source: doc/generic_types/generic_types.tex @ 1c2c253

ADTaaron-thesisarm-ehast-experimentalcleanup-dtorsdeferred_resndemanglerenumforall-pointer-decayjacob/cs343-translationjenkins-sandboxnew-astnew-ast-unique-exprnew-envno_listpersistent-indexerpthread-emulationqualifiedEnumresolv-newwith_gc
Last change on this file since 1c2c253 was 1c2c253, checked in by Aaron Moss <a3moss@…>, 7 years ago

Added polymorphic functions and traits background to generic types paper from comp II submission, also make CFA code formatting work

  • Property mode set to 100644
File size: 10.5 KB
Line 
1% take of review (for line numbers) and anonymous (for anonymization) on submission
2\documentclass[format=acmlarge, anonymous, review]{acmart}
3
4\usepackage{listings}   % For code listings
5
6% Useful macros
7\newcommand{\CFA}{C$\mathbf\forall$} % Cforall symbolic name
8\newcommand{\CC}{\rm C\kern-.1em\hbox{+\kern-.25em+}} % C++ symbolic name
9\newcommand{\CCeleven}{\rm C\kern-.1em\hbox{+\kern-.25em+}11} % C++11 symbolic name
10\newcommand{\CCfourteen}{\rm C\kern-.1em\hbox{+\kern-.25em+}14} % C++14 symbolic name
11\newcommand{\CCseventeen}{\rm C\kern-.1em\hbox{+\kern-.25em+}17} % C++17 symbolic name
12
13\newcommand{\TODO}{\textbf{TODO}}
14\newcommand{\eg}{\textit{e}.\textit{g}.}
15\newcommand{\ie}{\textit{i}.\textit{e}.}
16\newcommand{\etc}{\textit{etc}.}
17
18% CFA programming language, based on ANSI C (with some gcc additions)
19\lstdefinelanguage{CFA}[ANSI]{C}{
20        morekeywords={_Alignas,_Alignof,__alignof,__alignof__,asm,__asm,__asm__,_At,_Atomic,__attribute,__attribute__,auto,
21                _Bool,catch,catchResume,choose,_Complex,__complex,__complex__,__const,__const__,disable,dtype,enable,__extension__,
22                fallthrough,fallthru,finally,forall,ftype,_Generic,_Imaginary,inline,__label__,lvalue,_Noreturn,one_t,otype,restrict,_Static_assert,
23                _Thread_local,throw,throwResume,trait,try,typeof,__typeof,__typeof__,zero_t},
24}%
25
26\lstset{
27language=CFA,
28columns=fullflexible,
29basicstyle=\linespread{0.9}\sf,                                                 % reduce line spacing and use sanserif font
30stringstyle=\tt,                                                                                % use typewriter font
31tabsize=4,                                                                                              % 4 space tabbing
32xleftmargin=\parindent,                                                                 % indent code to paragraph indentation
33% extendedchars=true,                                                                   % allow ASCII characters in the range 128-255
34% escapechar=§,                                                                                 % LaTeX escape in CFA code §...§ (section symbol), emacs: C-q M-'
35mathescape=true,                                                                                % LaTeX math escape in CFA code $...$
36keepspaces=true,                                                                                %
37showstringspaces=false,                                                                 % do not show spaces with cup
38showlines=true,                                                                                 % show blank lines at end of code
39aboveskip=4pt,                                                                                  % spacing above/below code block
40belowskip=3pt,
41% replace/adjust listing characters that look bad in sanserif
42literate={-}{\raisebox{-0.15ex}{\texttt{-}}}1 {^}{\raisebox{0.6ex}{$\scriptscriptstyle\land\,$}}1
43        {~}{\raisebox{0.3ex}{$\scriptstyle\sim\,$}}1 {_}{\makebox[1.2ex][c]{\rule{1ex}{0.1ex}}}1 {`}{\ttfamily\upshape\hspace*{-0.1ex}`}1
44        {<-}{$\leftarrow$}2 {=>}{$\Rightarrow$}2,
45% moredelim=**[is][\color{red}]{®}{®},                                  % red highlighting ®...® (registered trademark symbol) emacs: C-q M-.
