\input texinfo @c -*-texinfo-*- @c %**start of header @setfilename rust.info @settitle Rust Documentation @setchapternewpage odd @c %**end of header @syncodeindex fn cp @include version.texi @ifinfo This manual is for the ``Rust'' programming language. @uref{http://github.com/graydon/rust} Version: @gitversion Copyright 2006-2010 Graydon Hoare Copyright 2009-2010 Mozilla Foundation See accompanying LICENSE.txt for terms. @end ifinfo @dircategory Programming @direntry * rust: (rust). Rust programming language @end direntry @titlepage @title Rust @subtitle A safe, concurrent, practical language. @author Graydon Hoare @author Mozilla Foundation @page @vskip 0pt plus 1filll @uref{http://github.com/graydon/rust} Version: @gitversion @sp 2 Copyright @copyright{} 2006-2010 Graydon Hoare Copyright @copyright{} 2009-2010 Mozilla Foundation See accompanying LICENSE.txt for terms. @end titlepage @everyfooting @| @emph{-- Draft @today --} @| @ifnottex @node Top @top Top Rust Documentation @end ifnottex @menu * Disclaimer:: Notes on a work in progress. * Introduction:: Background, intentions, lineage. * Tutorial:: Gentle introduction to reading Rust code. * Reference:: Systematic reference of language elements. * Index:: Index @end menu @ifnottex Complete table of contents @end ifnottex @contents @c ############################################################ @c Disclaimer @c ############################################################ @node Disclaimer @chapter Disclaimer To the reader, Rust is a work in progress. The language continues to evolve as the design shifts and is fleshed out in working code. Certain parts work, certain parts do not, certain parts will be removed or changed. This manual is a snapshot written in the present tense. Some features described do not yet exist in working code. Some may be temporary. It is a @emph{draft}, and we ask that you not take anything you read here as either definitive or final. The manual is to help you get a sense of the language and its organization, not to serve as a complete specification. At least not yet. If you have suggestions to make, please try to focus them on @emph{reductions} to the language: possible features that can be combined or omitted. At this point, every ``additive'' feature we're likely to support is already on the table. The task ahead involves combining, trimming, and implementing. @c ############################################################ @c Introduction @c ############################################################ @node Introduction @chapter Introduction @quotation We have to fight chaos, and the most effective way of doing that is to prevent its emergence. @flushright - Edsger Dijkstra @end flushright @end quotation @sp 2 Rust is a curly-brace, block-structured statement language. It visually resembles the C language family, but differs significantly in syntactic and semantic details. Its design is oriented toward concerns of ``programming in the large'', that is, of creating and maintaining @emph{boundaries} -- both abstract and operational -- that preserve large-system @emph{integrity}, @emph{availability} and @emph{concurrency}. It supports a mixture of imperative procedural, concurrent actor, object oriented and pure functional styles. Rust also supports generic programming and metaprogramming, in both static and dynamic styles. @menu * Goals:: Intentions, motivations. * Sales Pitch:: A summary for the impatient. * Influences:: Relationship to past languages. @end menu @node Goals @section Goals The language design pursues the following goals: @sp 1 @itemize @item Compile-time error detection and prevention. @item Run-time fault tolerance and containment. @item System building, analysis and maintenance affordances. @item Clarity and precision of expression. @item Implementation simplicity. @item Run-time efficiency. @item High concurrency. @end itemize @sp 1 Note that most of these goals are @emph{engineering} goals, not showcases for sophisticated language technology. Most of the technology in Rust is @emph{old} and has been seen decades earlier in other languages. All new languages are developed in a technological context. Rust's goals arise from the context of writing large programs that interact with the internet -- both servers and clients -- and are thus much more concerned with @emph{safety} and @emph{concurrency} than older generations of program. Our experience is that these two forces do not conflict; rather they drive system design decisions toward extensive use of @emph{partitioning} and @emph{statelessness}. Rust aims to make these a more natural part of writing programs, within the niche of lower-level, practical, resource-conscious languages. @page @node Sales Pitch @section Sales Pitch The following comprises a brief ``sales pitch'' overview of the salient features of Rust, relative to other languages. @itemize @sp 1 @item No @code{null} pointers The initialization state of every slot is statically computed as part of the typestate system (see below), and requires that all slots are initialized before use. There is no @code{null} value; uninitialized slots are uninitialized, and can only be written to, not read. The common use for @code{null} in other languages -- as a sentinel value -- is subsumed into the more general facility of disjoint union types. A program must explicitly model its use of such types. @sp 1 @item Lightweight tasks with no shared mutable state Like many @emph{actor} languages, Rust provides an isolation (and concurrency) model based on lightweight tasks scheduled by the language runtime. These tasks are very inexpensive and statically unable to mutate one another's local memory. Breaking the rule of task isolation is only possible by calling external (C/C++) code. Inter-task communication is typed, asynchronous and simplex, based on passing messages over channels to ports. Transmission can be rate-limited or rate-unlimited. Selection between multiple senders is pseudo-randomized on the receiver side. @sp 1 @item Predictable native code, simple runtime @cindex DWARF The meaning and cost of every operation within a Rust program is intended to be easy to model for the reader. The code should not ``surprise'' the programmer once it has been compiled. Rust compiles to native code. Rust compilation units are large and the compilation model is designed around multi-file, whole-library or whole-program optimization. The compiled units are standard loadable objects (ELF, PE, Mach-O) containing standard metadata (DWARF) and are compatible with existing, standard low-level tools (disassemblers, debuggers, profilers, dynamic loaders). The Rust runtime library is a small collection of support code for scheduling, memory management, inter-task communication, reflection and runtime linkage. This library is written in standard C++ and is quite straightforward. It presents a simple interface to embeddings. No research-level virtual machine, JIT or garbage collection technology is required. It should be relatively easy to adapt a Rust front-end on to many existing native toolchains. @sp 1 @item Integrated system-construction facility The units of compilation of Rust are multi-file amalgamations called @emph{crates}. A crate is described by a separate, declarative type of source file that guides the compilation of the crate, its packaging, its versioning, and its external dependencies. Crates are also the units of distribution and loading. Significantly: the dependency graph of crates is @emph{acyclic} and @emph{anonymous}: there is no global namespace for crates, and module-level recursion cannot cross crate barriers. Unlike many languages, individual modules do @emph{not} carry all the mechanisms or restrictions of crates. Modules and crates serve different roles. @sp 1 @item Static control over memory allocation, packing and aliasing. Many values in Rust are allocated @emph{within} their containing stack-frame or parent strucure. Numbers, records, tuples and tags are all allocated this way. To allocate such values in the heap, they must be explicitly @emph{boxed}. A @dfn{box} is a pointer to a heap allocation that holds another value, its @emph{content}. If the content of a box is a @emph{state} value -- the sort that may contain mutable members -- then the heap allocation is also subject to garbage collection. Boxing and unboxing in Rust is explicit, though in many cases (arithmetic operations, name-component dereferencing) Rust will automatically ``reach through'' the box to access its content. Box values can be passed and assigned independently, like pointers in C; the difference is that in Rust they always point to live contents, and are not subject to pointer arithmetic. In addition to boxes, Rust supports a kind of pass-by-reference slot called an alias. Forming or releasing an alias does not perform reference-count operations; aliases can only be formed on referents that will provably outlive the alias, and are therefore only used for passing arguments to functions. Aliases are not ``general values'', in the sense that they cannot be independently manipulated. They are more like C++ references, except that like boxes, aliases are safe: they always point to live values. In addition, every slot (stack-local allocation or alias) has a static initialization state that is calculated by the typestate system. This permits late initialization of slots in functions with complex control-flow, while still guaranteeing that every use of a slot occurs after it has been initialized. @sp 1 @item Static control over mutability. Types in Rust are classified as either immutable or mutable. By default, all types are immutable. If a type is declared as @code{mutable}, then the type is a @code{state} type and must be declared as such. Any type directly marked as @code{mutable} @emph{or indirectly containing} a state type is also a state type. This classification of data types in Rust interacts with the memory allocation and transmission rules. In particular: @itemize @item Only immutable (non-state) values can be sent over channels. @item Only immutable (non-state) objects can have destructor functions. @end itemize Boxed state values are subject to local (per-task) garbage-collection. Garbage collection costs are therefore also task-local and do not interrupt or suspend other tasks. Boxed immutable values are reference-counted and have a deterministic destruction order: top-down, immediately upon release of the last live reference. State values can refer to non-state values, but not vice-versa. Rust therefore encourages the programmer to write in a style that consists primarily of immutable types, but also permits limited, local (per-task) mutability. @sp 1 @item Stack-based iterators Rust provides a type of function-like multiple-invocation iterator that is very efficient: the iterator state lives only on the stack and is tightly coupled to the loop that invoked it. @sp 1 @item Direct interface to C code Rust can load and call many C library functions simply by declaring them. Calling a C function statically marks a function as ``unsafe'', unless the task calling the unsafe function is further isolated within an external ``heavyweight'' operating-system subprocess. Every ``unsafe'' function or module in a Rust compilation unit must be explicitly authorized in the crate file. @sp 1 @item Structural algebraic data types The Rust type system is primarily structural, and contains the standard assortment of useful ``algebraic'' type constructors from functional languages, such as function types, tuples, record types, vectors, and nominally-tagged disjoint unions. Such values may be @emph{pattern-matched} in an @code{alt} statement. @sp 1 @item Generic code Rust supports a simple form of parametric polymorphism: functions, iterators, types and objects can be parametrized by other types. @sp 1 @item Argument binding Rust provides a mechanism of partially binding arguments to functions, producing new functions that accept the remaining un-bound arguments. This mechanism combines some of the features of lexical closures with some of the features of