeping
1395}
1396\end{cfa}
1397The finally clause is always executed, i.e., if the try block ends normally or if an exception is raised.
1398If an exception is raised and caught, the handler is run before the finally clause.
1399Like a destructor (see Section~\ref{s:ConstructorsDestructors}), a finally clause can raise an exception but not if there is an exception being propagated.
1400Mimicking the @finally@ clause with mechanisms like RAII is non-trivially when there are multiple types and local accesses.
1401
1402
1403\subsection{\texorpdfstring{\LstKeywordStyle{with} Clause / Statement}{with Clause / Statement}}
1404\label{s:WithClauseStatement}
1405
1406Grouping heterogeneous data into \newterm{aggregate}s (structure/union) is a common programming practice, and an aggregate can be further organized into more complex structures, such as arrays and containers:
1407\begin{cfa}
1408struct S {                                                                      $\C{// aggregate}$
1409        char c;                                                                 $\C{// fields}$
1410        int i;
1411        double d;
1412};
1413S s, as[10];
1414\end{cfa}
1415However, functions manipulating aggregates must repeat the aggregate name to access its containing fields:
1416\begin{cfa}
1417void f( S s ) {
1418        `s.`c; `s.`i; `s.`d;                                    $\C{// access containing fields}$
1419}
1420\end{cfa}
1421which extends to multiple levels of qualification for nested aggregates.
1422A similar situation occurs in object-oriented programming, \eg \CC:
1423\begin{C++}
1424class C {
1425        char c;                                                                 $\C{// fields}$
1426        int i;
1427        double d;
1428        int f() {                                                               $\C{// implicit ``this'' aggregate}$
1429                `this->`c; `this->`i; `this->`d;        $\C{// access containing fields}$
1430        }
1431}
1432\end{C++}
1433Object-oriented nesting of member functions in a \lstinline[language=C++]@class@ allows eliding \lstinline[language=C++]@this->@ because of lexical scoping.
1434However, for other aggregate parameters, qualification is necessary:
1435\begin{cfa}
1436struct T { double m, n; };
1437int C::f( T & t ) {                                                     $\C{// multiple aggregate parameters}$
1438        c; i; d;                                                                $\C{\color{red}// this-\textgreater.c, this-\textgreater.i, this-\textgreater.d}$
1439        `t.`m; `t.`n;                                                   $\C{// must qualify}$
1440}
1441\end{cfa}
1442
1443To simplify the programmer experience, \CFA provides a @with@ statement (see Pascal~\cite[\S~4.F]{Pascal}) to elide aggregate qualification to fields by opening a scope containing the field identifiers.
1444Hence, the qualified fields become variables with the side-effect that it is easier to optimizing field references in a block.
1445\begin{cfa}
1446void f( S & this ) `with ( this )` {            $\C{// with statement}$
1447        c; i; d;                                                                $\C{\color{red}// this.c, this.i, this.d}$
1448}
1449\end{cfa}
1450with the generality of opening multiple aggregate-parameters:
1451\begin{cfa}
1452int f( S & s, T & t ) `with ( s, t )` {         $\C{// multiple aggregate parameters}$
1453        c; i; d;                                                                $\C{\color{red}// s.c, s.i, s.d}$
1454        m; n;                                                                   $\C{\color{red}// t.m, t.n}$
1455}
1456\end{cfa}
1457
1458In detail, the @with@ statement has the form:
1459\begin{cfa}
1460$\emph{with-statement}$:
1461        'with' '(' $\emph{expression-list}$ ')' $\emph{compound-statement}$
1462\end{cfa}
1463and may appear as the body of a function or nested within a function body.
1464Each expression in the expression-list provides a type and object.
1465The type must be an aggregate type.
1466(Enumerations are already opened.)
1467The object is the implicit qualifier for the open structure-fields.
1468
1469All expressions in the expression list are open in parallel within the compound statement.
1470This semantic is different from Pascal, which nests the openings from left to right.
1471The difference between parallel and nesting occurs for fields with the same name and type:
1472\begin{cfa}
1473struct S { int `i`; int j; double m; } s, w;
1474struct T { int `i`; int k; int m; } t, w;
1475with ( s, t ) {
1476        j + k;                                                                  $\C{// unambiguous, s.j + t.k}$
1477        m = 5.0;                                                                $\C{// unambiguous, t.m = 5.0}$
1478        m = 1;                                                                  $\C{// unambiguous, s.m = 1}$
1479        int a = m;                                                              $\C{// unambiguous, a = s.i }$
1480        double b = m;                                                   $\C{// unambiguous, b = t.m}$
1481        int c = s.i + t.i;                                              $\C{// unambiguous, qualification}$
1482        (double)m;                                                              $\C{// unambiguous, cast}$
1483}
1484\end{cfa}
1485For parallel semantics, both @s.i@ and @t.i@ are visible, so @i@ is ambiguous without qualification;
1486for nested semantics, @t.i@ hides @s.i@, so @i@ implies @t.i@.
1487\CFA's ability to overload variables means fields with the same name but different types are automatically disambiguated, eliminating most qualification when opening multiple aggregates.
1488Qualification or a cast is used to disambiguate.
1489
1490There is an interesting problem between parameters and the function-body @with@, \eg:
1491\begin{cfa}
1492void ?{}( S & s, int i ) with ( s ) {           $\C{// constructor}$
1493        `s.i = i;`  j = 3;  m = 5.5;                    $\C{// initialize fields}$
1494}
1495\end{cfa}
1496Here, the assignment @s.i = i@ means @s.i = s.i@, which is meaningless, and there is no mechanism to qualify the parameter @i@, making the assignment impossible using the function-body @with@.
1497To solve this problem, parameters are treated like an initialized aggregate:
1498\begin{cfa}
1499struct Params {
1500        S & s;
1501        int i;
1502} params;
1503\end{cfa}
1504and implicitly opened \emph{after} a function-body open, to give them higher priority:
1505\begin{cfa}
1506void ?{}( S & s, int `i` ) with ( s ) `with( $\emph{\color{red}params}$ )` {
1507        s.i = `i`; j = 3; m = 5.5;
1508}
1509\end{cfa}
1510Finally, a cast may be used to disambiguate among overload variables in a @with@ expression:
1511\begin{cfa}
1512with ( w ) { ... }                                                      $\C{// ambiguous, same name and no context}$
1513with ( (S)w ) { ... }                                           $\C{// unambiguous, cast}$
1514\end{cfa}
1515and @with@ expressions may be complex expressions with type reference (see Section~\ref{s:References}) to aggregate:
1516\begin{cfa}
1517struct S { int i, j; } sv;
1518with ( sv ) {                                                           $\C{implicit reference}$
1519        S & sr = sv;
1520        with ( sr ) {                                                   $\C{explicit reference}$
1521                S * sp = &sv;
1522                with ( *sp ) {                                          $\C{computed reference}$
1523                        i = 3; j = 4;                                   $\C{\color{red}// sp-{\textgreater}i, sp-{\textgreater}j}$
1524                }
1525                i = 2; j = 3;                                           $\C{\color{red}// sr.i, sr.j}$
1526        }
1527        i = 1; j = 2;                                                   $\C{\color{red}// sv.i, sv.j}$
1528}
1529\end{cfa}
1530
1531
1532\section{Declarations}
1533
1534Declarations in C have weaknesses and omissions.
1535\CFA attempts to correct and add to C declarations, while ensuring \CFA subjectively ``feels like'' C.
1536An important part of this subjective feel is maintaining C's syntax and procedural paradigm, as opposed to functional and object-oriented approaches in other systems languages such as \CC and Rust.
1537Maintaining the C approach means that C coding-patterns remain not only useable but idiomatic in \CFA, reducing the mental burden of retraining C programmers and switching between C and \CFA development.
1538Nevertheless, some features from other approaches are undeniably convenient;
1539\CFA attempts to adapt these features to the C paradigm.
1540
1541
1542\subsection{Alternative Declaration Syntax}
1543
1544C declaration syntax is notoriously confusing and error prone.
1545For example, many C programmers are confused by a declaration as simple as:
1546\begin{cquote}
1547\lstDeleteShortInline@%
1548\begin{tabular}{@{}ll@{}}
1549\begin{cfa}
1550int * x[5]
1551\end{cfa}
1552&
1553\raisebox{-0.75\totalheight}{\input{Cdecl}}
1554\end{tabular}
1555\lstMakeShortInline@%
1556\end{cquote}
1557Is this an array of 5 pointers to integers or a pointer to an array of 5 integers?
1558If there is any doubt, it implies productivity and safety issues even for basic programs.
1559Another example of confusion results from the fact that a function name and its parameters are embedded within the return type, mimicking the way the return value is used at the function's call site.
1560For example, a function returning a pointer to an array of integers is defined and used in the following way:
1561\begin{cfa}
1562int `(*`f`())[`5`]` {...};                                      $\C{// definition}$
1563 ... `(*`f`())[`3`]` += 1;                                      $\C{// usage}$
1564\end{cfa}
1565Essentially, the return type is wrapped around the function name in successive layers (like an onion).
1566While attempting to make the two contexts consistent is a laudable goal, it has not worked out in practice.
1567
1568\CFA provides its own type, variable and function declarations, using a different syntax~\cite[pp.~856--859]{Buhr94a}.
1569The new declarations place qualifiers to the left of the base type, while C declarations place qualifiers to the right.
1570The qualifiers have the same meaning but are ordered left to right to specify a variable's type.
1571\begin{cquote}
1572\lstDeleteShortInline@%
1573\begin{tabular}{@{}l@{\hspace{\parindentlnth}}l@{\hspace{\parindentlnth}}l@{}}
1574\multicolumn{1}{c@{\hspace{\parindentlnth}}}{\textbf{\CFA}}     & \multicolumn{1}{c}{\textbf{C}}        \\
1575\begin{cfa}
1576`[5] *` int x1;
1577`* [5]` int x2;
1578`[* [5] int]` f( int p );
1579\end{cfa}
1580&
1581\begin{cfa}
1582int `*` x1 `[5]`;
1583int `(*`x2`)[5]`;
1584`int (*`f( int p )`)[5]`;
1585\end{cfa}
1586&
1587\begin{cfa}
1588// array of 5 pointers to int
1589// pointer to an array of 5 int
1590// function returning pointer to an array of 5 int and taking an int
1591\end{cfa}
1592\end{tabular}
1593\lstMakeShortInline@%
1594\end{cquote}
1595The only exception is bit field specification, which always appear to the right of the base type.
1596% Specifically, the character @*@ is used to indicate a pointer, square brackets @[@\,@]@ are used to represent an array or function return value, and parentheses @()@ are used to indicate a function parameter.
1597However, unlike C, \CFA type declaration tokens are distributed across all variables in the declaration list.
1598For instance, variables @x@ and @y@ of type pointer to integer are defined in \CFA as follows:
1599\begin{cquote}
1600\lstDeleteShortInline@%
1601\begin{tabular}{@{}l@{\hspace{\parindentlnth}}l@{}}
1602\multicolumn{1}{c@{\hspace{\parindentlnth}}}{\textbf{\CFA}}     & \multicolumn{1}{c}{\textbf{C}}        \\
1603\begin{cfa}
1604`*` int x, y;
1605int y;
1606\end{cfa}
1607&
1608\begin{cfa}
1609int `*`x, `*`y;
1610
1611\end{cfa}
1612\end{tabular}
1613\lstMakeShortInline@%
1614\end{cquote}
1615The downside of the \CFA semantics is the need to separate regular and pointer declarations.