46% moredelim=**[is][\color{blue}]{ß}{ß},                                 % blue highlighting ß...ß (sharp s symbol) emacs: C-q M-_
47% moredelim=**[is][\color{OliveGreen}]{¢}{¢},                   % green highlighting ¢...¢ (cent symbol) emacs: C-q M-"
48% moredelim=[is][\lstset{keywords={}}]{¶}{¶},                   % keyword escape ¶...¶ (pilcrow symbol) emacs: C-q M-^
49}% lstset
50
51% inline code @...@
52\lstMakeShortInline@
53
54% ACM Information
55\citestyle{acmauthoryear}
56
57\acmJournal{PACMPL}
58
59\title{Generic Types with Efficient Dynamic Layout in \CFA{}}
60
61\author{Aaron Moss}
62\affiliation{%
63        \institution{University of Waterloo}
64        \department{David R. Cheriton School of Computer Science}
65        \streetaddress{Davis Centre, University of Waterloo}
66        \city{Waterloo}
67        \state{ON}
68        \postcode{N2L 3G1}
69        \country{Canada}
70}
71\email{a3moss@uwaterloo.ca}
72
73\terms{generic, types}
74\keywords{generic types, polymorphic functions, Cforall}
75
76\begin{CCSXML}
77<ccs2012>
78<concept>
79<concept_id>10011007.10011006.10011008.10011024.10011025</concept_id>
80<concept_desc>Software and its engineering~Polymorphism</concept_desc>
81<concept_significance>500</concept_significance>
82</concept>
83<concept>
84<concept_id>10011007.10011006.10011008.10011024.10011028</concept_id>
85<concept_desc>Software and its engineering~Data types and structures</concept_desc>
86<concept_significance>500</concept_significance>
87</concept>
88<concept>
89<concept_id>10011007.10011006.10011041.10011047</concept_id>
90<concept_desc>Software and its engineering~Source code generation</concept_desc>
91<concept_significance>300</concept_significance>
92</concept>
93</ccs2012>
94\end{CCSXML}
95
96\ccsdesc[500]{Software and its engineering~Polymorphism}
97\ccsdesc[500]{Software and its engineering~Data types and structures}
98\ccsdesc[300]{Software and its engineering~Source code generation}
99
100\begin{abstract}
101\TODO{} Write abstract.
102\end{abstract}
103
104\begin{document}
105
106\maketitle
107
108\section{Introduction \& Background}
109
110\CFA{}\footnote{Pronounced ``C-for-all'', and written \CFA{} or Cforall.} is an evolutionary extension of the C programming language which aims to add modern language features to C while maintaining both source compatibility with C and a familiar mental model for programmers. This paper describes how generic types are designed and implemented in \CFA{}, and how they interact with \CFA{}'s polymorphic functions.
111
112\subsection{Polymorphic Functions}
113
114\CFA{}'s polymorphism was originally formalized by \citet{Ditchfield92}, and first implemented by \citet{Bilson03}. The signature feature of \CFA{} is parametric-polymorphic functions; such functions are written using a @forall@ clause (which gives the language its name):
115\begin{lstlisting}
116forall(otype T)
117T identity(T x) {
118    return x;
119}
120
121int forty_two = identity(42); // T is bound to int, forty_two == 42
122\end{lstlisting}
123The @identity@ function above can be applied to any complete object type (or ``@otype@''). The type variable @T@ is transformed into a set of additional implicit parameters to @identity@, which encode sufficient information about @T@ to create and return a variable of that type. The \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. Here, the runtime cost of polymorphism is spread over each polymorphic call, due to passing more arguments to polymorphic functions; preliminary experiments have shown this overhead to be similar to \CC{} virtual function calls.