currying, in a smaller and simpler package. @sp 1 @item Local type inference To save some quantity of programmer key-pressing, Rust supports local type inference: signatures of functions, objects and iterators always require type annotation, but within the body of a function or iterator many slots can be declared @code{auto} and Rust will infer the slot's type from its uses. @sp 1 @item Structural object system Rust has a lightweight object system based on structural object types: there is no ``class hierarchy'' nor any concept of inheritance. Method overriding and object restriction are performed explicitly on object values, which are little more than order-insensitive records of methods sharing a common private value. Objects can be state or non-state, and only non-state objects can have destructors. @sp 1 @item Dynamic type Rust includes support for values of a top type, @code{any}, that can hold any type of value whatsoever. An @code{any} value is a pair of a type code and a boxed value of that type. Injection into an @code{any} and projection by type-case-selection is integrated into the language. @sp 1 @item Dynamic metaprogramming (reflection) Rust supports run-time reflection on the structure of a crate, using a combination of custom descriptor structures and the DWARF metadata tables used to support crate linkage and other runtime services. @sp 1 @item Static metaprogramming (syntactic extension) Rust supports a system for syntactic extensions that can be loaded into the compiler, to implement user-defined notations, macros, program-generators and the like. These notations are @emph{marked} using a special form of bracketing, such that a reader unfamiliar with the extension can still parse the surrounding text by skipping over the bracketed ``extension text''. @sp 1 @item Idempotent failure If a task fails due to a signal, or if it executes the special @code{fail} statement, it enters the @emph{failing} state. A failing task unwinds its control stack, frees all of its owned resources (executing destructors) and enters the @emph{dead} state. Failure is idempotent and non-recoverable. @sp 1 @item Signal handling Rust has a system for propagating task-failures and other spontaneous events between tasks. Some signals can be trapped and redirected to channels; other signals are fatal and result in task-failure. Tasks can designate other tasks to handle signals for them. This permits organizing tasks into mutually-supervising or mutually-failing groups. @sp 1 @item Deterministic destruction Immutable objects can have destructor functions, which are executed deterministically in top-down ownership order, as control frames are exited and/or objects are otherwise freed from data structures holding them. The same destructors are run in the same order whether the object is deleted by unwinding during failure or normal execution. Similarly, the rules for freeing immutable values are deterministic and predictable: on scope-exit or structure-release, local slots are released immediately. Referenced boxes have their reference count decreased and are released if the count drops to zero. Aliases are silently forgotten. State values are local to a task, and are subject to per-task garbage collection. As a result, unreferenced state boxes are not necessarily freed immediately; if an unreferenced state box is part of an acyclic graph, it is freed when the last reference to it drops, but if it is part of a reference cycle it will be freed when the GC collects it (or when the owning task terminates, at the latest). State values can point to immutable values but not vice-versa. Doing so merely delays (to an undefined future time) the moment when the deterministic, top-down destruction sequence for the referenced immutable values @emph{start}. In other words, the immutable ``leaves'' of a state value are released in a locally-predictable order, even if the ``interior'' of the state value is released in an unpredictable order. @sp 1 @item Typestate system Every storage slot in a Rust frame participates in not only a conventional structural static type system, describing the interpretation of memory in the slot, but also a @emph{typestate} system. The static typestates of a program describe the set of @emph{pure, dynamic predicates} that provably hold over some set of slots, at each point in the program's control-flow graph within each frame. The static calculation of the typestates of a program is a function-local dataflow problem, and handles user-defined predicates in a similar fashion to the way the type system permits user-defined types. A short way of thinking of this is: types statically model values, typestates statically model @emph{assertions that hold} before and after statements. @end itemize @page @node Influences @section Influences @sp 2 @quotation The essential problem that must be solved in making a fault-tolerant software system is therefore that of fault-isolation. Different programmers will write different modules, some modules will be correct, others will have errors. We do not want the errors in one module to adversely affect the behaviour of a module which does not have any errors. @flushright - Joe Armstrong @end flushright @end quotation @sp 2 @quotation In our approach, all data is private to some process, and processes can only communicate through communications channels. @emph{Security}, as used in this paper, is the property which guarantees that processes in a system cannot affect each other except by explicit communication. When security is absent, nothing which can be proven about a single module in isolation can be guaranteed to hold when that module is embedded in a system [...] @flushright - Robert Strom and Shaula Yemini @end flushright @end quotation @sp 2 @quotation Concurrent and applicative programming complement each other. The ability to send messages on channels provides I/O without side effects, while the avoidance of shared data helps keep concurrent processes from colliding. @flushright - Rob Pike @end flushright @end quotation @sp 2 @page Rust is not a particularly original language. It may however appear unusual by contemporary standards, as its design elements are drawn from a number of ``historical'' languages that have, with a few exceptions, fallen out of favour. Five prominent lineages contribute the most: @itemize @sp 1 @item The NIL (1981) and Hermes (1990) family. These languages were developed by Robert Strom, Shaula Yemini, David Bacon and others in their group at IBM Watson Research Center (Yorktown Heights, NY, USA). @sp 1 @item The Erlang (1987) language, developed by Joe Armstrong, Robert Virding, Claes Wikstr@"om, Mike Williams and others in their group at the Ericsson Computer Science Laboratory (@"Alvsj@"o, Stockholm, Sweden) . @sp 1 @item The Sather (1990) language, developed by Stephen Omohundro, Chu-Cheow Lim, Heinz Schmidt and others in their group at The International Computer Science Institute of the University of California, Berkeley (Berkeley, CA, USA). @sp 1 @item The Newsqueak (1988), Alef (1995), and Limbo (1996) family. These languages were developed by Rob Pike, Phil Winterbottom, Sean Dorward and others in their group at Bell labs Computing Sciences Reserch Center (Murray Hill, NJ, USA). @sp 1 @item The Napier (1985) and Napier88 (1988) family. These languages were developed by Malcolm Atkinson, Ron Morrison and others in their group at the University of St. Andrews (St. Andrews, Fife, UK). @end itemize @sp 1 Additional specific influences can be seen from the following languages: @itemize @item The structural algebraic types and compilation manager of SML. @item The syntax-extension systems of Camlp4 and the Common Lisp readtable. @item The deterministic destructor system of C++. @end itemize @c ############################################################ @c Tutorial @c ############################################################ @node Tutorial @chapter Tutorial @emph{TODO}. @c ############################################################ @c Reference @c ############################################################ @node Reference @chapter Reference @menu * Ref.Lex:: Lexical structure. * Ref.Path:: References to slots and items. * Ref.Gram:: Grammar. * Ref.Comp:: Compilation and component model. * Ref.Mem:: Semantic model of memory. * Ref.Task:: Semantic model of tasks. * Ref.Item:: The components of a module. * Ref.Type:: The types of values held in memory. * Ref.Expr:: Parsed and primitive expressions. * Ref.Stmt:: Executable statements. * Ref.Run:: Organization of runtime services. @end menu @node Ref.Lex @section Ref.Lex @c * Ref.Lex:: Lexical structure. @cindex Lexical structure @cindex Token The lexical structure of a Rust source file or crate file is defined in terms of Unicode character codes and character properties. Groups of Unicode character codes and characters are organized into @emph{tokens}. Tokens are defined as the longest contiguous sequence of characters within the same token type (identifier, keyword, literal, symbol), or interrupted by ignored characters. Most tokens in Rust follow rules similar to the C family. Most tokens (including identifiers, whitespace, keywords, operators and structural symbols) are drawn from the ASCII-compatible range of Unicode. String and character literals, however, may include the full range of Unicode characters. @emph{TODO: formalize this section much more}. @menu * Ref.Lex.Ignore:: Ignored characters. * Ref.Lex.Ident:: Identifier tokens. * Ref.Lex.Key:: Keyword tokens. * Ref.Lex.Res:: Reserved tokens. * Ref.Lex.Num:: Numeric tokens. * Ref.Lex.Text:: String and character tokens. * Ref.Lex.Syntax:: Syntactic extension tokens. * Ref.Lex.Sym:: Special symbol tokens. @end menu @node Ref.Lex.Ignore @subsection Ref.Lex.Ignore @c * Ref.Lex.Ignore:: Ignored tokens. Characters considered to be @emph{whitespace} or @emph{comment} are ignored, and are not considered as tokens. They serve only to delimit tokens. Rust is otherwise a free-form language. @dfn{Whitespace} is any of the following Unicode characters: U+0020 (space), U+0009 (tab, @code{'\t'}), U+000A (LF, @code{'\n'}), U+000D (CR, @code{'\r'}). @dfn{Comments} are @emph{single-line comments} or @emph{multi-line comments}. A @dfn{single-line comment} is any sequence of Unicode characters beginning with U+002F U+002F (@code{"//"}) and extending to the next U+000A character, @emph{excluding} cases in which such a sequence occurs within a string literal token or a syntactic extension token. A @dfn{multi-line comments} is any sequence of Unicode characters beginning with U+002F U+002A (@code{"/*"}) and ending with U+002A U+002F (@code{"*/"}), @emph{excluding} cases in which such a sequence occurs within a string literal token or a syntactic extension token. Multi-line comments may be nested. @node Ref.Lex.Ident @subsection Ref.Lex.Ident @c * Ref.Lex.Ident:: Identifier tokens. @cindex Identifier token Identifiers follow the pattern of C identifiers: they begin with a @emph{letter} or @emph{underscore}, and continue with any combination of @emph{letters}, @emph{decimal digits} and underscores, and must not be equal to any keyword or reserved token. @xref{Ref.Lex.Key}. @xref{Ref.Lex.Res}. A @emph{letter} is a Unicode character in the ranges U+0061-U+007A and U+0041-U+005A (@code{'a'}-@code{'z'} and @code{'A'}-@code{'Z'}). An @dfn{underscore} is the character U+005F ('_'). A @dfn{decimal digit} is a character in the range U+0030-U+0039 (@code{'0'}-@code{'9'}). @node Ref.Lex.Key @subsection Ref.Lex.Key @c * Ref.Lex.Key:: Keyword tokens. The keywords are: @cindex Keywords @sp 2 @multitable @columnfractions .15 .15 .15 .15 .15 @item @code{use} @tab @code{meta} @tab @code{syntax} @tab @code{mutable} @tab @code{native} @item @code{mod} @tab @code{import} @tab @code{export} @tab @code{let} @tab @code{auto} @item @code{state} @tab @code{gc} @tab @code{abs} @item @code{auth} @tab @code{impure} @tab @code{unsafe} @item @code{bind} @tab @code{type} @tab @code{true} @tab @code{false} @tab @code{any} @item @code{int} @tab @code{uint} @tab @code{float} @tab @code{char} @tab @code{bool} @item @code{u8} @tab @code{u16} @tab @code{u32} @tab @code{u64} @tab @code{f32} @item @code{i8} @tab @code{i16} @tab @code{i32} @tab @code{i64} @tab @code{f64} @item @code{rec} @tab @code{tup} @tab @code{tag} @tab @code{vec} @tab @code{str} @item @code{fn} @tab @code{iter} @tab @code{obj} @tab @code{as} @tab @code{drop} @item @code{task} @tab @code{port} @tab @code{chan} @tab @code{spawn} @tab @code{with} @item @code{if} @tab @code{else} @tab @code{alt} @tab @code{case} @tab @code{in} @item @code{do} @tab @code{while} @tab @code{break} @tab @code{cont} @tab @code{fail} @item @code{log} @tab @code{note} @tab @code{claim} @tab @code{check} @tab @code{prove} @item @code{for} @tab @code{each} @tab @code{ret} @tab @code{put} @tab @code{be} @end multitable @node Ref.Lex.Res @subsection Ref.Lex.Res @c * Ref.Lex.Res:: Reserved tokens. The reserved tokens are: @cindex Reserved @sp 2 @multitable @columnfractions .15 .15 .15 .15 .15 @item @code{f16} @tab @code{f80} @tab @code{f128} @item @code{m32} @tab @code{m64} @tab @code{m128} @tab @code{dec} @end multitable @sp 2 At present these tokens have no defined meaning in the Rust language. These tokens may correspond, in some current or future implementation, to additional built-in types for decimal floating-point, extended binary and interchange floating-point formats, as defined in the IEEE 754-1985 and IEEE 754-2008 specifications. @node Ref.Lex.Num @subsection Ref.Lex.Num @c * Ref.Lex.Num:: Numeric tokens. @cindex Number token @cindex Hex token @cindex Decimal token @cindex Binary token @cindex Floating-point token A @dfn{number literal} is either an @emph{integer literal} or a @emph{floating-point literal}. @sp 1 An @dfn{integer literal} has one of three forms: @enumerate @item A @dfn{decimal literal} starts with a @emph{decimal digit} and continues with any mixture of @emph{decimal digits} and @emph{underscores}. @item A @dfn{hex literal} starts with the character sequence U+0030 U+0078 (@code{"0x"}) and continues as any mixture @emph{hex digits} and @emph{underscores}. @item A @dfn{binary literal} starts with the character sequence U+0030 U+0062 (@code{"0b"}) and continues as any mixture @emph{binary digits} and @emph{underscores}. @end enumerate By default, an integer literal is of type @code{int}. An integer literal may be followed (immediately, without any spaces) by a @dfn{integer suffix}, which changes the type of the literal. There are three kinds of integer literal suffix: @enumerate @item The @code{u} suffix gives the literal type @code{uint}. @item The @code{g} suffix gives the literal type @code{big}. @item Each of the signed and unsigned machine types @code{u8}, @code{i8}, @code{u16}, @code{i16}, @code{u32}, @code{i32}, @code{u64} and @code{i64} give the literal the corresponding machine type. @end enumerate @sp 1 A @dfn{floating-point literal} has one of two forms: @enumerate @item Two @emph{decimal literals} separated by a period character U+002E ('.'), with an optional @emph{exponent} trailing after the second @emph{decimal literal}. @item A single @emph{decimal literal} followed by an @emph{exponent}. @end enumerate By default, a floating-point literal is of type @code{float}. A floating-point literal may be followed (immediately, without any spaces) by a @dfn{floating-point suffix}, which changes the type of the literal. There are only two floating-point suffixes: @code{f32} and @code{f64}. Each of these gives the floating point literal the associated type, rather than @code{float}. A set of suffixes are also reserved to accommodate literal support for types corresponding to reserved tokens. The reserved suffixes are @code{f16}, @code{f80}, @code{f128}, @code{m}, @code{m32}, @code{m64} and @code{m128}. @sp 1 A @dfn{hex digit} is either a @emph{decimal digit} or else a character in the ranges U+0061-U+0066 and U+0041-U+0046 (@code{'a'}-@code{'f'}, @code{'A'}-@code{'F'}). A @dfn{binary digit} is either the character U+0030 or U+0031 (@code{'0'} or @code{'1'}). An @dfn{exponent} begins with either of the characters U+0065 or U+0045 (@code{'e'} or @code{'E'}), followed by an optional @emph{sign character}, followed by a trailing @emph{decimal literal}. A @dfn{sign character} is either U+002B or U+002D (@code{'+'} or @code{'-'}). Examples of integer literals of various forms: @example 123; // type int 123u; // type uint 123_u; // type uint 0xff00; // type int 0xffu8; // type u8 0b1111_1111_1001_0000_i32; // type i32 0xffff_ffff_ffff_ffff_ffff_ffffg; // type big @end example Examples of floating-point literals of various forms: @example 123.0; // type float 0.1; // type float 0.1f32; // type f32 12E+99_f64; // type f64 @end example @node Ref.Lex.Text @subsection Ref.Lex.Text @c * Ref.Lex.Key:: String and character tokens. @cindex String token @cindex Character token @cindex Escape sequence @cindex Unicode A @dfn{character literal} is a single Unicode character enclosed within two U+0027 (single-quote) characters, with the exception of U+0027 itself, which must be @emph{escaped} by a preceding U+005C character ('\'). A @dfn{string literal} is a sequence of any Unicode characters enclosed within two U+0022 (double-quote) characters, with the exception of U+0022 itself, which must be @emph{escaped} by a preceding U+005C character ('\'). Some additional @emph{escapes} are available in either character or string literals. An escape starts with a U+005C ('\') and continues with one of the following forms: @itemize @item An @dfn{8-bit codepoint escape} escape starts with U+0078 ('x') and is followed by exactly two @dfn{hex digits}. It denotes the Unicode codepoint equal to the provided hex value. @item A @dfn{16-bit codepoint escape} starts with U+0075 ('u') and is followed by exactly four @dfn{hex digits}. It denotes the Unicode codepoint equal to the provided hex value. @item A @dfn{32-bit codepoint escape} starts with U+0055 ('U') and is followed by exactly eight @dfn{hex digits}. It denotes the Unicode codepoint equal to the provided hex value. @item A @dfn{whitespace escape} is one of the characters U+006E, U+0072, or U+0074, denoting the unicode values U+000A (LF), U+000D (CR) or U+0009 (HT) respectively. @item The @dfn{backslash escape} is the character U+005C ('\') which must be escaped in order to denote @emph{itself}. @end itemize @node Ref.Lex.Syntax @subsection Ref.Lex.Syntax @c * Ref.Lex.Syntax:: Syntactic extension tokens. Syntactic extensions are marked with the @emph{pound} sigil U+0023 (@code{#}), followed by a qualified name of a compile-time imported module item, an optional parenthesized list of @emph{parsed expressions}, and an optional brace-enclosed region of free-form text (with brace-matching and brace-escaping used to determine the limit of the region). @xref{Ref.Comp.Syntax}. @emph{TODO: formalize those terms more}. @node Ref.Lex.Sym @subsection Ref.Lex.Sym @c * Ref.Lex.Sym:: Special symbol tokens. @cindex Symbol @cindex Operator The special symbols are: @sp 2 @multitable @columnfractions .1 .1 .1 .1 .1 .1 @item @code{@@} @tab @code{_} @item @code{#} @tab @code{:} @tab @code{.} @tab @code{;} @tab @code{,} @item @code{[} @tab @code{]} @tab @code{@{} @tab @code{@}} @tab @code{(} @tab @code{)} @item @code{=} @tab @code{<-} @tab @code{<|} @tab @code{<+} @tab @code{->} @item @code{+} @tab @code{++} @tab @code{+=} @tab @code{-} @tab @code{--} @tab @code{-=} @item @code{*} @tab @code{/} @tab @code{%} @tab @code{*=} @tab @code{/=} @tab @code{%=} @item @code{&} @tab @code{|} @tab @code{!} @tab @code{~} @tab @code{^} @item @code{&=} @tab @code{|=} @tab @code{^=} @tab @code{!=} @item @code{>>} @tab @code{>>>} @tab @code{<<} @tab @code{<<=} @tab @code{>>=} @tab @code{>>>=} @item @code{<} @tab @code{<=} @tab @code{==} @tab @code{>=} @tab @code{>} @item @code{&&} @tab @code{||} @end multitable @page @page @node Ref.Path @section Ref.Path @c * Ref.Path:: References to slots and items. @cindex Names of items or slots @cindex Path name @cindex Type parameters A @dfn{path} is a ubiquitous syntactic form in Rust that deserves special attention. A path denotes a slot or an item. @xref{Ref.Mem.Slot}. @xref{Ref.Item}. Every slot and item in a Rust crate has a @emph{canonical path} that refers to it from the crate top-level, as well as a number of shorter @emph{relative paths} that may also denote it in inner scopes of the crate. There is no way to define a slot or item without a canonical path within its crate (with the exception of the crate's implicit top-level module). Paths have meaning only within a specific crate. @xref{Ref.Comp.Crate}. Paths consist of period-separated components. In the simplest form, path components are identifiers. @xref{Ref.Lex.Ident}. Two examples of simple paths consisting of only identifier components: @example x; x.y.z; @end example Paths fall into two important categories: @emph{names} and @emph{lvals}. A @dfn{name} denotes an item, and is statically resolved to its referent at compile time. An @dfn{lval} denotes a slot or some component of a value held within a slot, and is statically resolved at compile time to a sequence of memory operations and primitive (arithmetic) expressions that will be executed to load or store the associated value, starting from the task stack frame, at run time. In some contexts, the Rust grammar accepts a general @emph{path}, but a subsequent syntactic restriction requires the path to be an lval or a name. In other words: in some contexts an lval is required (for example, on the left hand side of the copy operator, @pxref{Ref.Stmt.Copy}) and in other contexts a name is required (for example, as a type parameter, @pxref{Ref.Item}). In no case is the grammar made ambiguous by accepting a general path and restricting allowed paths to names or lvals after parsing. These restrictions are noted in the grammar. @xref{Ref.Gram}. A name component may include type parameters. Type parameters are denoted by square brackets. Square brackets are used @emph{only} to denote type parameters in Rust. If a name component includes a type parameter, the type parameter must also resolve statically to a type in the environment of the name. Type parameters are only part of the names of items. @xref{Ref.Item}. An example of a name with type parameters: @example m.map[int,str]; @end example An lval component may include an indexing operator. Index operators are enclosed in parentheses and can include any integral expression. Indexing operators can only be applied to vectors or strings, and imply a run-time bounds-check. @xref{Ref.Type.Vec}. An example of an lval with a dynamic indexing operator: @example x.y.(1 + v).z; @end example @page @node Ref.Gram @section Ref.Gram @c * Ref.Gram:: Grammar. @emph{TODO: mostly LL(1), it reads like C, Alef and bits of Napier; formalize here}. @page @node Ref.Comp @section Ref.Comp @c * Ref.Comp:: Compilation and component model. @cindex Compilation model Rust is a @emph{compiled} language. Its semantics are divided along a @emph{phase distinction} between compile-time and run-time. Those semantic rules that have a @emph{static interpretation} govern the success or failure of compilation. A program that fails to compile due to violation of a compile-time rule has no defined semantics at run-time; the compiler should halt with an error report, and produce no executable artifact. The compilation model centres on artifacts called @emph{crates}. Each compilation is directed towards a single crate in source form, and if successful produces a single crate in executable form. @menu * Ref.Comp.Crate:: Units of compilation and linking. * Ref.Comp.Meta:: Metadata about a crate. * Ref.Comp.Syntax:: Syntax extensions. @end menu @node Ref.Comp.Crate @subsection Ref.Comp.Crate @c * Ref.Comp.Crate:: Units of compilation and linking. @cindex Crate A @dfn{crate} is a unit of compilation and linking, as well as versioning, distribution and runtime loading. Crates are defined by @emph{crate source files}, which are a type of source file written in a special declarative language: @emph{crate language}.@footnote{A crate is somewhat analogous to an @emph{assembly} in the ECMA-335 CLI model, a @emph{library} in the SML/NJ Compilation Manager, a @emph{unit} in the Owens and Flatt module system, or a @emph{configuration} in Mesa.