1616
1617\begin{comment}
1618Other examples are:
1619\begin{cquote}
1620\lstDeleteShortInline@%
1621\begin{tabular}{@{}l@{\hspace{\parindentlnth}}l@{\hspace{\parindentlnth}}l@{}}
1622\multicolumn{1}{c@{\hspace{\parindentlnth}}}{\textbf{\CFA}}     & \multicolumn{1}{c@{\hspace{\parindentlnth}}}{\textbf{C}}      \\
1623\begin{cfa}
1624[ 5 ] int z;
1625[ 5 ] * char w;
1626* [ 5 ] double v;
1627struct s {
1628        int f0:3;
1629        * int f1;
1630        [ 5 ] * int f2;
1631};
1632\end{cfa}
1633&
1634\begin{cfa}
1635int z[ 5 ];
1636char * w[ 5 ];
1637double (* v)[ 5 ];
1638struct s {
1639        int f0:3;
1640        int * f1;
1641        int * f2[ 5 ]
1642};
1643\end{cfa}
1644&
1645\begin{cfa}
1646// array of 5 integers
1647// array of 5 pointers to char
1648// pointer to array of 5 doubles
1649
1650// common bit field syntax
1651
1652
1653
1654\end{cfa}
1655\end{tabular}
1656\lstMakeShortInline@%
1657\end{cquote}
1658\end{comment}
1659
1660All specifiers (@extern@, @static@, \etc) and qualifiers (@const@, @volatile@, \etc) are used in the normal way with the new declarations and also appear left to right, \eg:
1661\begin{cquote}
1662\lstDeleteShortInline@%
1663\begin{tabular}{@{}l@{\hspace{1em}}l@{\hspace{1em}}l@{}}
1664\multicolumn{1}{c@{\hspace{1em}}}{\textbf{\CFA}}        & \multicolumn{1}{c@{\hspace{1em}}}{\textbf{C}} \\
1665\begin{cfa}
1666extern const * const int x;
1667static const * [ 5 ] const int y;
1668\end{cfa}
1669&
1670\begin{cfa}
1671int extern const * const x;
1672static const int (* const y)[ 5 ]
1673\end{cfa}
1674&
1675\begin{cfa}
1676// external const pointer to const int
1677// internal const pointer to array of 5 const int
1678\end{cfa}
1679\end{tabular}
1680\lstMakeShortInline@%
1681\end{cquote}
1682Specifiers must appear at the start of a \CFA function declaration\footnote{\label{StorageClassSpecifier}
1683The placement of a storage-class specifier other than at the beginning of the declaration specifiers in a declaration is an obsolescent feature.~\cite[\S~6.11.5(1)]{C11}}.
1684
1685The new declaration syntax can be used in other contexts where types are required, \eg casts and the pseudo-function @sizeof@:
1686\begin{cquote}
1687\lstDeleteShortInline@%
1688\begin{tabular}{@{}l@{\hspace{\parindentlnth}}l@{}}
1689\multicolumn{1}{c@{\hspace{\parindentlnth}}}{\textbf{\CFA}}     & \multicolumn{1}{c}{\textbf{C}}        \\
1690\begin{cfa}
1691y = (* int)x;
1692i = sizeof([ 5 ] * int);
1693\end{cfa}
1694&
1695\begin{cfa}
1696y = (int *)x;
1697i = sizeof(int * [ 5 ]);
1698\end{cfa}
1699\end{tabular}
1700\lstMakeShortInline@%
1701\end{cquote}
1702
1703The syntax of the new function-prototype declaration follows directly from the new function-definition syntax;
1704as well, parameter names are optional, \eg:
1705\begin{cfa}
1706[ int x ] f ( /* void */ );                                     $\C{// returning int with no parameters}$
1707[ int x ] f (...);                                                      $\C{// returning int with unknown parameters}$
1708[ * int ] g ( int y );                                          $\C{// returning pointer to int with int parameter}$
1709[ void ] h ( int, char );                                       $\C{// returning no result with int and char parameters}$
1710[ * int, int ] j ( int );                                       $\C{// returning pointer to int and int, with int parameter}$
1711\end{cfa}
1712This syntax allows a prototype declaration to be created by cutting and pasting source text from the function-definition header (or vice versa).
1713Like C, it is possible to declare multiple function-prototypes in a single declaration, where the return type is distributed across \emph{all} function names in the declaration list, \eg:
1714\begin{cquote}
1715\lstDeleteShortInline@%
1716\begin{tabular}{@{}l@{\hspace{\parindentlnth}}l@{}}
1717\multicolumn{1}{c@{\hspace{\parindentlnth}}}{\textbf{\CFA}}     & \multicolumn{1}{c}{\textbf{C}}        \\
1718\begin{cfa}
1719[double] foo(), foo( int ), foo( double ) {...}
1720\end{cfa}
1721&
1722\begin{cfa}
1723double foo1( void ), foo2( int ), foo3( double );
1724\end{cfa}
1725\end{tabular}
1726\lstMakeShortInline@%
1727\end{cquote}
1728where \CFA allows the last function in the list to define its body.
1729
1730The syntax for pointers to \CFA functions specifies the pointer name on the right, \eg:
1731\begin{cfa}
1732* [ int x ] () fp;                                                      $\C{// pointer to function returning int with no parameters}$
1733* [ * int ] ( int y ) gp;                                       $\C{// pointer to function returning pointer to int with int parameter}$
1734* [ ] ( int, char ) hp;                                         $\C{// pointer to function returning no result with int and char parameters}$
1735* [ * int, int ] ( int ) jp;                            $\C{// pointer to function returning pointer to int and int, with int parameter}$
1736\end{cfa}
1737Note, a function name cannot be specified:
1738\begin{cfa}
1739* [ int x ] f () fp;                                            $\C{// function name "f" is disallowed}$
1740\end{cfa}
1741
1742Finally, new \CFA declarations may appear together with C declarations in the same program block, but cannot be mixed within a specific declaration.
1743Therefore, a programmer has the option of either continuing to use traditional C declarations or take advantage of the new style.
1744Clearly, both styles need to be supported for some time due to existing C-style header-files, particularly for UNIX-like systems.
1745
1746
1747\subsection{References}
1748\label{s:References}
1749
1750All variables in C have an \newterm{address}, a \newterm{value}, and a \newterm{type};
1751at the position in the program's memory denoted by the address, there exists a sequence of bits (the value), with the length and semantic meaning of this bit sequence defined by the type.
1752The C type-system does not always track the relationship between a value and its address;
1753a value that does not have a corresponding address is called a \newterm{rvalue} (for ``right-hand value''), while a value that does have an address is called a \newterm{lvalue} (for ``left-hand value'').
1754For example, in @int x; x = 42;@ the variable expression @x@ on the left-hand-side of the assignment is a lvalue, while the constant expression @42@ on the right-hand-side of the assignment is a rvalue.
1755Despite the nomenclature of ``left-hand'' and ``right-hand'', an expression's classification as lvalue or rvalue is entirely dependent on whether it has an address or not; in imperative programming, the address of a value is used for both reading and writing (mutating) a value, and as such, lvalues can be converted to rvalues and read from, but rvalues cannot be mutated because they lack a location to store the updated value.
1756
1757Within a lexical scope, lvalue expressions have an \newterm{address interpretation} for writing a value or a \newterm{value interpretation} to read a value.
1758For example, in @x = y@, @x@ has an address interpretation, while @y@ has a value interpretation.
1759While this duality of interpretation is useful, C lacks a direct mechanism to pass lvalues between contexts, instead relying on \newterm{pointer types} to serve a similar purpose.
1760In C, for any type @T@ there is a pointer type @T *@, the value of which is the address of a value of type @T@.
1761A pointer rvalue can be explicitly \newterm{dereferenced} to the pointed-to lvalue with the dereference operator @*?@, while the rvalue representing the address of a lvalue can be obtained with the address-of operator @&?@.
1762
1763\begin{cfa}
1764int x = 1, y = 2, * p1, * p2, ** p3;
1765p1 = &x;                                                                        $\C{// p1 points to x}$
1766p2 = &y;                                                                        $\C{// p2 points to y}$
1767p3 = &p1;                                                                       $\C{// p3 points to p1}$
1768*p2 = ((*p1 + *p2) * (**p3 - *p1)) / (**p3 - 15);
1769\end{cfa}
1770
1771Unfortunately, the dereference and address-of operators introduce a great deal of syntactic noise when dealing with pointed-to values rather than pointers, as well as the potential for subtle bugs because of pointer arithmetic.
1772For both brevity and clarity, it is desirable for the compiler to figure out how to elide the dereference operators in a complex expression such as the assignment to @*p2@ above.
1773However, since C defines a number of forms of \newterm{pointer arithmetic}, two similar expressions involving pointers to arithmetic types (\eg @*p1 + x@ and @p1 + x@) may each have well-defined but distinct semantics, introducing the possibility that a programmer may write one when they mean the other, and precluding any simple algorithm for elision of dereference operators.
1774To solve these problems, \CFA introduces reference types @T &@;
1775a @T &@ has exactly the same value as a @T *@, but where the @T *@ takes the address interpretation by default, a @T &@ takes the value interpretation by default, as below:
1776
1777\begin{cfa}
1778int x = 1, y = 2, & r1, & r2, && r3;
1779&r1 = &x;                                                                       $\C{// r1 points to x}$
1780&r2 = &y;                                                                       $\C{// r2 points to y}$
1781&&r3 = &&r1;                                                            $\C{// r3 points to r2}$
1782r2 = ((r1 + r2) * (r3 - r1)) / (r3 - 15);       $\C{// implicit dereferencing}$
1783\end{cfa}
1784
1785Except for auto-dereferencing by the compiler, this reference example is exactly the same as the previous pointer example.
1786Hence, a reference behaves like a variable name -- an lvalue expression which is interpreted as a value -- but also has the type system track the address of that value.
1787One way to conceptualize a reference is via a rewrite rule, where the compiler inserts a dereference operator before the reference variable for each reference qualifier in the reference variable declaration, so the previous example implicitly acts like:
1788
1789\begin{cfa}
1790`*`r2 = ((`*`r1 + `*`r2) * (`**`r3 - `*`r1)) / (`**`r3 - 15);
1791\end{cfa}
1792
1793References in \CFA are similar to those in \CC, with important improvements, which can be seen in the example above.
1794Firstly, \CFA does not forbid references to references.
1795This provides a much more orthogonal design for library implementors, obviating the need for workarounds such as @std::reference_wrapper@.
1796Secondly, \CFA references are rebindable, whereas \CC references have a fixed address.
