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doc/theses/aaron_moss_PhD/phd/background.tex
rf845e80 r69c37cc 13 13 Notable features added during this period include generic types (Chapter~\ref{generic-chap}), constructors and destructors \cite{Schluntz17}, improved support for tuples \cite{Schluntz17}, reference types \cite{Moss18}, first-class concurrent and parallel programming support \cite{Delisle18}, as well as numerous pieces of syntactic sugar and the start of an idiomatic standard library \cite{Moss18}. 14 14 15 \cbstart 15 16 This thesis is primarily concerned with the \emph{expression resolution} portion of \CFA{} type-checking; resolution is discussed in more detail in Chapter~\ref{resolution-chap}, but is essentially determining which declarations the identifiers in each expression correspond to. 16 In C, no simultaneously-visible declarations share identifiers, hence expression resolution in C is not difficult.17 In \CFA{}, multiple added features make the resolution process significantly more complex. 18 Due to this complexity, the expression-resolution pass in \CFACC{} requires 95\% of compiler runtime on some source files, making a new, more efficient procedure for expression resolution a requirement for a performant \CFA{} compiler. 17 Given that no simultaneously-visible C declarations share identifiers, expression resolution in C is not difficult, but the added features of \CFA{} make its resolution algorithms significantly more complex. 18 Due to this complexity, the expression-resolution pass in \CFACC{} requires 95\% of compiler runtime on some source files, making an efficient procedure for expression resolution a requirement for a performant \CFA{} compiler. 19 \cbend 19 20 20 21 The features presented in this chapter are chosen to elucidate the design constraints of the work presented in this thesis. … … 207 208 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 !int!, while !(*it).value = 42;! interprets !*it! as !list!). 208 209 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, 209 I am unaware of any nominal-inheritance system that can model both relationships simultaneously. 210 \cbstart 211 the author is unaware of any nominal-inheritance system that could model both relationships simultaneously. 210 212 Further comparison of \CFA{} polymorphism with other languages can be found in Section~\ref{generic-related-sec}. 213 \cbend 211 214 212 215 The flexibility of \CFA{}'s implicit trait-satisfaction mechanism provides programmers with a great deal of power, but also blocks some optimization approaches for expression resolution. 213 216 The ability of types to begin or cease to satisfy traits when declarations go into or out of scope makes caching of trait satisfaction judgments difficult, and the ability of traits to take multiple type parameters can lead to a combinatorial explosion of work in any attempt to pre-compute trait satisfaction relationships. 214 217 218 \cbstart 215 219 \subsection{Deleted Declarations} 216 220 217 Particular type combinations can be exempted from matching a given polymorphic function through theuse of a \emph{deleted function declaration}:221 Particular type combinations can be exempted from matching a given polymorphic function through use of a \emph{deleted function declaration}: 218 222 219 223 \begin{cfa} … … 224 228 Deleted function declarations are implemented in \CFACC{} by adding them to the symbol table as usual, but with a flag set that indicates that the function is deleted. 225 229 If this deleted declaration is selected as the unique minimal-cost interpretation of an expression then an error is produced, allowing \CFA{} programmers to guide the expression resolver away from undesirable solutions. 230 \cbend 226 231 227 232 \section{Implicit Conversions} \label{implicit-conv-sec} … … 249 254 Given some type !T!, a !T&! (``reference to !T!'') is essentially an automatically dereferenced pointer. 250 255 These types allow seamless pass-by-reference for function parameters, without the extraneous dereferencing syntax present in C; they also allow easy aliasing of nested values with a similarly convenient syntax. 256 \cbstart 251 257 The addition of reference types also eliminated two syntactic special-cases present in previous versions of \CFA{}. 252 Consider the a call !a += b! to a compound assignment operator. 253 The previous declaration for that operator is !lvalue int ?+=?(int*, int)!. 254 To mutate the left argument, the built-in operators were special-cased to implicitly take the address of that argument, while the special !lvalue! syntax was used to mark the return type of a function as a mutable reference. 255 With references, this declaration is re-written as !int& ?+=?(int&, int)!. 256 The reference semantics generalize the implicit address-of on the left argument and allow it to be used in user-declared functions, while also subsuming the (now removed) !lvalue! syntax for function return types. 258 Considering a call !a += b! to a compound assignment operator, the previous declaration for that operator was !lvalue int ?+=?(int*, int)! -- to mutate the left argument, the built-in operators were special-cased to implicitly take the address of that argument, while the special !lvalue! syntax was used to mark the return type of a function as a mutable reference. 259 With references, this declaration can be re-written as !int& ?+=?(int&, int)! -- the reference semantics generalize the implicit address-of on the left argument and allow it to be used in user-declared functions, while also subsuming the (now removed) !lvalue! syntax for function return types. 260 \cbend 257 261 258 262 The C standard makes heavy use of the concept of \emph{lvalue}, an expression with a memory address; its complement, \emph{rvalue} (a non-addressable expression) is not explicitly named in the standard. … … 277 281 \CFA{} supports all of these use cases without further added syntax. 278 282 The key to this syntax-free feature support is an observation made by the author that the address of a reference is a lvalue. 279 In C, the address-of operator !&x! can only be applied to lvalue expressions, and always produces an immutable rvalue; \CFA{} supports reference re-binding by assignment to the address of a reference\footnote{ The syntactic difference between reference initialization and reference assignment is unfortunate, but preserves the ability to pass function arguments by reference (a reference initialization context) without added syntax.}, and pointers to references by repeating the address-of operator:283 In C, the address-of operator !&x! can only be applied to lvalue expressions, and always produces an immutable rvalue; \CFA{} supports reference re-binding by assignment to the address of a reference\footnote{\cbstart The syntactic difference between reference initialization and reference assignment is unfortunate, but preserves the ability to pass function arguments by reference (a reference initialization context) without added syntax. \cbend }, and pointers to references by repeating the address-of operator: 280 284 281 285 \begin{cfa}
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