} A crate source file describes: @itemize @item Metadata about the crate, such as author, name, version, and copyright. @item The source-file and directory modules that make up the crate. @item The set of syntax extensions to enable for the crate. @item Any external crates or native modules that the crate imports to its top level. @item The organization of the crate's internal namespace. @item The set of names exported from the crate. @end itemize A single crate source file may describe the compilation of a large number of Rust source files; it is compiled in its entirety, as a single indivisible unit. The compilation phase attempts to transform a single crate source file, and its referenced contents, into a single compiled crate. Crate source files and compiled crates have a 1:1 relationship. The syntactic form of a crate is a sequence of @emph{directives}, some of which have nested sub-directives. A crate defines an implicit top-level anonymous module: within this module, all members of the crate have canonical path names. @xref{Ref.Path}. The @code{mod} directives within a crate file specify sub-modules to include in the crate: these are either directory modules, corresponding to directories in the filesystem of the compilation environment, or file modules, corresponding to Rust source files. The names given to such modules in @code{mod} directives become prefixes of the paths of items and slots defined within any included Rust source files. The @code{use} directives within the crate specify @emph{other crates} to scan for, locate, import into the crate's module namespace during compilation, and link against at runtime. Use directives may also occur independently in rust source files. These directives may specify loose or tight ``matching criteria'' for imported crates, depending on the preferences of the crate developer. In the simplest case, a @code{use} directive may only specify a symbolic name and leave the task of locating and binding an appropriate crate to a compile-time heuristic. In a more controlled case, a @code{use} directive may specify any metadata as matching criteria, such as a URI, an author name or version number, a checksum or even a cryptographic signature, in order to select an an appropriate imported crate. @xref{Ref.Comp.Meta}. The compiled form of a crate is a loadable and executable object file full of machine code, in a standard loadable operating-system format such as ELF, PE or Mach-O. The loadable object contains extensive DWARF metadata, describing: @itemize @item Metadata required for type reflection. @item The publicly exported module structure of the crate. @item Any metadata about the crate, defined by @code{meta} directives. @item The crates to dynamically link with at run-time, with matching criteria derived from the same @code{use} directives that guided compile-time imports. @end itemize The @code{syntax} directives of a crate are similar to the @code{use} directives, except they govern the syntax extension namespace (accessed through the syntax-extension sigil @code{#}, @pxref{Ref.Comp.Syntax}) available only at compile time. A @code{syntax} directive also makes its extension available to all subsequent directives in the crate file. An example of a crate: @example // Metadata about this crate meta (author = "Jane Doe", name = "projx" desc = "Project X", ver = "2.5"); // Import a module. use std (ver = "1.0"); // Activate a syntax-extension. syntax re; // Define some modules. mod foo = "foo.rs"; mod bar @{ mod quux = "quux.rs"; @} @end example @node Ref.Comp.Meta @subsection Ref.Comp.Meta @cindex Metadata, in crates In a crate, a @code{meta} directive associates free form key-value metadata with the crate. This metadata can, in turn, be used in providing partial matching parameters to syntax-extension loading and crate importing directives, denoted by @code{syntax} and @code{use} keywords respectively. Alternatively, metadata can serve as a simple form of documentation. @node Ref.Comp.Syntax @subsection Ref.Comp.Syntax @c * Ref.Comp.Syntax:: Syntax extension. @cindex Syntax extension Rust provides a notation for @dfn{syntax extension}. The notation is a marked syntactic form that can appear as an expression, statement or item in the body of a Rust program, or as a directive in a Rust crate, and which causes the text enclosed within the marked form to be translated through a named extension function loaded into the compiler at compile-time. The compile-time extension function must return a value of the corresponding Rust AST type, either an expression node, a statement node or an item node. @footnote{The syntax-extension system is analogous to the extensible reader system provided by Lisp @emph{readtables}, or the Camlp4 system of Objective Caml.} @xref{Ref.Lex.Syntax}. A syntax extension is enabled by a @code{syntax} directive, which must occur in a crate file. When the Rust compiler encounters a @code{syntax} directive in a crate file, it immediately loads the named syntax extension, and makes it available for all subsequent crate directives within the enclosing block scope of the crate file, and all Rust source files referenced as modules from the enclosing block scope of the crate file. For example, this extension might provide a syntax for regular expression literals: @example // In a crate file: // Requests the 're' syntax extension from the compilation environment. syntax re; // Also declares an import dependency on the module 're'. use re; // Reference to a Rust source file as a module in the crate. mod foo = "foo.rs"; @dots{} // In the source file "foo.rs", use the #re syntax extension and // the re module at run-time. let str s = get_string(); let regex pattern = #re.pat@{ aa+b? @}; let bool matched = re.match(pattern, s); @end example @page @node Ref.Mem @section Ref.Mem @c * Ref.Mem:: Semantic model of memory. @cindex Memory model @cindex Box @cindex Slot A Rust task's memory consists of a static set of @emph{items}, a set of tasks each with its own @emph{stack}, and a @emph{heap}. Immutable portions of the heap may be shared between tasks, mutable portions may not. Allocations in the stack consist of @emph{slots}, and allocations in the heap consist of @emph{boxes}. @menu * Ref.Mem.Alloc:: Memory allocation model. * Ref.Mem.Own:: Memory ownership model. * Ref.Mem.Slot:: Stack memory model. * Ref.Mem.Box:: Heap memory model. * Ref.Mem.Acct:: Memory accounting model. @end menu @node Ref.Mem.Alloc @subsection Ref.Mem.Alloc @c * Ref.Mem.Alloc:: Memory allocation model. @cindex Item @cindex Stack @cindex Heap @cindex Shared box @cindex Task-local box The @dfn{items} of a program are those functions, iterators, objects, modules and types that have their value calculated at compile-time and stored uniquely in the memory image of the rust process. Items are neither dynamically allocated nor freed. A task's @dfn{stack} consists of activation frames automatically allocated on entry to each function as the task executes. A stack allocation is reclaimed when control leaves the frame containing it. The @dfn{heap} is a general term that describes two separate sets of boxes: @emph{task-local} state boxes and the @emph{shared} non-state boxes. State boxes are @dfn{task-local}, owned by the task. Like any other state value, they cannot pass over channels. State boxes do not outlive the task that owns them. When unreferenced, they are collected using a general (cycle-aware) garbage-collector local to each task. Garbage collection within a local heap does not interrupt execution of other tasks. Non-state boxes are @dfn{shared}, and can be multiply-referenced by many different tasks. Like any other immutable type, they can pass over channels, and live as long as the last task referencing them within a given domain. When unreferenced, they are destroyed immediately (due to reference-counting) and returned to the heap memory allocator. Destruction of an immutable box also executes within the context of the task that drops the last reference to a shared heap allocation, so executing a long-running destructor does not interrupt execution of other tasks. @node Ref.Mem.Own @subsection Ref.Mem.Own @c * Ref.Mem.Own:: Memory ownership model. @cindex Ownership A task @emph{owns} all the @emph{stack-local} slot allocations in its stack and @emph{task-local} boxes accessible from its stack. A task @emph{shares} ownership of @emph{shared} boxes accessible from its stack. A task does not own any items. @dfn{Ownership} of an allocation means that the owning task is the only task that can access the allocation. @dfn{Sharing} of an allocation means that the same allocation may be concurrently read by multiple tasks. The only shared allocations are those that are non-state. When a stack frame is exited, its local allocations are all released, and its references to boxes (both shared and owned) are dropped. When a task finishes, its stack is necessarily empty and it therefore has no references to any boxes. @node Ref.Mem.Slot @subsection Ref.Mem.Slot @c * Ref.Mem.Slot:: Stack memory model. @cindex Stack @cindex Slot @cindex Local slot @cindex Alias slot A task's stack contains slots. A @dfn{slot} is a component of a stack frame. A slot is either @emph{local} or an @emph{alias}. A @dfn{local} slot (or @emph{stack-local} allocation) holds a value directly, allocated within the stack's memory. The value is a part of the stack frame. An @dfn{alias} references a value outside the frame. An alias may refer to a value allocated in another frame @emph{or} a boxed value in the heap. The alias-formation rules ensure that the referent of an alias will outlive the alias. Local slots are always implicitly mutable. Local slots are not initialized when allocated; the entire frame worth of local slots are allocated at once, on frame-entry, in an uninitialized state. Subsequent statements within a function may or may not initialize the local slots. Local slots can only be used after they have been initialized; this condition is guaranteed by the typestate system. Aliases can @emph{only} be declared as arguments in a function or iterator signature, bound to the lifetime of a stack frame. Aliases are not general values and cannot be held in boxed allocations or other general data types. Alias slots are indicated by the @emph{ampersand} sigil @code{&}. An example function that accepts an alias parameter: @example type point3d = rec(int x, int y, int z); fn extract_z(&point3d p) -> int @{ ret p.z; @} @end example An example function that accepts an alias to a mutable value: @example fn incr(& mutable int i) @{ i = i + 1; @} @end example @node Ref.Mem.Box @subsection Ref.Mem.Box @c * Ref.Mem.Box:: Heap memory model. @cindex Box @cindex Dereference operator A @dfn{box} is a reference to a reference-counted heap allocation holding another value. Box types and values are constructed by the @emph{at} sigil @code{@@}. An example of constructing a box type and value: @example let @@int x = @@10; @end example Some operations implicitly dereference boxes. Examples of such @dfn{implicit dereference} operations are: @itemize @item arithmetic operators (@code{x + y - z}) @item name-component selection (@code{x.y.z}) @end itemize An example of an implicit-dereference operation performed on box values: @example let @@int x = @@10; let @@int y = @@12; check (x + y == 22); @end example Other operations act on box values as single-word-sized address values, automatically adjusting reference counts on the associated heap allocation. For these operations, to access the value held in the box requires an explicit dereference of the box value. Explicitly dereferencing a box is indicated with the unary @emph{star} operator @code{*}. Examples of such @dfn{explicit dererence} operations are: @itemize @item copying box values (@code{x = y}) @item passing box values to functions (@code{f(x,y)}) @end itemize An example of an explicit-dereference operation performed on box values: @example fn takes_boxed(@@int b) @{ @} fn takes_unboxed(int b) @{ @} fn main() @{ let @@int x = @@10; takes_boxed(x); takes_unboxed(*x); @} @end example @node Ref.Mem.Acct @subsection Ref.Mem.Acct @c * Ref.Mem.Acct:: Memory accounting model. @cindex Domain @cindex Accounting @cindex Memory budget Every task belongs to a domain, and that domain tracks the amount of memory allocated and not yet released by tasks within it. @xref{Ref.Task.Dom}. Each domain has a memory budget. The @dfn{budget} of a domain is the maximum amount of memory that can be simultaneously allocated in the domain. If a task tries to allocate memory within a domain with an exceeded budget, the task will receive a signal. Within a task, accounting is strictly enforced: all memory allocated through the runtime library, both user data, sub-domains and runtime-support structures such as channel and signal queues, are charged to a task's domain. When a communication channel crosses from one domain to another, any value sent over the channel is guaranteed to have been @emph{detached} from the domain's memory graph (singly referenced, and/or deep-copied), so its memory cost is transferred to the receiving domain. @page @node Ref.Task @section Ref.Task @c * Ref.Task:: Semantic model of tasks. @cindex Task @cindex Process An executing Rust program consists of a tree of tasks. A Rust @dfn{task} consists of an entry function, a stack, a set of outgoing communication channels and incoming communication ports, and ownership of some portion of the heap of a single operating-system process. Multiple Rust tasks may coexist in a single operating-system process. Execution of multiple Rust tasks in a single operating-system process may be either truly concurrent or interleaved by the runtime scheduler. Rust tasks are lightweight: each consumes less memory than an operating-system process, and switching between Rust tasks is faster than switching between operating-system processes. @menu * Ref.Task.Comm:: Inter-task communication. * Ref.Task.Life:: Task lifecycle and state transitions. * Ref.Task.Dom:: Task domains. * Ref.Task.Sched:: Task scheduling model. @end menu @node Ref.Task.Comm @subsection Ref.Task.Comm @c * Ref.Task.Comm:: Inter-task communication. @cindex Communication @cindex Port @cindex Channel @cindex Message passing @cindex Send statement @cindex Receive statement With the exception of @emph{unsafe} constructs, Rust tasks are isolated from interfering with one another's memory directly. Instead of manipulating shared storage, Rust tasks communicate with one another using a typed, asynchronous, simplex message-passing system. A @dfn{port} is a communication endpoint that can @emph{receive} messages. Ports receive messages from channels. A @dfn{channel} is a communication endpoint that can @emph{send} messages. Channels send messages to ports. Each port is implicitly boxed and mutable; as such a port has has a unique per-task identity and cannot be replicated or transmitted. If a port value is copied, both copies refer to the @emph{same} port. New ports can be constructed dynamically and stored in data structures. Each channel is bound to a port when the channel is constructed, so the destination port for a channel must exist before the channel itself. A channel cannot be rebound to a different port from the one it was constructed with. Many channels can be bound to the same port, but each channel is bound to a single port. In other words, channels and ports exist in an N:1 relationship, N channels to 1 port. @footnote{It may help to remember nautical terminology when differentiating channels from ports. Many different waterways -- channels -- may lead to the same port.} Each port and channel can carry only one type of message. The message type is encoded as a parameter of the channel or port type. The message type of a channel is equal to the message type of the port it is bound to. Messages are sent asynchronously or semi-synchronously. A channel contains a message queue and asynchronously sending a message merely inserts it into the sending channel's queue; message receipt is the responsibility of the receiving task. Queued messages in channels are charged to the domain of the @emph{sending} task. If too many messages are queued for transmission from a single sending task, without being received by a receiving task, the sending task may exceed its memory budget, which causes a run-time signal. To help control this possibility, a semi-synchronous send operation is possible, which blocks until there is room in the existing queue and then executes an asynchronous send. The asynchronous message-send operator is @code{<+}. The semi-synchronous message-send operator is @code{<|}. @xref{Ref.Stmt.Send}. The message-receive operator is @code{<-}. @xref{Ref.Stmt.Recv}. @node Ref.Task.Life @subsection Ref.Task.Life @c * Ref.Task.Life:: Task lifecycle and state transitions. @cindex Lifecycle of task @cindex Scheduling @cindex Running, task state @cindex Blocked, task state @cindex Failing, task state @cindex Dead, task state @cindex Soft failure @cindex Hard failure The @dfn{lifecycle} of a task consists of a finite set of states and events that cause transitions between the states. The lifecycle states of a task are: @itemize @item running @item blocked @item failing @item dead @end itemize A task begins its lifecycle -- once it has been spawned -- in the @emph{running} state. In this state it executes the statements of its entry function, and any functions called by the entry function. A task may transition from the @emph{running} state to the @emph{blocked} state any time it executes a communication statement on a port or channel that cannot be immediately completed. When the communication statement can be completed -- when a message arrives at a sender, or a queue drains sufficiently to complete a semi-synchronous send -- then the blocked task will unblock and transition back to @emph{running}. A task may transition to the @emph{failing} state at any time, due to an un-trapped signal or the execution of a @code{fail} statement. Once @emph{failing}, a task unwinds its stack and transitions to the @emph{dead} state. Unwinding the stack of a task is done by the task itself, on its own control stack. If a value with a destructor is freed during unwinding, the code for the destructor is run, also on the task's control stack. Running the destructor code causes a temporary transition to a @emph{running} state, and allows the destructor code to cause any subsequent state transitions. The original task of unwinding and failing thereby may suspend temporarily, and may involve (recursive) unwinding of the stack of a failed destructor. Nonetheless, the outermost unwinding activity will continue until the stack is unwound and the task transitions to the @emph{dead} state. There is no way to ``recover'' from task failure. Once a task has temporarily suspended its unwinding in the @emph{failing} state, failure occurring from within this destructor results in @emph{hard} failure. The unwinding procedure of hard failure frees resources but does not execute destructors. The original (soft) failure is still resumed at the point where it was temporarily suspended. A task in the @emph{dead} state cannot transition to other states; it exists only to have its termination status inspected by other tasks, and/or to await reclamation when the last reference to it drops. @node Ref.Task.Dom @subsection Ref.Task.Dom @c * Ref.Task.Dom:: Task domains @cindex Domain @cindex Process @cindex Thread Every task belongs to a domain. A @dfn{domain} is a structure that owns tasks, schedules tasks, tracks memory allocation within tasks and manages access to runtime services on behalf of tasks. Typically each domain runs on a separate operating-system @emph{thread}, or within an isolated operating-system @emph{process}. An easy way to think of a domain is as an abstraction over either an operating-system thread @emph{or} a process. The key feature of a domain is that it isolates memory references created by the Rust tasks within it. No Rust task can refer directly to memory outside its domain. Tasks can own sub-domains, which in turn own their own tasks. Every domain owns one @emph{root task}, which is the root of the tree of tasks owned by the domain. @node Ref.Task.Sched @subsection Ref.Task.Sched @c * Ref.Task.Sched:: Task scheduling model. @cindex Scheduling @cindex Preemption @cindex Yielding control Every task is @emph{scheduled} within its domain. @xref{Ref.Task.Dom}. The currently scheduled task is given a finite @emph{time slice} in which to execute, after which it is @emph{descheduled} at a loop-edge or similar preemption point, and another task within the domain is scheduled, pseudo-randomly. An executing task can @code{yield} control at any time, which deschedules it immediately. Entering any other non-executing state (blocked, dead) similarly deschedules the task. @page @node Ref.Item @section Ref.Item @c * Ref.Item:: The components of a module. @cindex Item @cindex Type parameters @cindex Module item An @dfn{item} is a component of a module. Items are entirely determined at compile-time, remain constant during execution, and may reside in read-only memory. There are five primary kinds of item: modules, functions, iterators, objects and types. All items form an implicit scope for the declaration of sub-items. In other words, within a function, object or iterator, declarations of items can (in many cases) be mixed with the statements, control blocks, and similar artifacts that otherwise compose the item body. The meaning of these scoped items is the same as if the item was declared outside the scope, except that the item's @emph{path name} within the module namespace is qualified by the name of the enclosing item. The exact locations in which sub-items may be declared is given by the grammar. @xref{Ref.Gram}. Functions, iterators, objects and types may be @emph{parametrized} by type. Type parameters are given as a comma-separated list of identifiers enclosed in square brackets (@code{[]}), after the name of the item and before its definition. The type parameters of an item are part of the name, not the type of the item; in order to refer to the type-parametrized item, a referencing name must in general provide type arguments as a list of comma-separated types enclosed within square brackets (though the type-inference system can often infer such argument types from context). There are no general parametric types. @menu * Ref.Item.Mod:: Items defining modules. * Ref.Item.Fn:: Items defining functions. * Ref.Item.Iter:: Items defining iterators. * Ref.Item.Obj:: Items defining objects. * Ref.Item.Type:: Items defining the types of values and slots. * Ref.Item.Tag:: Items defining the constructors of a tag type. @end menu @node Ref.Item.Mod @subsection Ref.Item.Mod @c * Ref.Item.Mod:: Items defining sub-modules. @cindex Module item @cindex Importing names @cindex Exporting names @cindex Visibility control A @dfn{module item} contains declarations of other @emph{items}. The items within a module may be functions, modules, objects or types. These declarations have both static and dynamic interpretation. The purpose of a module is to organize @emph{names} and control @emph{visibility}. Modules are declared with the keyword @code{mod}. An example of a module: @example mod math @{ type complex = (f64,f64); fn sin(f64) -> f64 @{ @dots{} @} fn cos(f64) -> f64 @{ @dots{} @} fn tan(f64) -> f64 @{ @dots{} @} @dots{} @} @end example Modules may also include any number of @dfn{import and export declarations}. These declarations must precede any module item declarations within the module, and control the visibility of names both within the module and outside of it. @menu * Ref.Item.Mod.Import:: Declarations for module-local synonyms. * Ref.Item.Mod.Export:: Declarations for restricting visibility. @end menu @node Ref.Item.Mod.Import @subsubsection Ref.Item.Mod.Import @c * Ref.Item.Mod.Import:: Declarations for module-local synonyms. @cindex Importing names @cindex Visibility control An @dfn{import declaration} creates one or more local name bindings synonymous with some other name. Usually