1797\newsavebox{\LstBox}
1798\begin{lrbox}{\LstBox}
1799\lstset{basicstyle=\footnotesize\linespread{0.9}\sf}
1800\begin{cfa}
1801int & r = *new( int );
1802...                                                                                     $\C{// non-null reference}$
1803delete &r;                                                                      $\C{// unmanaged (programmer) memory-management}$
1804r += 1;                                                                         $\C{// undefined reference}$
1805\end{cfa}
1806\end{lrbox}
1807Rebinding allows \CFA references to be default-initialized (\eg to a null pointer\footnote{
1808While effort has been made into non-null reference checking in \CC and Java, the exercise seems moot for any non-managed languages (C/\CC), given that it only handles one of many different error situations:
1809\begin{cquote}
1810\usebox{\LstBox}
1811\end{cquote}
1812}%
1813) and point to different addresses throughout their lifetime, like pointers.
1814Rebinding is accomplished by extending the existing syntax and semantics of the address-of operator in C.
1815
1816In C, the address of a lvalue is always a rvalue, as in general that address is not stored anywhere in memory, and does not itself have an address.
1817In \CFA, the address of a @T &@ is a lvalue @T *@, as the address of the underlying @T@ is stored in the reference, and can thus be mutated there.
1818The result of this rule is that any reference can be rebound using the existing pointer assignment semantics by assigning a compatible pointer into the address of the reference, \eg @&r1 = &x;@ above.
1819This rebinding occurs to an arbitrary depth of reference nesting;
1820loosely speaking, nested address-of operators produce a nested lvalue pointer up to the depth of the reference.
1821These explicit address-of operators can be thought of as ``cancelling out'' the implicit dereference operators, \eg @(&`*`)r1 = &x@ or @(&(&`*`)`*`)r3 = &(&`*`)r1@ or even @(&`*`)r2 = (&`*`)`*`r3@ for @&r2 = &r3@.
1822More precisely:
1823\begin{itemize}
1824\item
1825if @R@ is an rvalue of type {@T &@$_1 \cdots$@ &@$_r$} where $r \ge 1$ references (@&@ symbols) than @&R@ has type {@T `*`&@$_{\color{red}2} \cdots$@ &@$_{\color{red}r}$}, \\ \ie @T@ pointer with $r-1$ references (@&@ symbols).
1826       
1827\item
1828if @L@ is an lvalue of type {@T &@$_1 \cdots$@ &@$_l$} where $l \ge 0$ references (@&@ symbols) then @&L@ has type {@T `*`&@$_{\color{red}1} \cdots$@ &@$_{\color{red}l}$}, \\ \ie @T@ pointer with $l$ references (@&@ symbols).
1829\end{itemize}
1830Since pointers and references share the same internal representation, code using either is equally performant; in fact the \CFA compiler converts references to pointers internally, and the choice between them is made solely on convenience, \eg many pointer or value accesses.
1831
1832By analogy to pointers, \CFA references also allow cv-qualifiers such as @const@:
1833\begin{cfa}
1834const int cx = 5;                                                       $\C{// cannot change cx}$
1835const int & cr = cx;                                            $\C{// cannot change cr's referred value}$
1836&cr = &cx;                                                                      $\C{// rebinding cr allowed}$
1837cr = 7;                                                                         $\C{// ERROR, cannot change cr}$
1838int & const rc = x;                                                     $\C{// must be initialized, like in \CC}$
1839&rc = &x;                                                                       $\C{// ERROR, cannot rebind rc}$
1840rc = 7;                                                                         $\C{// x now equal to 7}$
1841\end{cfa}
1842Given that a reference is meant to represent a lvalue, \CFA provides some syntactic shortcuts when initializing references.
1843There are three initialization contexts in \CFA: declaration initialization, argument/parameter binding, and return/temporary binding.
1844In each of these contexts, the address-of operator on the target lvalue is elided.
1845The syntactic motivation is clearest when considering overloaded operator-assignment, \eg @int ?+=?(int &, int)@; given @int x, y@, the expected call syntax is @x += y@, not @&x += y@.
1846
1847More generally, this initialization of references from lvalues rather than pointers is an instance of a ``lvalue-to-reference'' conversion rather than an elision of the address-of operator;
1848this conversion is used in any context in \CFA where an implicit conversion is allowed.
1849Similarly, use of a the value pointed to by a reference in an rvalue context can be thought of as a ``reference-to-rvalue'' conversion, and \CFA also includes a qualifier-adding ``reference-to-reference'' conversion, analogous to the @T *@ to @const T *@ conversion in standard C.
1850The final reference conversion included in \CFA is ``rvalue-to-reference'' conversion, implemented by means of an implicit temporary.
1851When an rvalue is used to initialize a reference, it is instead used to initialize a hidden temporary value with the same lexical scope as the reference, and the reference is initialized to the address of this temporary.
1852\begin{cfa}
1853struct S { double x, y; };
1854int i, j;
1855void f( int & i, int & j, S & s, int v[] );
1856f( 3, i + j, (S){ 1.0, 7.0 }, (int [3]){ 1, 2, 3 } );   $\C{// pass rvalue to lvalue \(\Rightarrow\) implicit temporary}$
1857\end{cfa}
1858This allows complex values to be succinctly and efficiently passed to functions, without the syntactic overhead of explicit definition of a temporary variable or the runtime cost of pass-by-value.
1859\CC allows a similar binding, but only for @const@ references; the more general semantics of \CFA are an attempt to avoid the \newterm{const hell} problem, in which addition of a @const@ qualifier to one reference requires a cascading chain of added qualifiers.
1860
1861
1862\subsection{Type Nesting}
1863
1864Nested types provide a mechanism to organize associated types and refactor a subset of fields into a named aggregate (\eg sub-aggregates @name@, @address@, @department@, within aggregate @employe@).
1865Java nested types are dynamic (apply to objects), \CC are static (apply to the \lstinline[language=C++]@class@), and C hoists (refactors) nested types into the enclosing scope, meaning there is no need for type qualification.
1866Since \CFA in not object-oriented, adopting dynamic scoping does not make sense;
1867instead \CFA adopts \CC static nesting, using the field-selection operator ``@.@'' for type qualification, as does Java, rather than the \CC type-selection operator ``@::@'' (see Figure~\ref{f:TypeNestingQualification}).
1868\begin{figure}
1869\centering
1870\lstDeleteShortInline@%
1871\begin{tabular}{@{}l@{\hspace{3em}}l|l@{}}
1872\multicolumn{1}{c@{\hspace{3em}}}{\textbf{C Type Nesting}}      & \multicolumn{1}{c}{\textbf{C Implicit Hoisting}}      & \multicolumn{1}{|c}{\textbf{\CFA}}    \\
1873\hline
1874\begin{cfa}
1875struct S {
1876        enum C { R, G, B };
1877        struct T {
1878                union U { int i, j; };
1879                enum C c;
1880                short int i, j;
1881        };
1882        struct T t;
1883} s;
1884
1885int rtn() {
1886        s.t.c = R;
1887        struct T t = { R, 1, 2 };
1888        enum C c;
1889        union U u;
1890}
1891\end{cfa}
1892&
1893\begin{cfa}
1894enum C { R, G, B };
1895union U { int i, j; };
1896struct T {
1897        enum C c;
1898        short int i, j;
1899};
1900struct S {
1901        struct T t;
1902} s;
1903       
1904
1905
1906
1907
1908
1909
1910\end{cfa}
1911&
1912\begin{cfa}
1913struct S {
1914        enum C { R, G, B };
1915        struct T {
1916                union U { int i, j; };
1917                enum C c;
1918                short int i, j;
1919        };
1920        struct T t;
1921} s;
1922
1923int rtn() {
1924        s.t.c = `S.`R;  // type qualification
1925        struct `S.`T t = { `S.`R, 1, 2 };
1926        enum `S.`C c;
1927        union `S.T.`U u;
1928}
1929\end{cfa}
1930\end{tabular}
1931\lstMakeShortInline@%
1932\caption{Type Nesting / Qualification}
1933\label{f:TypeNestingQualification}
1934\end{figure}
1935In the C left example, types @C@, @U@ and @T@ are implicitly hoisted outside of type @S@ into the containing block scope.
1936In the \CFA right example, the types are not hoisted and accessible.
1937
1938
1939\subsection{Constructors and Destructors}
1940\label{s:ConstructorsDestructors}
1941
1942One of the strengths (and weaknesses) of C is memory-management control, allowing resource release to be precisely specified versus unknown release with garbage-collected memory-management.
1943However, this manual approach is often verbose, and it is useful to manage resources other than memory (\eg file handles) using the same mechanism as memory.
1944\CC addresses these issues using Resource Aquisition Is Initialization (RAII), implemented by means of \newterm{constructor} and \newterm{destructor} functions;
1945\CFA adopts constructors and destructors (and @finally@) to facilitate RAII.
1946While constructors and destructors are a common feature of object-oriented programming-languages, they are an independent capability allowing \CFA to adopt them while retaining a procedural paradigm.
1947Specifically, \CFA constructors and destructors are denoted by name and first parameter-type versus name and nesting in an aggregate type.
1948Constructor calls seamlessly integrate with existing C initialization syntax, providing a simple and familiar syntax to C programmers and allowing constructor calls to be inserted into legacy C code with minimal code changes.
1949
1950In \CFA, a constructor is named @?{}@ and a destructor is named @^?{}@\footnote{%
1951The symbol \lstinline+^+ is used for the destructor name because it was the last binary operator that could be used in a unary context.}.
1952The name @{}@ comes from the syntax for the initializer: @struct S { int i, j; } s = `{` 2, 3 `}`@.
1953Like other \CFA operators, these names represent the syntax used to call the constructor or destructor, \eg @?{}(x, ...)@ or @^{}(x, ...)@.
1954The constructor and destructor have return type @void@, and the first parameter is a reference to the object type to be constructed or destructed.
1955While the first parameter is informally called the @this@ parameter, as in object-oriented languages, any variable name may be used.
1956Both constructors and destructors allow additional parametes after the @this@ parameter for specifying values for initialization/de-initialization\footnote{
1957Destruction parameters are useful for specifying storage-management actions, such as de-initialize but not deallocate.}.
1958\begin{cfa}
1959struct VLA { int len, * data; };
1960void ?{}( VLA & vla ) with ( vla ) { len = 10;  data = alloc( len ); }  $\C{// default constructor}$
1961void ^?{}( VLA & vla ) with ( vla ) { free( data ); } $\C{// destructor}$
1962{
1963        VLA x;                                                                  $\C{// implicit:  ?\{\}( x );}$
1964}                                                                                       $\C{// implicit:  ?\^{}\{\}( x );}$
1965\end{cfa}
1966@VLA@ is a \newterm{managed type}\footnote{
1967A managed type affects the runtime environment versus a self-contained type.}: a type requiring a non-trivial constructor or destructor, or with a field of a managed type.
1968A managed type is implicitly constructed at allocation and destructed at deallocation to ensure proper interaction with runtime resources, in this case, the @data@ array in the heap.
1969For details of the code-generation placement of implicit constructor and destructor calls among complex executable statements see~\cite[\S~2.2]{Schluntz17}.
1970
1971\CFA also provides syntax for \newterm{initialization} and \newterm{copy}:
1972\begin{cfa}
1973void ?{}( VLA & vla, int size, char fill ) with ( vla ) {  $\C{// initialization}$
1974        len = size;  data = alloc( len, fill );
1975}
1976void ?{}( VLA & vla, VLA other ) {                      $\C{// copy, shallow}$
1977        vla.len = other.len;  vla.data = other.data;
1978}
1979\end{cfa}
1980(Note, the example is purposely simplified using shallow-copy semantics.)
