# Changeset f845e80 for doc/theses/aaron_moss_PhD/phd/background.tex

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Apr 25, 2019, 2:23:48 PM (3 years ago)
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aaron-thesis, arm-eh, cleanup-dtors, jacob/cs343-translation, jenkins-sandbox, master, new-ast, new-ast-unique-expr
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thesis: apply round 2 revisions and strip change bars

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 r69c37cc 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}. \cbstart 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. 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. 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. \cbend In C, no simultaneously-visible declarations share identifiers, hence expression resolution in C is not difficult. In \CFA{}, multiple added features make the resolution process significantly more complex. 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. The features presented in this chapter are chosen to elucidate the design constraints of the work presented in this thesis. 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!). 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, \cbstart the author is unaware of any nominal-inheritance system that could model both relationships simultaneously. I am unaware of any nominal-inheritance system that can model both relationships simultaneously. Further comparison of \CFA{} polymorphism with other languages can be found in Section~\ref{generic-related-sec}. \cbend 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. 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. \cbstart \subsection{Deleted Declarations} Particular type combinations can be exempted from matching a given polymorphic function through use of a \emph{deleted function declaration}: Particular type combinations can be exempted from matching a given polymorphic function through the use of a \emph{deleted function declaration}: \begin{cfa} 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. 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. \cbend \section{Implicit Conversions} \label{implicit-conv-sec} Given some type !T!, a !T&! (reference to !T!'') is essentially an automatically dereferenced pointer. 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. \cbstart The addition of reference types also eliminated two syntactic special-cases present in previous versions of \CFA{}. 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. 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. \cbend Consider the a call !a += b! to a compound assignment operator. The previous declaration for that operator is !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. With references, this declaration is 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. 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. \CFA{} supports all of these use cases without further added syntax. The key to this syntax-free feature support is an observation made by the author that the address of a reference is a lvalue. 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: 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: \begin{cfa}