an import declaration is used to shorten the path required to refer to a module item. @emph{Note}: unlike many languages, Rust's @code{import} declarations do @emph{not} declare linkage-dependency with external crates. Linkage dependencies are independently declared with @code{use} declarations. @xref{Ref.Comp.Crate}. An example of an import: @example import std.math.sin; fn main() @{ // Equivalent to 'log std.math.sin(1.0);' log sin(1.0); @} @end example @node Ref.Item.Mod.Export @subsubsection Ref.Item.Mod.Export @c * Ref.Item.Mod.Import:: Declarations for restricting visibility. @cindex Exporting names @cindex Visibility control An @dfn{export declaration} restricts the set of local declarations within a module that can be accessed from code outside the module. By default, all local declarations in a module are exported. If a module contains an export declaration, this declaration replaces the default export with the export specified. An example of an export: @example mod foo @{ export primary; fn primary() @{ helper(1, 2); helper(3, 4); @} fn helper(int x, int y) @{ @dots{} @} @} fn main() @{ foo.primary(); // Will compile. foo.helper(2,3) // ERROR: will not compile. @} @end example @node Ref.Item.Fn @subsection Ref.Item.Fn @c * Ref.Item.Fn:: Items defining functions. @cindex Functions @cindex Slots, function input and output @cindex Effect of a function @cindex Predicate A @dfn{function item} defines a sequence of statements associated with a name and a set of parameters. Functions are declared with the keyword @code{fn}. Functions declare a set of @emph{input slots} as parameters, through which the caller passes arguments into the function, and an @emph{output slot} through which the function passes results back to the caller. A function may also be copied into a first class @emph{value}, in which case the value has the corresponding @emph{function type}, and can be used otherwise exactly as a function item (with a minor additional cost of calling the function, as such a call is indirect). @xref{Ref.Type.Fn}. Every control path in a function ends with either a @code{ret} or @code{be} statement. If a control path lacks a @code{ret} statement in source code, an implicit @code{ret} statement is appended to the end of the control path during compilation, returning the implicit @code{()} value. A function may have an @emph{effect}, which may be either @code{io}, @code{state}, @code{unsafe}. If no effect is specified, the function is said to be @dfn{pure}. Any pure boolean function is also called a @emph{predicate}, and may be used as part of the static typestate system. @xref{Ref.Stmt.Stat.Constr}. An example of a function: @example fn add(int x, int y) -> int @{ ret x + y; @} @end example @node Ref.Item.Iter @subsection Ref.Item.Iter @c * Ref.Item.Iter:: Items defining iterators. @cindex Iterators @cindex Put statement @cindex Put each statement @cindex Foreach statement Iterators are function-like items that can @code{put} multiple values during their execution before returning or tail-calling. Putting a value is similar to returning a value -- the argument to @code{put} is copied into the caller's frame and control transfers back to the caller -- but the iterator frame is only @emph{suspended} during the put, and will be @emph{resumed} at the statement after the @code{put}, on the next iteration of the caller's loop. The output type of an iterator is the type of value that the function will @code{put}, before it eventually executes a @code{ret} or @code{be} statement of type @code{()} and completes its execution. An iterator can only be called in the loop header of a matching @code{for each} loop or as the argument in a @code{put each} statement. @xref{Ref.Stmt.Foreach}. An example of an iterator: @example iter range(int lo, int hi) -> int @{ let int i = lo; while (i < hi) @{ put i; i = i + 1; @} @} let int sum = 0; for each (int x = range(0,100)) @{ sum += x; @} @end example @node Ref.Item.Obj @subsection Ref.Item.Obj @c * Ref.Item.Obj:: Items defining objects. @cindex Objects @cindex Object constructors An @dfn{object item} defines the @emph{state} and @emph{methods} of a set of @emph{object values}. Object values have object types. @xref{Ref.Type.Obj}. An @emph{object item} declaration -- in addition to providing a scope for state and method declarations -- implicitly declares a static function called the @emph{object constructor}, as well as a named @emph{object type}. The name given to the object item is resolved to a type when used in type context, or a constructor function when used in value context (such as a call). Example of an object item: @example obj counter(int state) @{ fn incr() @{ state += 1; @} fn get() -> int @{ ret state; @} @} let counter c = counter(1); c.incr(); c.incr(); check (c.get() == 3); @end example There is no @emph{this} or @emph{self} available inside an object's methods, either implicitly or explicitly, so you can't directly call other methods. For example: @example obj my_obj() @{ fn get() -> int @{ ret 3; @} fn foo() @{ auto c = get(); // Fails @} @} @end example The current replacement is to write a factory function for your type, which provides any private helper functions: @example type my_obj = obj @{ fn get() -> int; fn foo(); @}; fn mk_my_obj() -> my_obj @{ fn get_helper() -> int @{ ret 3; @} obj impl() @{ fn get() -> int @{ ret get_helper(); @} fn foo() @{ auto c = get_helper(); // Works @} @} ret impl(); @} @end example This factory function also allows you to bind the object's state variables to initial values. @node Ref.Item.Type @subsection Ref.Item.Type @c * Ref.Item.Type:: Items defining the types of values and slots. @cindex Types A @dfn{type} defines an @emph{interpretation} of a value in memory. @xref{Ref.Type}. Types are declared with the keyword @code{type}. A type's interpretation is used for the values held in any slot with that type. @xref{Ref.Mem.Slot}. The interpretation of a value includes: @itemize @item Whether the value is composed of sub-values or is indivisible. @item Whether the value represents textual or numerical information. @item Whether the value represents integral or floating-point information. @item The sequence of memory operations required to access the value. @item Whether the value is mutable or immutable. @end itemize For example, the type @code{rec(u8 x, u8 y)} defines the interpretation of values that are composite records, each containing two unsigned 8-bit integers accessed through the components @code{x} and @code{y}, and laid out in memory with the @code{x} component preceding the @code{y} component. @node Ref.Item.Tag @subsection Ref.Item.Tag @c * Ref.Item.Type:: Items defining the constructors of a tag type. @cindex Tag types A tag item simultaneously declares a new nominal tag type (@pxref{Ref.Type.Tag}) as well as a set of @emph{constructors} that can be used to create or pattern-match values of the corresponding tag type. The constructors of a @code{tag} type may be recursive: that is, each constructor may take an argument that refers, directly or indirectly, to the tag type the constructor is a member of. Such recursion has restrictions: @itemize @item Recursive types can only be introduced through @code{tag} constructors. @item A recursive @code{tag} item must have at least one non-recursive constructor (in order to give the recursion a basis case). @item The recursively argument of recursive tag constructors must be @emph{box} values (in order to bound the in-memory size of the constructor). @item Recursive type definitions can cross module boundaries, but not module @emph{visibility} boundaries, nor crate boundaries (in order to simplify the module system). @end itemize An example of a @code{tag} item and its use: @example tag animal @{ dog(); cat(); @} let animal a = dog(); a = cat(); @end example An example of a @emph{recursive} @code{tag} item and its use: @example tag list[T] @{ nil(); cons(T, @@list[T]); @} let list[int] a = cons(7, cons(13, nil())); @end example @page @node Ref.Type @section Ref.Type @cindex Types Every slot and value in a Rust program has a type. The @dfn{type} of a @emph{value} defines the interpretation of the memory holding it. The type of a @emph{slot} may also include constraints. @xref{Ref.Type.Constr}. Built-in types and type-constructors are tightly integrated into the language, in nontrivial ways that are not possible to emulate in user-defined types. User-defined types have limited capabilities. In addition, every built-in type or type-constructor name is reserved as a @emph{keyword} in Rust; they cannot be used as user-defined identifiers in any context. @menu * Ref.Type.Any:: An open union of every possible type. * Ref.Type.Mach:: Machine-level types. * Ref.Type.Int:: The machine-dependent integer types. * Ref.Type.Float:: The machine-dependent floating-point types. * Ref.Type.Prim:: Primitive types. * Ref.Type.Big:: The arbitrary-precision integer type. * Ref.Type.Text:: Strings and characters. * Ref.Type.Rec:: Labeled products of heterogeneous types. * Ref.Type.Tup:: Unlabeled products of homogeneous types. * Ref.Type.Vec:: Open products of homogeneous types. * Ref.Type.Tag:: Disjoint unions of heterogeneous types. * Ref.Type.Fn:: Subroutine types. * Ref.Type.Iter:: Scoped coroutine types. * Ref.Type.Port:: Unique inter-task message-receipt endpoints. * Ref.Type.Chan:: Copyable inter-task message-send capabilities. * Ref.Type.Task:: General coroutine-instance types. * Ref.Type.Obj:: Abstract types. * Ref.Type.Constr:: Constrained types. * Ref.Type.Type:: Types describing types. @end menu @node Ref.Type.Any @subsection Ref.Type.Any @cindex Any type @cindex Dynamic type, see @i{Any type} @cindex Reflection @cindex Alt type statement The type @code{any} is the union of all possible Rust types. A value of type @code{any} is represented in memory as a pair consisting of a boxed value of some non-@code{any} type @var{T} and a reflection of the type @var{T}. Values of type @code{any} can be used in an @code{alt type} statement, in which the reflection is used to select a block corresponding to a particular type extraction. @xref{Ref.Stmt.Alt}. @node Ref.Type.Mach @subsection Ref.Type.Mach @cindex Machine types @cindex Floating-point types @cindex Integer types @cindex Word types The machine types are the following: @itemize @item The unsigned word types @code{u8}, @code{u16}, @code{u32} and @code{u64}, with values drawn from the integer intervals @iftex @math{[0, 2^8 - 1]}, @math{[0, 2^{16} - 1]}, @math{[0, 2^{32} - 1]} and @math{[0, 2^{64} - 1]} @end iftex @ifhtml @html [0, 28-1], [0, 216-1], [0, 232-1] and [0, 264-1] @end html @end ifhtml respectively. @item The signed two's complement word types @code{i8}, @code{i16}, @code{i32} and @code{i64}, with values drawn from the integer intervals @iftex @math{[-(2^7),(2^7)-1)]}, @math{[-(2^{15}),2^{15}-1)]}, @math{[-(2^{31}),2^{31}-1)]} and @math{[-(2^{63}),2^{63}-1)]} @end iftex @ifhtml @html [-(27), 27-1], [-(215), 215-1], [-(231), 231-1] and [-(263), 263-1] @end html @end ifhtml respectively. @item The IEEE 754-2008 @code{binary32} and @code{binary64} floating-point types: @code{f32} and @code{f64}, respectively. @end itemize @node Ref.Type.Int @subsection Ref.Type.Int @cindex Machine-dependent types @cindex Integer types @cindex Word types The Rust type @code{uint}@footnote{A Rust @code{uint} is analogous to a C99 @code{uintptr_t}.} is an unsigned integer type with with target-machine-dependent size. Its size, in bits, is equal to the number of bits required to hold any memory address on the target machine. The Rust type @code{int}@footnote{A Rust @code{int} is analogous to a C99 @code{intptr_t}.