1981An initialization constructor-call has the same syntax as a C initializer, except the initialization values are passed as arguments to a matching constructor (number and type of paremeters).
1982\begin{cfa}
1983VLA va = `{` 20, 0 `}`,  * arr = alloc()`{` 5, 0 `}`;
1984\end{cfa}
1985Note, the use of a \newterm{constructor expression} to initialize the storage from the dynamic storage-allocation.
1986Like \CC, the copy constructor has two parameters, the second of which is a value parameter with the same type as the first parameter;
1987appropriate care is taken to not recursively call the copy constructor when initializing the second parameter.
1988
1989\CFA constructors may be explicitly call, like Java, and destructors may be explicitly called, like \CC.
1990Explicit calls to constructors double as a \CC-style \emph{placement syntax}, useful for construction of member fields in user-defined constructors and reuse of existing storage allocations.
1991While existing call syntax works for explicit calls to constructors and destructors, \CFA also provides a more concise \newterm{operator syntax} for both:
1992\begin{cfa}
1993{
1994        VLA  x,            y = { 20, 0x01 },     z = y; $\C{// z points to y}$
1995        //      ?{}( x );  ?{}( y, 20, 0x01 );  ?{}( z, y );
1996        ^x{};                                                                   $\C{// deallocate x}$
1997        x{};                                                                    $\C{// reallocate x}$
1998        z{ 5, 0xff };                                                   $\C{// reallocate z, not pointing to y}$
1999        ^y{};                                                                   $\C{// deallocate y}$
2000        y{ x };                                                                 $\C{// reallocate y, points to x}$
2001        x{};                                                                    $\C{// reallocate x, not pointing to y}$
2002        // ^?{}(z);  ^?{}(y);  ^?{}(x);
2003}
2004\end{cfa}
2005
2006To provide a uniform type interface for @otype@ polymorphism, the \CFA compiler automatically generates a default constructor, copy constructor, assignment operator, and destructor for all types.
2007These default functions can be overridden by user-generated versions.
2008For compatibility with the standard behaviour of C, the default constructor and destructor for all basic, pointer, and reference types do nothing, while the copy constructor and assignment operator are bitwise copies;
2009if default zero-initialization is desired, the default constructors can be overridden.
2010For user-generated types, the four functions are also automatically generated.
2011@enum@ types are handled the same as their underlying integral type, and unions are also bitwise copied and no-op initialized and destructed.
2012For compatibility with C, a copy constructor from the first union member type is also defined.
2013For @struct@ types, each of the four functions are implicitly defined to call their corresponding functions on each member of the struct.
2014To better simulate the behaviour of C initializers, a set of \newterm{field constructors} is also generated for structures.
2015A constructor is generated for each non-empty prefix of a structure's member-list to copy-construct the members passed as parameters and default-construct the remaining members.
2016To allow users to limit the set of constructors available for a type, when a user declares any constructor or destructor, the corresponding generated function and all field constructors for that type are hidden from expression resolution;
2017similarly, the generated default constructor is hidden upon declaration of any constructor.
2018These semantics closely mirror the rule for implicit declaration of constructors in \CC\cite[p.~186]{ANSI98:C++}.
2019
2020In some circumstance programmers may not wish to have constructor and destructor calls.
2021In these cases, \CFA provides the initialization syntax \lstinline|S x @= {}|, and the object becomes unmanaged, so implicit constructor and destructor calls are not generated.
2022Any C initializer can be the right-hand side of an \lstinline|@=| initializer, \eg \lstinline|VLA a @= { 0, 0x0 }|, with the usual C initialization semantics.
2023The point of \lstinline|@=| is to provide a migration path from legacy C code to \CFA, by providing a mechanism to incrementally convert to implicit initialization.
2024
2025
2026% \subsection{Default Parameters}
2027
2028
2029\section{Literals}
2030
2031C already includes limited polymorphism for literals -- @0@ can be either an integer or a pointer literal, depending on context, while the syntactic forms of literals of the various integer and float types are very similar, differing from each other only in suffix.
2032In keeping with the general \CFA approach of adding features while respecting the ``C-style'' of doing things, C's polymorphic constants and typed literal syntax are extended to interoperate with user-defined types, while maintaining a backwards-compatible semantics.
2033
2034A simple example is allowing the underscore, as in Ada, to separate prefixes, digits, and suffixes in all \CFA constants, \eg @0x`_`1.ffff`_`ffff`_`p`_`128`_`l@, where the underscore is also the standard separator in C identifiers.
2035\CC uses a single quote as a separator but it is restricted among digits, precluding its use in the literal prefix or suffix, \eg @0x1.ffff@@`'@@ffffp128l@, and causes problems with most IDEs, which must be extended to deal with this alternate use of the single quote.
2036
2037
2038\subsection{Integral Suffixes}
2039
2040Additional integral suffixes are added to cover all the integral types and lengths.
2041\begin{cquote}
2042\lstDeleteShortInline@%
2043\begin{tabular}{@{}l@{\hspace{\parindentlnth}}l@{\hspace{\parindentlnth}}l@{}}
2044\begin{cfa}
204520_`hh`     // signed char
204621_`hhu`   // unsigned char
204722_`h`       // signed short int
204823_`uh`     // unsigned short int
204924_`z`       // size_t
2050\end{cfa}
2051&
2052\begin{cfa}
205320_`L8`      // int8_t
205421_`ul8`     // uint8_t
205522_`l16`     // int16_t
205623_`ul16`   // uint16_t
205724_`l32`     // int32_t
2058\end{cfa}
2059&
2060\begin{cfa}
206125_`ul32`      // uint32_t
206226_`l64`        // int64_t
206327_`l64u`      // uint64_t
206426_`L128`     // int128
206527_`L128u`   // unsigned int128
2066\end{cfa}
2067\end{tabular}
2068\lstMakeShortInline@%
2069\end{cquote}
2070
2071
2072\subsection{0/1}
2073
2074In C, @0@ has the special property that it is the only ``false'' value;
2075from the standard, any value that compares equal to @0@ is false, while any value that compares unequal to @0@ is true.
2076As such, an expression @x@ in any boolean context (such as the condition of an @if@ or @while@ statement, or the arguments to @&&@, @||@, or @?:@\,) can be rewritten as @x != 0@ without changing its semantics.
2077Operator overloading in \CFA provides a natural means to implement this truth-value comparison for arbitrary types, but the C type system is not precise enough to distinguish an equality comparison with @0@ from an equality comparison with an arbitrary integer or pointer.
2078To provide this precision, \CFA introduces a new type @zero_t@ as the type of literal @0@ (somewhat analagous to @nullptr_t@ and @nullptr@ in \CCeleven);
2079@zero_t@ can only take the value @0@, but has implicit conversions to the integer and pointer types so that C code involving @0@ continues to work.
2080With this addition, \CFA rewrites @if (x)@ and similar expressions to @if ((x) != 0)@ or the appropriate analogue, and any type @T@ is ``truthy'' by defining an operator overload @int ?!=?(T, zero_t)@.
2081\CC makes types truthy by adding a conversion to @bool@;
2082prior to the addition of explicit cast operators in \CCeleven, this approach had the pitfall of making truthy types transitively convertable to any numeric type;
2083\CFA avoids this issue.
2084
2085Similarly, \CFA also has a special type for @1@, @one_t@;
2086like @zero_t@, @one_t@ has built-in implicit conversions to the various integral types so that @1@ maintains its expected semantics in legacy code for operations @++@ and @--@.
2087The addition of @one_t@ allows generic algorithms to handle the unit value uniformly for types where it is meaningful.
2088\TODO{Make this sentence true}
2089In particular, polymorphic functions 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)@;
2090analogous overloads for the decrement operators are present as well.
2091
2092
2093\subsection{User Literals}
2094
2095For readability, it is useful to associate units to scale literals, \eg weight (stone, pound, kilogram) or time (seconds, minutes, hours).
2096The left of Figure~\ref{f:UserLiteral} shows the \CFA alternative call-syntax (literal argument before function name), using the backquote, to convert basic literals into user literals.
2097The backquote is a small character, making the unit (function name) predominate.
2098For examples, the multi-precision integer-type in Section~\ref{s:MultiPrecisionIntegers} has user literals:
2099{\lstset{language=CFA,moredelim=**[is][\color{red}]{|}{|},deletedelim=**[is][]{`}{`}}
2100\begin{cfa}
2101y = 9223372036854775807L|`mp| * 18446744073709551615UL|`mp|;
2102y = "12345678901234567890123456789"|`mp| + "12345678901234567890123456789"|`mp|;
2103\end{cfa}
2104Because \CFA uses a standard function, all types and literals are applicable, as well as overloading and conversions.
2105}%
2106
2107The right of Figure~\ref{f:UserLiteral} shows the equivalent \CC version using the underscore for the call-syntax.
2108However, \CC restricts the types, \eg @unsigned long long int@ and @long double@ to represent integral and floating literals.
2109After which, user literals must match (no conversions);
2110hence, it is necessary to overload the unit with all appropriate types.
2111
2112\begin{figure}
2113\centering
2114\lstset{language=CFA,moredelim=**[is][\color{red}]{|}{|},deletedelim=**[is][]{`}{`}}
2115\lstDeleteShortInline@%
2116\begin{tabular}{@{}l@{\hspace{\parindentlnth}}l@{}}
2117\multicolumn{1}{c@{\hspace{\parindentlnth}}}{\textbf{\CFA}}     & \multicolumn{1}{c}{\textbf{\CC}}      \\
2118\begin{cfa}
2119struct W {
2120        double stones;
2121};
2122void ?{}( W & w ) { w.stones = 0; }
2123void ?{}( W & w, double w ) { w.stones = w; }
2124W ?+?( W l, W r ) {
2125        return (W){ l.stones + r.stones };
2126}
2127W |?`st|( double w ) { return (W){ w }; }
2128W |?`lb|( double w ) { return (W){ w / 14.0 }; }
2129W |?`kg|( double w ) { return (W) { w * 0.16 }; }
2130
2131
2132
2133int main() {
2134        W w, heavy = { 20 };
2135        w = 155|`lb|;
2136        w = 0b_1111|`st|;
2137        w = 0_233|`lb|;
2138        w = 0x_9b_u|`kg|;
2139        w = 5.5|`st| + 8|`kg| + 25.01|`lb| + heavy;
2140}
2141\end{cfa}
2142&
2143\begin{cfa}
2144struct W {
2145    double stones;
2146    W() { stones = 0.0; }
2147    W( double w ) { stones = w; }
2148};
2149W operator+( W l, W r ) {
2150        return W( l.stones + r.stones );
2151}
2152W |operator"" _st|( unsigned long long int w ) { return W( w ); }
2153W |operator"" _lb|( unsigned long long int w ) { return W( w / 14.0 ); }
2154W |operator"" _kg|( unsigned long long int w ) { return W( w * 0.16 ); }
2155W |operator"" _st|( long double w ) { return W( w ); }
2156W |operator"" _lb|( long double w ) { return W( w / 14.0 ); }
2157W |operator"" _kg|( long double w ) { return W( w * 0.16 ); }
2158int main() {
2159        W w, heavy = { 20 };
2160        w = 155|_lb|;
2161        w = 0b1111|_lb|;       // error, binary unsupported
2162        w = 0${\color{red}\LstBasicStyle{'}}$233|_lb|;          // quote separator
2163        w = 0x9b|_kg|;
2164        w = 5.5d|_st| + 8|_kg| + 25.01|_lb| + heavy;
2165}
2166\end{cfa}
2167\end{tabular}
2168\lstMakeShortInline@%
2169\caption{User Literal}
2170\label{f:UserLiteral}
2171\end{figure}
2172
2173
2174\section{Libraries}
2175\label{sec:libraries}
2176
2177As stated in Section~\ref{sec:poly-fns}, \CFA inherits a large corpus of library code, where other programming languages must rewrite or provide fragile inter-language communication with C.