124
125Since bare polymorphic types do not provide a great range of available operations, \CFA{} provides a \emph{type assertion} mechanism to provide further information about a type:
126\begin{lstlisting}
127forall(otype T | { T twice(T); })
128T four_times(T x) {
129    return twice( twice(x) );
130}
131
132double twice(double d) { return d * 2.0; } // (1)
133
134double magic = four_times(10.5); // T is bound to double, uses (1) to satisfy type assertion
135\end{lstlisting}
136These type assertions may be either variable or function declarations that depend on a polymorphic type variable. @four_times@ can only be called with an argument for which there exists a function named @twice@ that can take that argument and return another value of the same type; a pointer to the appropriate @twice@ function is passed as an additional implicit parameter to the call of @four_times@.
137
138Monomorphic specializations of polymorphic functions can themselves be used to satisfy type assertions. For instance, @twice@ could have been defined using the \CFA{} syntax for operator overloading as:
139\begin{lstlisting}
140forall(otype S | { S ?+?(S, S); })
141S twice(S x) { return x + x; }  // (2)
142\end{lstlisting} 
143This version of @twice@ works for any type @S@ that has an addition operator defined for it, and it could have been used to satisfy the type assertion on @four_times@.
144The compiler accomplishes this by creating a wrapper function calling @twice // (2)@ with @S@ bound to @double@, then providing this wrapper function to @four_times@\footnote{\lstinline@twice // (2)@ could also have had a type parameter named \lstinline@T@; \CFA{} specifies renaming of the type parameters, which would avoid the name conflict with the type variable \lstinline@T@ of \lstinline@four_times@.}.
145
146\subsection{Traits}
147
148\CFA{} provides \emph{traits} as a means to name a group of type assertions, as in the example below:
149\begin{lstlisting}
150trait has_magnitude(otype T) {
151    bool ?<?(T, T);  // comparison operator for T
152    T -?(T);  // negation operator for T
153    void ?{}(T*, zero_t);  // constructor from 0 literal
154};
155
156forall(otype M | has_magnitude(M))
157M abs( M m ) {
158    M zero = { 0 };  // uses zero_t constructor from trait
159    return m < zero ? -m : m;
160}
161
162forall(otype M | has_magnitude(M))
163M max_magnitude( M a, M b ) {
164    return abs(a) < abs(b) ? b : a;
165}
166\end{lstlisting}
167
168Semantically, traits are simply a named lists of type assertions, but they may be used for many of the same purposes that interfaces in Java or abstract base classes in \CC{} are used for. Unlike Java interfaces or \CC{} base classes, \CFA{} types do not explicitly state any inheritance relationship to traits they satisfy; this can be considered a form of structural inheritance, similar to implementation of an interface in Go, as opposed to the nominal inheritance model of Java and \CC{}. Nominal inheritance can be simulated with traits using marker variables or functions:
169\begin{lstlisting}
170trait nominal(otype T) {
171    T is_nominal;
172};
173
174int is_nominal;  // int now satisfies the nominal trait
175{
176    char is_nominal; // char satisfies the nominal trait
177}
178// char no longer satisfies the nominal trait here 
179\end{lstlisting}
180
181Traits, however, are significantly more powerful than nominal-inheritance interfaces; firstly, due to the scoping rules of the declarations that satisfy a trait's type assertions, a type may not satisfy a trait everywhere that the type is declared, as with @char@ and the @nominal@ trait above. Secondly, traits may be used to declare a relationship among multiple types, a property that may be difficult or impossible to represent in nominal-inheritance type systems:
182\begin{lstlisting}
183trait pointer_like(otype Ptr, otype El) {
184    lvalue El *?(Ptr); // Ptr can be dereferenced into a modifiable value of type El
185}
186
187struct list {
188    int value;
189    list *next;  // may omit "struct" on type names
190};
191
192typedef list *list_iterator;
193
194lvalue int *?( list_iterator it ) {
195    return it->value;
196}
197\end{lstlisting}
198
199In 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@).
200While 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.
201
202\bibliographystyle{ACM-Reference-Format}
203\bibliography{generic_types}
204
205\end{document}
Note: See TracBrowser for help on using the repository browser.