} is a two's complement signed integer type with target-machine-dependent size. Its size, in bits, is equal to the size of the rust type @code{uint} on the same target machine. @node Ref.Type.Float @subsection Ref.Type.Float @cindex Machine-dependent types @cindex Floating-point types The Rust type @code{float} is a machine-specific type equal to one of the supported Rust floating-point machine types (@code{f32} or @code{f64}). It is the largest floating-point type that is directly supported by hardware on the target machine, or if the target machine has no floating-point hardware support, the largest floating-point type supported by the software floating-point library used to support the other floating-point machine types. Note that due to the preference for hardware-supported floating-point, the type @code{float} may not be equal to the largest @emph{supported} floating-point type. @node Ref.Type.Prim @subsection Ref.Type.Prim @cindex Primitive types @cindex Integer types @cindex Floating-point types @cindex Character type @cindex Boolean type The primitive types are the following: @itemize @item The ``nil'' type @code{()}, having the single ``nil'' value @code{()}.@footnote{The ``nil'' value @code{()} is @emph{not} a sentinel ``null pointer'' value for alias slots; the ``nil'' type is the implicit return type from functions otherwise lacking a return type, and can be used in other contexts (such as message-sending or type-parametric code) as a zero-size type.} @item The boolean type @code{bool} with values @code{true} and @code{false}. @item The machine types. @item The machine-dependent integer and floating-point types. @end itemize @node Ref.Type.Big @subsection Ref.Type.Big @cindex Integer types @cindex Big integer type The Rust type @code{big}@footnote{A Rust @code{big} is analogous to a Lisp bignum or a Python long integer.} is an arbitrary precision integer type that fits in a machine word @emph{when possible} and transparently expands to a boxed ``big integer'' allocated in the run-time heap when it overflows or underflows outside of the range of a machine word. A Rust @code{big} grows to accommodate extra binary digits as they are needed, by taking extra memory from the memory budget available to each Rust task, and should only exhaust its range due to memory exhaustion. @node Ref.Type.Text @subsection Ref.Type.Text @cindex Text types @cindex String type @cindex Character type @cindex Unicode @cindex UCS-4 @cindex UTF-8 The types @code{char} and @code{str} hold textual data. A value of type @code{char} is a Unicode character, represented as a 32-bit unsigned word holding a UCS-4 codepoint. A value of type @code{str} is a Unicode string, represented as a vector of 8-bit unsigned bytes holding a sequence of UTF-8 codepoints. @node Ref.Type.Rec @subsection Ref.Type.Rec @cindex Record types @cindex Structure types, see @i{Record types} The record type-constructor @code{rec} forms a new heterogeneous product of values.@footnote{The @code{rec} type-constructor is analogous to the @code{struct} type-constructor in the Algol/C family, the @emph{record} types of the ML family, or the @emph{structure} types of the Lisp family.} Fields of a @code{rec} type are accessed by name and are arranged in memory in the order specified by the @code{rec} type. An example of a @code{rec} type and its use: @example type point = rec(int x, int y); let point p = rec(x=10, y=11); let int px = p.x; @end example @node Ref.Type.Tup @subsection Ref.Type.Tup @cindex Tuple types The tuple type-constructor @code{tup} forms a new heterogeneous product of values exactly as the @code{rec} type-constructor does, with the difference that tuple members are automatically assigned implicit field names, given by ascending integers prefixed by the underscore character: @code{_0}, @code{_1}, @code{_2}, etc. The members of a tuple are laid out in memory contiguously, like a record, in order specified by the tuple type. An example of a tuple type and its use: @example type pair = tup(int,str); let pair p = tup(10,"hello"); check (p._0 == 10); p._1 = "world"; check (p._1 == "world"); @end example @node Ref.Type.Vec @subsection Ref.Type.Vec @cindex Vector types @cindex Array types, see @i{Vector types} The vector type-constructor @code{vec} represents a homogeneous array of values of a given type. A vector has a fixed size. If the member-type of a vector is a state type, then vector is a @emph{state} type, like any type containing another type. Vectors can be sliced. A slice expression builds a new vector by copying a contiguous range -- given by a pair of indices representing a half-open interval -- out of the sliced vector. An example of a @code{vec} type and its use: @example let vec[int] v = vec(7, 5, 3); let int i = v.(2); let vec[int] v2 = v.(0,1); // Form a slice. @end example Vectors always @emph{allocate} a storage region sufficient to store the first power of two worth of elements greater than or equal to the size of the vector. This behaviour supports idiomatic in-place ``growth'' of a mutable slot holding a vector: @example let mutable vec[int] v = vec(1, 2, 3); v += vec(4, 5, 6); @end example Normal vector concatenation causes the allocation of a fresh vector to hold the result; in this case, however, the slot holding the vector recycles the underlying storage in-place (since the reference-count of the underlying storage is equal to 1). All accessible elements of a vector are always initialized, and access to a vector is always bounds-checked. @node Ref.Type.Tag @subsection Ref.Type.Tag @cindex Tag types @cindex Union types, see @i{Tag types} A @emph{tag type} is a nominal, heterogeneous disjoint union type.@footnote{The @code{tag} type is analogous to a @code{data} constructor declaration in ML or a @emph{pick ADT} in Limbo.} A @code{tag} @emph{item} consists of a number of @emph{constructors}, each of which is independently named and takes an optional tuple of arguments. Tag types cannot be denoted @emph{structurally} as types, but must be denoted by named reference to a @emph{tag item} declaration. @xref{Ref.Item.Tag}. @node Ref.Type.Fn @subsection Ref.Type.Fn @cindex Function types The function type-constructor @code{fn} forms new function types. A function type consists of a sequence of input slots, an optional set of input constraints (@pxref{Ref.Stmt.Stat.Constr}), an output slot, and an @emph{effect}. @xref{Ref.Item.Fn}. An example of a @code{fn} type: @example fn add(int x, int y) -> int @{ ret x + y; @} let int x = add(5,7); type binop = fn(int,int) -> int; let binop bo = add; x = bo(5,7); @end example @node Ref.Type.Iter @subsection Ref.Type.Iter @cindex Iterator types The iterator type-constructor @code{iter} forms new iterator types. An iterator type consists a sequence of input slots, an optional set of input constraints, an output slot, and an @emph{effect}. @xref{Ref.Item.Iter}. An example of an @code{iter} type: @example iter range(int x, int y) -> int @{ while (x < y) @{ put x; x += 1; @} @} for each (int i = range(5,7)) @{ @dots{}; @} @end example @node Ref.Type.Port @subsection Ref.Type.Port @cindex Port types @cindex Communication The port type-constructor @code{port} forms types that describe ports. A port is the @emph{receiving end} of a typed, asynchronous, simplex inter-task communication facility. @xref{Ref.Task.Comm}. A @code{port} type takes a single type parameter, denoting the type of value that can be received from a @code{port} value of that type. Ports are modeled as mutable native types with built-in meaning to the language. They cannot be transmitted over channels or otherwise replicated, and are always local to the task that creates them. An example of a @code{port} type: @example type port[vec[str]] svp; let svp p = get_port(); let vec[str] v; v <- p; @end example @node Ref.Type.Chan @subsection Ref.Type.Chan @cindex Channel types @cindex Communication The channel type-constructor @code{chan} forms types that describe channels. A channel is the @emph{sending end} of a typed, asynchronous, simplex inter-task communication facility. @xref{Ref.Task.Comm}. A @code{chan} type takes a single type parameter, denoting the type of value that can be sent to a channel of that type. Channels are immutable, and can be transmitted over channels to other tasks. They are modeled as immutable native types with built-in meaning to the language. When a task sends a message into a channel, the task forms an outgoing queue associated with that channel. The per-task queue @emph{associated} with a channel can be indirectly manipulated by the task, but is @emph{not} otherwise considered ``part of'' to the channel: the queue is ``part of'' the @emph{sending task}. Sending a channel to another task does not copy the queue associated with the channel. Channels are also @emph{weak}: a channel is directly coupled to a particular destination port on a particular task, but does not keep that port or task @emph{alive}. A channel may therefore fail to operate at any moment. If a task sends to a channel that is connected to a nonexistent port, it receives a signal. An example of a @code{chan} type: @example type chan[vec[str]] svc; let svc c = get_chan(); let vec[str] v = vec("hello", "world"); c <| v; @end example @node Ref.Type.Task @subsection Ref.Type.Task @cindex Task type The task type @code{task} describes values that are @emph{live tasks}. Tasks form an @emph{ownership tree} in which each task (except the root task) is directly owned by exactly one parent task. The purpose of a variable of @code{task} type is to manage the lifecycle of the associated task. Communication is carried out solely using channels and ports. Like ports, tasks are modeled as mutable native types with built-in meaning to the language. They cannot be transmitted over channels or otherwise replicated, and are always local to the task that spawns them. If all references to a task are dropped (due to the release of any structure holding those references), the released task immediately fails. @xref{Ref.Task.Life}. @node Ref.Type.Obj @subsection Ref.Type.Obj @c * Ref.Type.Obj:: Object types. @cindex Object types A @dfn{object type} describes values of abstract type, that carry some hidden @emph{fields} and are accessed through a set of un-ordered @emph{methods}. Every object item (@pxref{Ref.Item.Obj}) implicitly declares an object type carrying methods with types derived from all the methods of the object item. Object types can also be declared in isolation, independent of any object item declaration. Such a ``plain'' object type can be used to describe an interface that a variety of particular objects may conform to, by supporting a superset of the methods. An object type that can contain a state must be declared as a @code{state obj} like any other state type. And similarly a method type that performs I/O or makes native calls must be declared @code{io} or @code{unsafe}, like any other function. Moreover, @emph{all} methods of a state object are implicitly state functions -- as they all bind the same mutable state field(s) -- so implicitly have an effect lower than @code{io}. It is therefore unnecessary to declare methods within a state object type (or state object item) as @code{io}. An example of an object type with two separate object items supporting it, and a client function using both items via the object type: @example state type taker = state obj @{ fn take(int); @}; state obj adder(mutable int x) @{ fn take(int y) @{ x += y; @} @} obj sender(chan[int] c) @{ io fn take(int z) @{ c <| z; @} @} fn give_ints(taker t) @{ t.take(1); t.take(2); t.take(3); @} let port[int] p = port(); let taker t1 = adder(0); let taker t2 = sender(chan(p)); give_ints(t1); give_ints(t2); @end example @node Ref.Type.Constr @subsection Ref.Type.Constr @c * Ref.Type.Constr:: Constrained types. @cindex Constrained types A @dfn{constrained type} is a type that carries a @emph{formal constraint} (@pxref{Ref.Stmt.Stat.Constr}), which is similar to a normal constraint except that the @emph{base name} of any slots mentioned in the constraint must be the special @emph{formal symbol} @emph{*}. When a constrained type is instantiated in a particular slot declaration, the formal symbol in the constraint is replaced with the name of the declared slot and the resulting constraint is checked immediately after the slot is declared. @xref{Ref.Stmt.Check}. An example of a constrained type with two separate instantiations: @example type ordered_range = rec(int low, int high) : less_than(*.low, *.high); let ordered_range rng1 = rec(low=5, high=7); // implicit: 