2178\CFA has replacement libraries condensing hundreds of existing C names into tens of \CFA overloaded names, all without rewriting the actual computations.
2179In many cases, the interface is an inline wrapper providing overloading during compilation but zero cost at runtime.
2180The following sections give a glimpse of the interface reduction to many C libraries.
2181In many cases, @signed@/@unsigned@ @char@, @short@, and @_Complex@ functions are available (but not shown) to ensure expression computations remain in a single type, as conversions can distort results.
2182
2183
2184\subsection{Limits}
2185
2186C library @limits.h@ provides lower and upper bound constants for the basic types.
2187\CFA name overloading is used to condense these typed constants, \eg:
2188\begin{cquote}
2189\lstDeleteShortInline@%
2190\begin{tabular}{@{}l@{\hspace{\parindentlnth}}l@{}}
2191\multicolumn{1}{c@{\hspace{\parindentlnth}}}{\textbf{Definition}}       & \multicolumn{1}{c}{\textbf{Usage}}    \\
2192\begin{cfa}
2193const short int `MIN` = -32768;
2194const int `MIN` = -2147483648;
2195const long int `MIN` = -9223372036854775808L;
2196\end{cfa}
2197&
2198\begin{cfa}
2199short int si = `MIN`;
2200int i = `MIN`;
2201long int li = `MIN`;
2202\end{cfa}
2203\end{tabular}
2204\lstMakeShortInline@%
2205\end{cquote}
2206The result is a significant reduction in names to access typed constants, \eg:
2207\begin{cquote}
2208\lstDeleteShortInline@%
2209\lstset{basicstyle=\linespread{0.9}\sf\small}
2210\begin{tabular}{@{}l@{\hspace{0.5\parindentlnth}}l@{}}
2211\multicolumn{1}{c@{\hspace{0.5\parindentlnth}}}{\textbf{\CFA}}  & \multicolumn{1}{c}{\textbf{C}}        \\
2212\begin{cfa}
2213MIN
2214MAX
2215PI
2216E
2217\end{cfa}
2218&
2219\begin{cfa}
2220SCHAR_MIN, CHAR_MIN, SHRT_MIN, INT_MIN, LONG_MIN, LLONG_MIN, FLT_MIN, DBL_MIN, LDBL_MIN
2221SCHAR_MAX, UCHAR_MAX, SHRT_MAX, INT_MAX, LONG_MAX, LLONG_MAX, FLT_MAX, DBL_MAX, LDBL_MAX
2222M_PI, M_PIl
2223M_E, M_El
2224\end{cfa}
2225\end{tabular}
2226\lstMakeShortInline@%
2227\end{cquote}
2228
2229
2230\subsection{Math}
2231
2232C library @math.h@ provides many mathematical functions.
2233\CFA function overloading is used to condense these mathematical functions, \eg:
2234\begin{cquote}
2235\lstDeleteShortInline@%
2236\begin{tabular}{@{}l@{\hspace{\parindentlnth}}l@{}}
2237\multicolumn{1}{c@{\hspace{\parindentlnth}}}{\textbf{Definition}}       & \multicolumn{1}{c}{\textbf{Usage}}    \\
2238\begin{cfa}
2239float `log`( float x );
2240double `log`( double );
2241double _Complex `log`( double _Complex x );
2242\end{cfa}
2243&
2244\begin{cfa}
2245float f = `log`( 3.5 );
2246double d = `log`( 3.5 );
2247double _Complex dc = `log`( 3.5+0.5I );
2248\end{cfa}
2249\end{tabular}
2250\lstMakeShortInline@%
2251\end{cquote}
2252The result is a significant reduction in names to access math functions, \eg:
2253\begin{cquote}
2254\lstDeleteShortInline@%
2255\begin{tabular}{@{}l@{\hspace{\parindentlnth}}l@{}}
2256\multicolumn{1}{c@{\hspace{\parindentlnth}}}{\textbf{\CFA}}     & \multicolumn{1}{c}{\textbf{C}}        \\
2257\begin{cfa}
2258log
2259sqrt
2260sin
2261\end{cfa}
2262&
2263\begin{cfa}
2264logf, log, logl, clogf, clog, clogl
2265sqrtf, sqrt, sqrtl, csqrtf, csqrt, csqrtl
2266sinf, sin, sinl, csinf, csin, csinl
2267\end{cfa}
2268\end{tabular}
2269\lstMakeShortInline@%
2270\end{cquote}
2271While \Celeven has type-generic math~\cite[\S~7.25]{C11} in @tgmath.h@ to provide a similar mechanism, these macros are limited, matching a function name with a single set of floating type(s).
2272For example, it is impossible to overload @atan@ for both one and two arguments;
2273instead the names @atan@ and @atan2@ are required (see Section~\ref{s:NameOverloading}).
2274The key observation is that only a restricted set of type-generic macros are provided for a limited set of function names, which do not generalize across the type system, as in \CFA.
2275
2276
2277\subsection{Standard}
2278
2279C library @stdlib.h@ provides many general functions.
2280\CFA function overloading is used to condense these utility functions, \eg:
2281\begin{cquote}
2282\lstDeleteShortInline@%
2283\begin{tabular}{@{}l@{\hspace{\parindentlnth}}l@{}}
2284\multicolumn{1}{c@{\hspace{\parindentlnth}}}{\textbf{Definition}}       & \multicolumn{1}{c}{\textbf{Usage}}    \\
2285\begin{cfa}
2286unsigned int `abs`( int );
2287double `abs`( double );
2288double abs( double _Complex );
2289\end{cfa}
2290&
2291\begin{cfa}
2292unsigned int i = `abs`( -1 );
2293double d = `abs`( -1.5 );
2294double d = `abs`( -1.5+0.5I );
2295\end{cfa}
2296\end{tabular}
2297\lstMakeShortInline@%
2298\end{cquote}
2299The result is a significant reduction in names to access utility functions, \eg:
2300\begin{cquote}
2301\lstDeleteShortInline@%
2302\begin{tabular}{@{}l@{\hspace{\parindentlnth}}l@{}}
2303\multicolumn{1}{c@{\hspace{\parindentlnth}}}{\textbf{\CFA}}     & \multicolumn{1}{c}{\textbf{C}}        \\
2304\begin{cfa}
2305abs
2306strto
2307random
2308\end{cfa}
2309&
2310\begin{cfa}
2311abs, labs, llabs, fabsf, fabs, fabsl, cabsf, cabs, cabsl
2312strtol, strtoul, strtoll, strtoull, strtof, strtod, strtold
2313srand48, mrand48, lrand48, drand48
2314\end{cfa}
2315\end{tabular}
2316\lstMakeShortInline@%
2317\end{cquote}
2318In additon, there are polymorphic functions, like @min@ and @max@, that work on any type with operators @?<?@ or @?>?@.
2319
2320The following shows one example where \CFA \emph{extends} an existing standard C interface to reduce complexity and provide safety.
2321C/\Celeven provide a number of complex and overlapping storage-management operation to support the following capabilities:
2322\begin{description}[topsep=3pt,itemsep=2pt,parsep=0pt]
2323\item[fill]
2324an allocation with a specified character.
2325\item[resize]
2326an existing allocation to decreased or increased its size.
2327In either case, new storage may or may not be allocated and, if there is a new allocation, as much data from the existing allocation is copied.
2328For an increase in storage size, new storage after the copied data may be filled.
2329\item[align]
2330an allocation on a specified memory boundary, \eg, an address multiple of 64 or 128 for cache-line purposes.
2331\item[array]
2332allocation with a specified number of elements.
2333An array may be filled, resized, or aligned.
2334\end{description}
2335Table~\ref{t:StorageManagementOperations} shows the capabilities provided by C/\Celeven allocation-functions and how all the capabilities can be combined into two \CFA functions.
2336\CFA storage-management functions extend the C equivalents by overloading, providing shallow type-safety, and removing the need to specify the base allocation-size.
2337Figure~\ref{f:StorageAllocation} contrasts \CFA and C storage-allocation performing the same operations with the same type safety.
2338
2339\begin{table}
2340\centering
2341\lstDeleteShortInline@%
2342\lstMakeShortInline~%
2343\begin{tabular}{@{}r|r|l|l|l|l@{}}
2344\multicolumn{1}{c}{}&           & \multicolumn{1}{c|}{fill}     & resize        & align & array \\
2345\hline
2346C               & ~malloc~                      & no                    & no            & no            & no    \\
2347                & ~calloc~                      & yes (0 only)  & no            & no            & yes   \\
2348                & ~realloc~                     & no/copy               & yes           & no            & no    \\
2349                & ~memalign~            & no                    & no            & yes           & no    \\
2350                & ~posix_memalign~      & no                    & no            & yes           & no    \\
2351\hline
2352C11             & ~aligned_alloc~       & no                    & no            & yes           & no    \\
2353\hline
2354\CFA    & ~alloc~                       & yes/copy              & no/yes        & no            & yes   \\
2355                & ~align_alloc~         & yes                   & no            & yes           & yes   \\
2356\end{tabular}
2357\lstDeleteShortInline~%
2358\lstMakeShortInline@%
2359\caption{Storage-Management Operations}
2360\label{t:StorageManagementOperations}
2361\end{table}
2362
2363\begin{figure}
2364\centering
2365\begin{cquote}
2366\begin{cfa}[aboveskip=0pt]
2367size_t  dim = 10;                                                       $\C{// array dimension}$
2368char fill = '\xff';                                                     $\C{// initialization fill value}$
2369int * ip;
2370\end{cfa}
2371\lstDeleteShortInline@%
2372\begin{tabular}{@{}l@{\hspace{\parindentlnth}}l@{}}
2373\multicolumn{1}{c@{\hspace{\parindentlnth}}}{\textbf{\CFA}}     & \multicolumn{1}{c}{\textbf{C}}        \\
2374\begin{cfa}
2375ip = alloc();
2376ip = alloc( fill );
2377ip = alloc( dim );
2378ip = alloc( dim, fill );
2379ip = alloc( ip, 2 * dim );
2380ip = alloc( ip, 4 * dim, fill );
2381
2382ip = align_alloc( 16 );
2383ip = align_alloc( 16, fill );
2384ip = align_alloc( 16, dim );
2385ip = align_alloc( 16, dim, fill );
2386\end{cfa}
2387&
2388\begin{cfa}
2389ip = (int *)malloc( sizeof( int ) );
2390ip = (int *)malloc( sizeof( int ) ); memset( ip, fill, sizeof( int ) );
2391ip = (int *)malloc( dim * sizeof( int ) );
2392ip = (int *)malloc( sizeof( int ) ); memset( ip, fill, dim * sizeof( int ) );
2393ip = (int *)realloc( ip, 2 * dim * sizeof( int ) );
2394ip = (int *)realloc( ip, 4 * dim * sizeof( int ) ); memset( ip, fill, 4 * dim * sizeof( int ) );
2395
2396ip = memalign( 16, sizeof( int ) );
2397ip = memalign( 16, sizeof( int ) ); memset( ip, fill, sizeof( int ) );
2398ip = memalign( 16, dim * sizeof( int ) );
2399ip = memalign( 16, dim * sizeof( int ) ); memset( ip, fill, dim * sizeof( int ) );
2400\end{cfa}
2401\end{tabular}
2402\lstMakeShortInline@%
2403\end{cquote}
2404\caption{\CFA versus C Storage-Allocation}
2405\label{f:StorageAllocation}
2406\end{figure}
2407
2408Variadic @new@ (see Section~\ref{sec:variadic-tuples}) cannot support the same overloading because extra parameters are for initialization.
2409Hence, there are @new@ and @anew@ functions for single and array variables, and the fill value is the arguments to the constructor, \eg:
2410\begin{cfa}
2411struct S { int i, j; };
2412void ?{}( S & s, int i, int j ) { s.i = i; s.j = j; }
2413S * s = new( 2, 3 );                                            $\C{// allocate storage and run constructor}$
2414S * as = anew( dim, 2, 3 );                                     $\C{// each array element initialized to 2, 3}$
2415\end{cfa}
2416Note, \CC can only initialization array elements via the default constructor.
2417
2418Finally, the \CFA memory-allocator has \newterm{sticky properties} for dynamic storage: fill and alignment are remembered with an object's storage in the heap.
2419When a @realloc@ is performed, the sticky properties are respected, so that new storage is correctly aligned and initialized with the fill character.
2420
2421
2422\subsection{I/O}
2423\label{s:IOLibrary}
2424
2425The goal of \CFA I/O is to simplify the common cases, while fully supporting polymorphism and user defined types in a consistent way.
2426The approach combines ideas from \CC and Python.
2427The \CFA header file for the I/O library is @fstream@.
2428
2429The common case is printing out a sequence of variables separated by whitespace.
2430\begin{cquote}
2431\lstDeleteShortInline@%
2432\begin{tabular}{@{}l@{\hspace{\parindentlnth}}l@{}}
2433\multicolumn{1}{c@{\hspace{\parindentlnth}}}{\textbf{\CFA}}     & \multicolumn{1}{c}{\textbf{\CC}}      \\
2434\begin{cfa}
2435int x = 1, y = 2, z = 3;
2436sout | x `|` y `|` z | endl;
2437\end{cfa}
2438&
2439\begin{cfa}
2440
2441cout << x `<< " "` << y `<< " "` << z << endl;
2442\end{cfa}
2443\\
2444\begin{cfa}[showspaces=true,aboveskip=0pt,belowskip=0pt]
24451` `2` `3
2446\end{cfa}
2447&
2448\begin{cfa}[showspaces=true,aboveskip=0pt,belowskip=0pt]
24491 2 3
2450\end{cfa}
2451\end{tabular}
2452\lstMakeShortInline@%
2453\end{cquote}
2454The \CFA form has half the characters of the \CC form, and is similar to Python I/O with respect to implicit separators.
2455Similar simplification occurs for tuple I/O, which prints all tuple values separated by ``\lstinline[showspaces=true]@, @''.
2456\begin{cfa}
2457[int, [ int, int ] ] t1 = [ 1, [ 2, 3 ] ], t2 = [ 4, [ 5, 6 ] ];
2458sout | t1 | t2 | endl;                                  $\C{// print tuples}$
2459\end{cfa}
2460\begin{cfa}[showspaces=true,aboveskip=0pt]
24611`, `2`, `3 4`, `5`, `6
2462\end{cfa}
2463Finally, \CFA uses the logical-or operator for I/O as it is the lowest-priority overloadable operator, other than assignment.
2464Therefore, fewer output expressions require parenthesis.
2465\begin{cquote}
2466\lstDeleteShortInline@%
2467\begin{tabular}{@{}ll@{}}
2468\textbf{\CFA:}
2469&
2470\begin{cfa}
2471sout | x * 3 | y + 1 | z << 2 | x == y | (x | y) | (x || y) | (x > z ? 1 : 2) | endl;
2472\end{cfa}
2473\\
2474\textbf{\CC:}
2475&
2476\begin{cfa}
2477cout << x * 3 << y + 1 << `(`z << 2`)` << `(`x == y`)` << (x | y) << (x || y) << (x > z ? 1 : 2) << endl;
2478\end{cfa}
2479\\
2480&
2481\begin{cfa}[showspaces=true,aboveskip=0pt]
24823 3 12 0 3 1 2
2483\end{cfa}
2484\end{tabular}
2485\lstMakeShortInline@%
2486\end{cquote}
2487There is a weak similarity between the \CFA logical-or operator and the Shell pipe-operator for moving data, where data flows in the correct direction for input but the opposite direction for output.
2488
2489The implicit separator character (space/blank) is a separator not a terminator.
2490The rules for implicitly adding the separator are:
2491\begin{itemize}
2492\item
2493A separator does not appear at the start or end of a line.
2494\item
2495A separator does not appear before or after a character literal or variable.
2496\item
2497A separator does not appear before or after a null (empty) C string, which is a local mechanism to disable insertion of the separator character.
2498\item
2499A separator does not appear before a C string starting with the characters: \lstinline[mathescape=off,basicstyle=\tt]@([{=$@
2500\item
2501A separator does not appear after a C string ending with the characters: \lstinline[basicstyle=\tt]@,.;!?)]}%@
2502\item
2503{\lstset{language=CFA,deletedelim=**[is][]{`}{`}}
2504A separator does not appear before or after a C string beginning/ending with the quote or whitespace characters: \lstinline[basicstyle=\tt,showspaces=true]@`'": \t\v\f\r\n@
2505}%
2506\end{itemize}
2507There are functions to set and get the separator string, and manipulators to toggle separation on and off in the middle of output.
2508
2509
2510\subsection{Multi-precision Integers}
2511\label{s:MultiPrecisionIntegers}
2512
2513\CFA has an interface to the GMP multi-precision signed-integers~\cite{GMP}, similar to the \CC interface provided by GMP.
2514The \CFA interface wraps GMP functions into operator functions to make programming with multi-precision integers identical to using fixed-sized integers.
2515The \CFA type name for multi-precision signed-integers is @Int@ and the header file is @gmp@.
2516The following multi-precision factorial programs contrast using GMP with the \CFA and C interfaces.
2517\begin{cquote}
2518\lstDeleteShortInline@%
2519\begin{tabular}{@{}l@{\hspace{\parindentlnth}}@{\hspace{\parindentlnth}}l@{}}
2520\multicolumn{1}{c@{\hspace{\parindentlnth}}}{\textbf{\CFA}}     & \multicolumn{1}{@{\hspace{\parindentlnth}}c}{\textbf{C}}      \\
2521\begin{cfa}
2522#include <gmp>
2523int main( void ) {
2524        sout | "Factorial Numbers" | endl;
2525        Int fact = 1;
2526        sout | 0 | fact | endl;
2527        for ( unsigned int i = 1; i <= 40; i += 1 ) {
2528                fact *= i;
2529                sout | i | fact | endl;
2530        }
2531}
2532\end{cfa}
2533&
2534\begin{cfa}
2535#include <gmp.h>
2536int main( void ) {
2537        `gmp_printf`( "Factorial Numbers\n" );
2538        `mpz_t` fact;  `mpz_init_set_ui`( fact, 1 );
2539        `gmp_printf`( "%d %Zd\n", 0, fact );
2540        for ( unsigned int i = 1; i <= 40; i += 1 ) {
2541                `mpz_mul_ui`( fact, fact, i );
2542                `gmp_printf`( "%d %Zd\n", i, fact );
2543        }
2544}
2545\end{cfa}
2546\end{tabular}
2547\lstMakeShortInline@%
2548\end{cquote}
2549
2550
2551\section{Evaluation}
2552\label{sec:eval}
2553
2554Though \CFA provides significant added functionality over C, these features have a low runtime penalty.
2555In fact, \CFA's features for generic programming can enable faster runtime execution than idiomatic @void *@-based C code.
2556This claim is demonstrated through a set of generic-code-based micro-benchmarks in C, \CFA, and \CC (see stack implementations in Appendix~\ref{sec:BenchmarkStackImplementation}).
2557Since all these languages share a subset essentially comprising standard C, maximal-performance benchmarks would show little runtime variance, other than in length and clarity of source code.
2558A more illustrative benchmark measures the costs of idiomatic usage of each language's features.
2559Figure~\ref{fig:BenchmarkTest} shows the \CFA benchmark tests for a generic stack based on a singly linked-list, a generic pair-data-structure, and a variadic @print@ function similar to that in Section~\ref{sec:variadic-tuples}.
2560The benchmark test is similar for C and \CC.
2561The experiment uses element types @int@ and @pair(_Bool, char)@, and pushes $N=40M$ elements on a generic stack, copies the stack, clears one of the stacks, finds the maximum value in the other stack, and prints $N/2$ (to reduce graph height) constants.
2562
2563\begin{figure}
2564\begin{cfa}[xleftmargin=3\parindentlnth,aboveskip=0pt,belowskip=0pt]
2565int main( int argc, char * argv[] ) {
2566        int max = 0, val = 42;
2567        stack( int ) si, ti;
2568
2569        REPEAT_TIMED( "push_int", N, push( si, val ); )
2570        TIMED( "copy_int", ti = si; )
2571        TIMED( "clear_int", clear( si ); )
2572        REPEAT_TIMED( "pop_int", N, int x = pop( ti ); if ( x > max ) max = x; )
2573
2574        pair( _Bool, char ) max = { (_Bool)0, '\0' }, val = { (_Bool)1, 'a' };
2575        stack( pair( _Bool, char ) ) sp, tp;
2576
2577        REPEAT_TIMED( "push_pair", N, push( sp, val ); )
2578        TIMED( "copy_pair", tp = sp; )
2579        TIMED( "clear_pair", clear( sp ); )
2580        REPEAT_TIMED( "pop_pair", N, pair(_Bool, char) x = pop( tp ); if ( x > max ) max = x; )
2581}
2582\end{cfa}
2583\caption{\protect\CFA Benchmark Test}
2584\label{fig:BenchmarkTest}
2585\end{figure}
2586
2587The structure of each benchmark implemented is: C with @void *@-based polymorphism, \CFA with the presented features, \CC with templates, and \CC using only class inheritance for polymorphism, called \CCV.
2588The \CCV variant illustrates an alternative object-oriented idiom where all objects inherit from a base @object@ class, mimicking a Java-like interface;
2589hence runtime checks are necessary to safely down-cast objects.
2590The most notable difference among the implementations is in memory layout of generic types: \CFA and \CC inline the stack and pair elements into corresponding list and pair nodes, while C and \CCV lack such a capability and instead must store generic objects via pointers to separately-allocated objects.
2591For the print benchmark, idiomatic printing is used: the C and \CFA variants used @stdio.h@, while the \CC and \CCV variants used @iostream@; preliminary tests show this distinction has negligible runtime impact.
2592Note, the C benchmark uses unchecked casts as there is no runtime mechanism to perform such checks, while \CFA and \CC provide type-safety statically.
2593
2594Figure~\ref{fig:eval} and Table~\ref{tab:eval} show the results of running the benchmark in Figure~\ref{fig:BenchmarkTest} and its C, \CC, and \CCV equivalents.
2595The graph plots the median of 5 consecutive runs of each program, with an initial warm-up run omitted.
2596All code is compiled at \texttt{-O2} by gcc or g++ 6.2.0, with all \CC code compiled as \CCfourteen.
2597The benchmarks are run on an Ubuntu 16.04 workstation with 16 GB of RAM and a 6-core AMD FX-6300 CPU with 3.5 GHz maximum clock frequency.
2598
2599\begin{figure}
2600\centering
2601\input{timing}
2602\caption{Benchmark Timing Results (smaller is better)}
2603\label{fig:eval}
2604\end{figure}
2605
2606\begin{table}
2607\centering
2608\caption{Properties of benchmark code}
2609\label{tab:eval}
2610\newcommand{\CT}[1]{\multicolumn{1}{c}{#1}}
2611\begin{tabular}{rrrrr}
2612                                                                        & \CT{C}        & \CT{\CFA}     & \CT{\CC}      & \CT{\CCV}             \\ \hline
2613maximum memory usage (MB)                       & 10001         & 2502          & 2503          & 11253                 \\
2614source code size (lines)                        & 247           & 222           & 165           & 339                   \\
2615redundant type annotations (lines)      & 39            & 2                     & 2                     & 15                    \\
2616binary size (KB)                                        & 14            & 229           & 18            & 38                    \\
2617\end{tabular}
2618\end{table}
2619
2620The C and \CCV variants are generally the slowest with the largest memory footprint, because of their less-efficient memory layout and the pointer-indirection necessary to implement generic types;
2621this inefficiency is exacerbated by the second level of generic types in the pair-based benchmarks.
2622By contrast, the \CFA and \CC variants run in roughly equivalent time for both the integer and pair of @_Bool@ and @char@ because the storage layout is equivalent, with the inlined libraries (\ie no separate compilation) and greater maturity of the \CC compiler contributing to its lead.
2623\CCV is slower than C largely due to the cost of runtime type-checking of down-casts (implemented with @dynamic_cast@);
2624There are two outliers in the graph for \CFA: all prints and pop of @pair@.
2625Both of these cases result from the complexity of the C-generated polymorphic code, so that the gcc compiler is unable to optimize some dead code and condense nested calls.
2626A compiler designed for \CFA could easily perform these optimizations.
2627Finally, the binary size for \CFA is larger because of static linking with the \CFA libraries.
2628
2629\CFA is also competitive in terms of source code size, measured as a proxy for programmer effort. The line counts in Table~\ref{tab:eval} include implementations of @pair@ and @stack@ types for all four languages for purposes of direct comparison, though it should be noted that \CFA and \CC have pre-written data structures in their standard libraries that programmers would generally use instead. Use of these standard library types has minimal impact on the performance benchmarks, but shrinks the \CFA and \CC benchmarks to 73 and 54 lines, respectively.
2630On the other hand, C does not have a generic collections-library in its standard distribution, resulting in frequent reimplementation of such collection types by C programmers.
2631\CCV does not use the \CC standard template library by construction, and in fact includes the definition of @object@ and wrapper classes for @bool@, @char@, @int@, and @const char *@ in its line count, which inflates this count somewhat, as an actual object-oriented language would include these in the standard library;
2632with their omission, the \CCV line count is similar to C.
2633We justify the given line count by noting that many object-oriented languages do not allow implementing new interfaces on library types without subclassing or wrapper types, which may be similarly verbose.
2634
2635Raw line-count, however, is a fairly rough measure of code complexity;
2636another important factor is how much type information the programmer must manually specify, especially where that information is not checked by the compiler.
2637Such unchecked type information produces a heavier documentation burden and increased potential for runtime bugs, and is much less common in \CFA than C, with its manually specified function pointers arguments and format codes, or \CCV, with its extensive use of un-type-checked downcasts (\eg @object@ to @integer@ when popping a stack, or @object@ to @printable@ when printing the elements of a @pair@).
2638To quantify this, the ``redundant type annotations'' line in Table~\ref{tab:eval} counts the number of lines on which the type of a known variable is re-specified, either as a format specifier, explicit downcast, type-specific function, or by name in a @sizeof@, struct literal, or @new@ expression.
2639The \CC benchmark uses two redundant type annotations to create a new stack nodes, while the C and \CCV benchmarks have several such annotations spread throughout their code.
2640The two instances in which the \CFA benchmark still uses redundant type specifiers are to cast the result of a polymorphic @malloc@ call (the @sizeof@ argument is inferred by the compiler).
2641These uses are similar to the @new@ expressions in \CC, though the \CFA compiler's type resolver should shortly render even these type casts superfluous.
2642
2643
2644\section{Related Work}
2645
2646
2647\subsection{Polymorphism}
2648
2649\CC is the most similar language to \CFA;
2650both are extensions to C with source and runtime backwards compatibility.
2651The fundamental difference is the engineering approach to maintain C compatibility and programmer expectation.
2652While \CC provides good compatibility with C, it has a steep learning curve for many of its extensions.
2653For example, polymorphism is provided via three disjoint mechanisms: overloading, inheritance, and templates.
2654The overloading is restricted because resolution does not use the return type, inheritance requires learning object-oriented programming and coping with a restricted nominal-inheritance hierarchy, templates cannot be separately compiled resulting in compilation/code bloat and poor error messages, and determining how these mechanisms interact and which to use is confusing.
2655In contrast, \CFA has a single facility for polymorphic code supporting type-safe separate-compilation of polymorphic functions and generic (opaque) types, which uniformly leverage the C procedural paradigm.
2656The key mechanism to support separate compilation is \CFA's \emph{explicit} use of assumed type properties.
2657Until \CC concepts~\cite{C++Concepts} are standardized (anticipated for \CCtwenty), \CC provides no way to specify the requirements of a generic function beyond compilation errors during template expansion;
2658furthermore, \CC concepts are restricted to template polymorphism.
2659
2660Cyclone~\cite{Grossman06} also provides capabilities for polymorphic functions and existential types, similar to \CFA's @forall@ functions and generic types.
2661Cyclone existential types can include function pointers in a construct similar to a virtual function-table, but these pointers must be explicitly initialized at some point in the code, a tedious and potentially error-prone process.
2662Furthermore, Cyclone's polymorphic functions and types are restricted to abstraction over types with the same layout and calling convention as @void *@, \ie only pointer types and @int@.
2663In \CFA terms, all Cyclone polymorphism must be dtype-static.
2664While the Cyclone design provides the efficiency benefits discussed in Section~\ref{sec:generic-apps} for dtype-static polymorphism, it is more restrictive than \CFA's general model.
2665Smith and Volpano~\cite{Smith98} present Polymorphic C, an ML dialect with polymorphic functions, C-like syntax, and pointer types; it lacks many of C's features, however, most notably structure types, and so is not a practical C replacement.
2666
2667Objective-C~\cite{obj-c-book} is an industrially successful extension to C.
2668However, Objective-C is a radical departure from C, using an object-oriented model with message-passing.
2669Objective-C did not support type-checked generics until recently \cite{xcode7}, historically using less-efficient runtime checking of object types.
2670The GObject~\cite{GObject} framework also adds object-oriented programming with runtime type-checking and reference-counting garbage-collection to C;
2671these features are more intrusive additions than those provided by \CFA, in addition to the runtime overhead of reference-counting.
2672Vala~\cite{Vala} compiles to GObject-based C, adding the burden of learning a separate language syntax to the aforementioned demerits of GObject as a modernization path for existing C code-bases.
2673Java~\cite{Java8} included generic types in Java~5, which are type-checked at compilation and type-erased at runtime, similar to \CFA's.
2674However, in Java, each object carries its own table of method pointers, while \CFA passes the method pointers separately to maintain a C-compatible layout.
2675Java is also a garbage-collected, object-oriented language, with the associated resource usage and C-interoperability burdens.
2676
2677D~\cite{D}, Go, and Rust~\cite{Rust} are modern, compiled languages with abstraction features similar to \CFA traits, \emph{interfaces} in D and Go and \emph{traits} in Rust.
2678However, each language represents a significant departure from C in terms of language model, and none has the same level of compatibility with C as \CFA.
2679D and Go are garbage-collected languages, imposing the associated runtime overhead.
2680The necessity of accounting for data transfer between managed runtimes and the unmanaged C runtime complicates foreign-function interfaces to C.
2681Furthermore, while generic types and functions are available in Go, they are limited to a small fixed set provided by the compiler, with no language facility to define more.
2682D restricts garbage collection to its own heap by default, while Rust is not garbage-collected, and thus has a lighter-weight runtime more interoperable with C.
2683Rust also possesses much more powerful abstraction capabilities for writing generic code than Go.
2684On the other hand, Rust's borrow-checker provides strong safety guarantees but is complex and difficult to learn and imposes a distinctly idiomatic programming style.
2685\CFA, with its more modest safety features, allows direct ports of C code while maintaining the idiomatic style of the original source.
2686
2687
2688\subsection{Tuples/Variadics}
2689
2690Many programming languages have some form of tuple construct and/or variadic functions, \eg SETL, C, KW-C, \CC, D, Go, Java, ML, and Scala.
2691SETL~\cite{SETL} is a high-level mathematical programming language, with tuples being one of the primary data types.
2692Tuples in SETL allow subscripting, dynamic expansion, and multiple assignment.
2693C provides variadic functions through @va_list@ objects, but the programmer is responsible for managing the number of arguments and their types, so the mechanism is type unsafe.
2694KW-C~\cite{Buhr94a}, a predecessor of \CFA, introduced tuples to C as an extension of the C syntax, taking much of its inspiration from SETL.
2695The main contributions of that work were adding MRVF, tuple mass and multiple assignment, and record-field access.
2696\CCeleven introduced @std::tuple@ as a library variadic template structure.
2697Tuples are a generalization of @std::pair@, in that they allow for arbitrary length, fixed-size aggregation of heterogeneous values.
2698Operations include @std::get<N>@ to extract values, @std::tie@ to create a tuple of references used for assignment, and lexicographic comparisons.
2699\CCseventeen proposes \emph{structured bindings}~\cite{Sutter15} to eliminate pre-declaring variables and use of @std::tie@ for binding the results.
2700This 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.
2701Furthermore, structured bindings are not a full replacement for @std::tie@, as it always declares new variables.
2702Like \CC, D provides tuples through a library variadic-template structure.
2703Go does not have tuples but supports MRVF.
2704Java's variadic functions appear similar to C's but are type-safe using homogeneous arrays, which are less useful than \CFA's heterogeneously-typed variadic functions.
2705Tuples are a fundamental abstraction in most functional programming languages, such as Standard ML~\cite{sml} and~\cite{Scala}, which decompose tuples using pattern matching.
2706
2707
2708\subsection{Control Structures / Declarations / Literals}
2709
2710Java has default fall through like C/\CC.
2711Pascal/Ada/Go/Rust do not have default fall through.
2712\Csharp does not have fall through but still requires a break.
2713Python uses dictionary mapping. \\
2714\CFA choose is like Rust match.
2715
2716Java has labelled break/continue. \\
2717Languages with and without exception handling.
2718
2719Alternative C declarations. \\
2720Different references \\
2721Constructors/destructors
2722
27230/1 Literals \\
2724user defined: D, Objective-C
2725
2726\section{Conclusion and Future Work}
2727
2728The goal of \CFA is to provide an evolutionary pathway for large C development-environments to be more productive and safer, while respecting the talent and skill of C programmers.
2729While other programming languages purport to be a better C, they are in fact new and interesting languages in their own right, but not C extensions.
2730The purpose of this paper is to introduce \CFA, and showcase language features that illustrate the \CFA type-system and approaches taken to achieve the goal of evolutionary C extension.
2731The contributions are a powerful type-system using parametric polymorphism and overloading, generic types, tuples, advanced control structures, and extended declarations, which all have complex interactions.
2732The work is a challenging design, engineering, and implementation exercise.
2733On the surface, the project may appear as a rehash of similar mechanisms in \CC.
2734However, every \CFA feature is different than its \CC counterpart, often with extended functionality, better integration with C and its programmers, and always supporting separate compilation.
2735All of these new features are being used by the \CFA development-team to build the \CFA runtime-system.
2736Finally, we demonstrate that \CFA performance for some idiomatic cases is better than C and close to \CC, showing the design is practically applicable.
2737
2738There is ongoing work on a wide range of \CFA feature extensions, including arrays with size, user-defined conversions, concurrent primitives, and modules.
2739(While all examples in the paper compile and run, a public beta-release of \CFA will take another 8--12 months to finalize these additional extensions.)
2740In addition, there are interesting future directions for the polymorphism design.
2741Notably, \CC template functions trade compile time and code bloat for optimal runtime of individual instantiations of polymorphic functions.
2742\CFA polymorphic functions use dynamic virtual-dispatch;
2743the runtime overhead of this approach is low, but not as low as inlining, and it may be beneficial to provide a mechanism for performance-sensitive code.
2744Two promising approaches are an @inline@ annotation at polymorphic function call sites to create a template-specialization of the function (provided the code is visible) or placing an @inline@ annotation on polymorphic function-definitions to instantiate a specialized version for some set of types (\CC template specialization).
2745These approaches are not mutually exclusive and allow performance optimizations to be applied only when necessary, without suffering global code-bloat.
2746In general, we believe separate compilation, producing smaller code, works well with loaded hardware-caches, which may offset the benefit of larger inlined-code.
2747
2748
2749\section{Acknowledgments}
2750
2751The authors would like to recognize the design assistance of Glen Ditchfield, Richard Bilson, Thierry Delisle, and Andrew Beach on the features described in this paper, and thank Magnus Madsen for feedback in the writing.
2752This work is supported through a corporate partnership with Huawei Ltd.\ (\url{http://www.huawei.com}), and Aaron Moss and Peter Buhr are partially funded by the Natural Sciences and Engineering Research Council of Canada.
2753
2754% the first author's \grantsponsor{NSERC-PGS}{NSERC PGS D}{http://www.nserc-crsng.gc.ca/Students-Etudiants/PG-CS/BellandPostgrad-BelletSuperieures_eng.asp} scholarship.
2755
2756
2757\bibliographystyle{plain}
2758\bibliography{pl}
2759
2760
2761\appendix
2762
2763\section{Benchmark Stack Implementation}
2764\label{sec:BenchmarkStackImplementation}
2765
2766\lstset{basicstyle=\linespread{0.9}\sf\small}
2767
2768Throughout, @/***/@ designates a counted redundant type annotation.
2769
2770\smallskip\noindent
2771\CFA
2772\begin{cfa}[xleftmargin=2\parindentlnth,aboveskip=0pt,belowskip=0pt]
2773forall(otype T) struct stack_node;
2774forall(otype T) struct stack {
2775        stack_node(T) * head;
2776};
2777forall(otype T) struct stack_node {
2778        T value;
2779        stack_node(T) * next;
2780};
2781forall(otype T) void ?{}( stack(T) & s ) { (s.head){ 0 }; }
2782forall(otype T) void ?{}( stack(T) & s, stack(T) t ) {
2783        stack_node(T) ** crnt = &s.head;
2784        for ( stack_node(T) * next = t.head; next; next = next->next ) {
2785                stack_node(T) * new_node = ((stack_node(T)*)malloc());
2786                (*new_node){ next->value }; /***/
2787                *crnt = new_node;
2788                stack_node(T) * acrnt = *crnt;
2789                crnt = &acrnt->next;
2790        }
2791        *crnt = 0;
2792}
2793forall(otype T) stack(T) ?=?( stack(T) & s, stack(T) t ) {
2794        if ( s.head == t.head ) return s;
2795        clear( s );
2796        s{ t };
2797        return s;
2798}
2799forall(otype T) void ^?{}( stack(T) & s) { clear( s ); }
2800forall(otype T) _Bool empty( const stack(T) & s ) { return s.head == 0; }
2801forall(otype T) void push( stack(T) & s, T value ) {
2802        stack_node(T) * new_node = ((stack_node(T)*)malloc());
2803        (*new_node){ value, s.head }; /***/
2804        s.head = new_node;
2805}
2806forall(otype T) T pop( stack(T) & s ) {
2807        stack_node(T) * n = s.head;
2808        s.head = n->next;
2809        T v = n->value;
2810        delete( n );
2811        return v;
2812}
2813forall(otype T) void clear( stack(T) & s ) {
2814        for ( stack_node(T) * next = s.head; next; ) {
2815                stack_node(T) * crnt = next;
2816                next = crnt->next;
2817                delete( crnt );
2818        }
2819        s.head = 0;
2820}
2821\end{cfa}
2822
2823\medskip\noindent
2824\CC
2825\begin{cfa}[xleftmargin=2\parindentlnth,aboveskip=0pt,belowskip=0pt]
2826template<typename T> class stack {
2827        struct node {
2828                T value;
2829                node * next;
2830                node( const T & v, node * n = nullptr ) : value(v), next(n) {}
2831        };
2832        node * head;
2833        void copy(const stack<T>& o) {
2834                node ** crnt = &head;
2835                for ( node * next = o.head;; next; next = next->next ) {
2836                        *crnt = new node{ next->value }; /***/
2837                        crnt = &(*crnt)->next;
2838                }
2839                *crnt = nullptr;
2840        }
2841  public:
2842        stack() : head(nullptr) {}
2843        stack(const stack<T>& o) { copy(o); }
2844        stack(stack<T> && o) : head(o.head) { o.head = nullptr; }
2845        ~stack() { clear(); }
2846        stack & operator= (const stack<T>& o) {
2847                if ( this == &o ) return *this;
2848                clear();
2849                copy(o);
2850                return *this;
2851        }
2852        stack & operator= (stack<T> && o) {
2853                if ( this == &o ) return *this;
2854                head = o.head;
2855                o.head = nullptr;
2856                return *this;
2857        }
2858        bool empty() const { return head == nullptr; }
2859        void push(const T & value) { head = new node{ value, head };  /***/ }
2860        T pop() {
2861                node * n = head;
2862                head = n->next;
2863                T x = std::move(n->value);
2864                delete n;
2865                return x;
2866        }
2867        void clear() {
2868                for ( node * next = head; next; ) {
2869                        node * crnt = next;
2870                        next = crnt->next;
2871                        delete crnt;
2872                }
2873                head = nullptr;
2874        }
2875};
2876\end{cfa}
2877
2878\medskip\noindent
2879C
2880\begin{cfa}[xleftmargin=2\parindentlnth,aboveskip=0pt,belowskip=0pt]
2881struct stack_node {
2882        void * value;
2883        struct stack_node * next;
2884};
2885struct stack new_stack() { return (struct stack){ NULL }; /***/ }
2886void copy_stack(struct stack * s, const struct stack * t, void * (*copy)(const void *)) {
2887        struct stack_node ** crnt = &s->head;
2888        for ( struct stack_node * next = t->head; next; next = next->next ) {
2889                *crnt = malloc(sizeof(struct stack_node)); /***/
2890                **crnt = (struct stack_node){ copy(next->value) }; /***/
2891                crnt = &(*crnt)->next;
2892        }
2893        *crnt = 0;
2894}
2895_Bool stack_empty(const struct stack * s) { return s->head == NULL; }
2896void push_stack(struct stack * s, void * value) {
2897        struct stack_node * n = malloc(sizeof(struct stack_node)); /***/
2898        *n = (struct stack_node){ value, s->head }; /***/
2899        s->head = n;
2900}
2901void * pop_stack(struct stack * s) {
2902        struct stack_node * n = s->head;
2903        s->head = n->next;
2904        void * x = n->value;
2905        free(n);
2906        return x;
2907}
2908void clear_stack(struct stack * s, void (*free_el)(void *)) {
2909        for ( struct stack_node * next = s->head; next; ) {
2910                struct stack_node * crnt = next;
2911                next = crnt->next;
2912                free_el(crnt->value);
2913                free(crnt);
2914        }
2915        s->head = NULL;
2916}
2917\end{cfa}
2918
2919\medskip\noindent
2920\CCV
2921\begin{cfa}[xleftmargin=2\parindentlnth,aboveskip=0pt,belowskip=0pt]
2922stack::node::node( const object & v, node * n ) : value( v.new_copy() ), next( n ) {}
2923void stack::copy(const stack & o) {
2924        node ** crnt = &head;
2925        for ( node * next = o.head; next; next = next->next ) {
2926                *crnt = new node{ *next->value };
2927                crnt = &(*crnt)->next;
2928        }
2929        *crnt = nullptr;
2930}
2931stack::stack() : head(nullptr) {}
2932stack::stack(const stack & o) { copy(o); }
2933stack::stack(stack && o) : head(o.head) { o.head = nullptr; }
2934stack::~stack() { clear(); }
2935stack & stack::operator= (const stack & o) {
2936        if ( this == &o ) return *this;
2937        clear();
2938        copy(o);
2939        return *this;
2940}
2941stack & stack::operator= (stack && o) {
2942        if ( this == &o ) return *this;
2943        head = o.head;
2944        o.head = nullptr;
2945        return *this;
2946}
2947bool stack::empty() const { return head == nullptr; }
2948void stack::push(const object & value) { head = new node{ value, head }; /***/ }
2949ptr<object> stack::pop() {
2950        node * n = head;
2951        head = n->next;
2952        ptr<object> x = std::move(n->value);
2953        delete n;
2954        return x;
2955}
2956void stack::clear() {
2957        for ( node * next = head; next; ) {
2958                node * crnt = next;
2959                next = crnt->next;
2960                delete crnt;
2961        }
2962        head = nullptr;
2963}
2964\end{cfa}
2965
2966
2967\end{document}
2968
2969% Local Variables: %
2970% tab-width: 4 %
2971% compile-command: "make" %
2972% End: %
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