'check less_than(rng1.low, rng1.high);' let ordered_range rng2 = rec(low=15, high=17); // implicit: 'check less_than(rng2.low, rng2.high);' @end example @node Ref.Type.Type @subsection Ref.Type.Type @c * Ref.Type.Type:: Types describing types. @cindex Type type @emph{TODO}. @page @node Ref.Expr @section Ref.Expr @c * Ref.Expr:: Parsed and primitive expressions. @cindex Expressions Rust has two kinds of expressions: @emph{parsed expressions} and @emph{primitive expressions}. The former are syntactic sugar and are eliminated during parsing. The latter are very minimal, consisting only of paths and primitive literals, possibly combined via a single level (non-recursive) unary or binary machine-level operation (ALU or FPU). @xref{Ref.Path}. For the most part, Rust semantics are defined in terms of @emph{statements}, which parsed expressions are desugared to. The desugaring is defined in the grammar. @xref{Ref.Gram}. The residual primitive statements appear only in the right hand side of copy statements, @xref{Ref.Stmt.Copy}. @page @node Ref.Stmt @section Ref.Stmt @c * Ref.Stmt:: Executable statements. @cindex Statements A @dfn{statement} is a component of a block, which is in turn a components of an outer block, a function or an iterator. When a function is spawned into a task, the task @emph{executes} statements in an order determined by the body of the enclosing structure. Each statement causes the task to perform certain actions. @menu * Ref.Stmt.Stat:: The static typestate system of statement analysis. * Ref.Stmt.Decl:: Statement declaring an item or slot. * Ref.Stmt.Copy:: Statement for copying a value. * Ref.Stmt.Spawn:: Statements for creating new tasks. * Ref.Stmt.Send:: Statements for sending a value into a channel. * Ref.Stmt.Recv:: Statement for receiving a value from a channel. * Ref.Stmt.Call:: Statement for calling a function. * Ref.Stmt.Bind:: Statement for binding arguments to functions. * Ref.Stmt.Ret:: Statement for stopping and producing a value. * Ref.Stmt.Be:: Statement for stopping and executing a tail call. * Ref.Stmt.Put:: Statement for pausing and producing a value. * Ref.Stmt.Fail:: Statement for causing task failure. * Ref.Stmt.Log:: Statement for logging values to diagnostic buffers. * Ref.Stmt.Note:: Statement for logging values during failure. * Ref.Stmt.While:: Statement for simple conditional looping. * Ref.Stmt.Break:: Statement for terminating a loop. * Ref.Stmt.Cont:: Statement for terminating a single loop iteration. * Ref.Stmt.For:: Statement for looping over strings and vectors. * Ref.Stmt.Foreach:: Statement for looping via an iterator. * Ref.Stmt.If:: Statement for simple conditional branching. * Ref.Stmt.Alt:: Statement for complex conditional branching. * Ref.Stmt.Prove:: Statement for static assertion of typestate. * Ref.Stmt.Check:: Statement for dynamic assertion of typestate. * Ref.Stmt.IfCheck:: Statement for dynamic testing of typestate. @end menu @node Ref.Stmt.Stat @subsection Ref.Stmt.Stat @c * Ref.Stmt.Stat:: The static typestate system of statement analysis. @cindex Typestate system Statements have a detailed static semantics. The static semantics determine, on a statement-by-statement basis, the @emph{effects} the statement has on its environment, as well the @emph{legality} of the statement in its environment. The legality of a statement is partly governed by syntactic rules, partly by its conformance to the types of value it affects, and partly by a statement-oriented static dataflow analysis. This section describes the statement-oriented static dataflow analysis, also called the @emph{typestate} system. @menu * Ref.Stmt.Stat.Point:: Inter-statement positions of logical judgements. * Ref.Stmt.Stat.CFG:: The control-flow graph formed by statements. * Ref.Stmt.Stat.Constr:: Predicates applied to slots. * Ref.Stmt.Stat.Cond:: Constraints required and implied by a statement. * Ref.Stmt.Stat.Typestate:: Constraints that hold at points. * Ref.Stmt.Stat.Check:: Relating dynamic state to static typestate. @end menu @node Ref.Stmt.Stat.Point @subsubsection Ref.Stmt.Stat.Point @c * Ref.Stmt.Stat.Point:: Inter-statement positions of logical judgements. @cindex Points A @dfn{point} exists before and after any statement in a Rust program. For example, this code: @example s = "hello, world"; print(s); @end example Consists of two statements and four points: @itemize @item the point before the first statement @item the point after the first statement @item the point before the second statement @item the point after the second statement @end itemize The typestate system reasons over points, rather than statements. This may seem counter-intuitive, but points are the more primitive concept. Another way of thinking about a point is as a set of @emph{instants in time} at which the state of a task is fixed. By contrast, a statement represents a @emph{duration in time}, during which the state of the task changes. The typestate system is concerned with constraining the possible states of a task's memory at @emph{instants}; it is meaningless to speak of the state of a task's memory ``at'' a statement, as each statement is likely to change the contents of memory. @node Ref.Stmt.Stat.CFG @subsubsection Ref.Stmt.Stat.CFG @c * Ref.Stmt.Stat.CFG:: The control-flow graph formed by statements. @cindex Control-flow graph Each @emph{point} can be considered a vertex in a directed @emph{graph}. Each kind of statement implies a single edge in this graph between the point before the statement and the point after it, as well as a set of zero or more edges from the points of the statement to points before other statements. The edges between points represent @emph{possible} indivisible control transfers that might occur during execution. This implicit graph is called the @dfn{control-flow graph}, or @dfn{CFG}. @node Ref.Stmt.Stat.Constr @subsubsection Ref.Stmt.Stat.Constr @c * Ref.Stmt.Stat.Constr:: Predicates applied to slots. @cindex Predicate @cindex Constraint A @dfn{predicate} is any pure boolean function. @xref{Ref.Item.Fn}. A @dfn{constraint} is a predicate applied to specific slots. For example, consider the following code: @example fn is_less_than(int a, int b) -> bool @{ ret a < b; @} fn test() @{ let int x = 10; let int y = 20; check is_less_than(x,y); @} @end example This example defines the predicate @code{is_less_than}, and applies it to the slots @code{x} and @code{y}. The constraint being checked on the third line of the function is @code{is_less_than(x,y)}. Predicates can only apply to slots holding immutable values. The slots a predicate applies to can themselves be mutable, but the types of values held in those slots must be immutable. @node Ref.Stmt.Stat.Cond @subsubsection Ref.Stmt.Stat.Cond @c * Ref.Stmt.Stat.Cond:: Constraints required and implied by a statement. @cindex Condition @cindex Precondition @cindex Postcondition A @dfn{condition} is a set of zero or more constraints. Each @emph{point} has an associated @emph{condition}: @itemize @item The @dfn{precondition} of a statement is the condition the statement requires in the point before the condition. @item The @dfn{postcondition} of a statement is the condition the statement enforces in the point after the statement. @end itemize Any constraint present in the precondition and @emph{absent} in the postcondition is considered to be @emph{dropped} by the statement. @node Ref.Stmt.Stat.Typestate @subsubsection Ref.Stmt.Stat.Typestate @c * Ref.Stmt.Stat.Typestate:: Constraints that hold at points. @cindex Typestate @cindex Prestate @cindex Poststate The typestate checking system @emph{calculates} an additional condition for each point called its typestate. For a given statement, we call the two typestates associated with its two points the prestate and a poststate. @itemize @item The @dfn{prestate} of a statement is the typestate of the point before the statement. @item The @dfn{poststate} of a statement is the typestate of the point after the statement. @end itemize A @dfn{typestate} is a condition that has @emph{been determined by the typestate algorithm} to hold at a point. This is a subtle but important point to understand: preconditions and postconditions are @emph{inputs} to the typestate algorithm; prestates and poststates are @emph{outputs} from the typestate algorithm. The typestate algorithm analyses the preconditions and postconditions of every statement in a block, and computes a condition for each typestate. Specifically: @itemize @item Initially, every typestate is empty. @item Each statement's poststate is given the union of the statement's prestate, precondition, and postcondition. @item Each statement's poststate has the difference between the statement's precondition and postcondition removed. @item Each statement's prestate is given the intersection of the poststates of every parent statement in the CFG. @item The previous three steps are repeated until no typestates in the block change. @end itemize The typestate algorithm is a very conventional dataflow calculation, and can be performed using bit-set operations, with one bit per predicate and one bit-set per condition. After the typestates of a block are computed, the typestate algorithm checks that every constraint in the precondition of a statement is satisfied by its prestate. If any preconditions are not satisfied, the mismatch is considered a static (compile-time) error. @node Ref.Stmt.Stat.Check @subsubsection Ref.Stmt.Stat.Check @c * Ref.Stmt.Stat.Check:: Relating dynamic state to static typestate. @cindex Check statement @cindex Assertions, see @i{Check statement} The key mechanism that connects run-time semantics and compile-time analysis of typestates is the use of @code{check} statements. @xref{Ref.Stmt.Check}. A @code{check} statement guarantees that @emph{if} control were to proceed past it, the predicate associated with the @code{check} would have succeeded, so the constraint being checked @emph{statically} holds in subsequent statements.@footnote{A @code{check} statement is similar to an @code{assert} call in a C program, with the significant difference that the Rust compiler @emph{tracks} the constraint that each @code{check} statement enforces. Naturally, @code{check} statements cannot be omitted from a ``production build'' of a Rust program the same way @code{asserts} are frequently disabled in deployed C programs.} It is important to understand that the typestate system has @emph{no insight} into the meaning of a particular predicate. Predicates and constraints are not evaluated in any way at compile time. Predicates are treated as specific (but unknown) functions applied to specific (also unknown) slots. All the typestate system does is track which of those predicates -- whatever they calculate -- @emph{must have been checked already} in order for program control to reach a particular point in the CFG. The fundamental building block, therefore, is the @code{check} statement, which tells the typestate system ``if control passes this statement, the checked predicate holds''. From this building block, constraints can be propagated to function signatures and constrained types, and the responsibility to @code{check} a constraint pushed further and further away from the site at which the program requires it to hold in order to execute properly. @node Ref.Stmt.Decl @subsection Ref.Stmt.Decl @c * Ref.Stmt.Decl:: Statement declaring an item or slot. @cindex Declaration statement A @dfn{declaration statement} is one that introduces a @emph{name} into the enclosing statement block. The declared name may denote a new slot or a new item. The scope of the name extends to the entire containing block, both before and after the declaration. @menu * Ref.Stmt.Decl.Item:: Statement declaring an item. * Ref.Stmt.Decl.Slot:: Statement declaring a slot. @end menu @node Ref.Stmt.Decl.Item @subsubsection Ref.Stmt.Decl.Item @c * Ref.Stmt.Decl.Item:: Statement declaring an item. An @dfn{item declaration statement} has a syntactic form identical to an item declaration within a module. Declaring an item -- a function, iterator, object, type or module -- locally within a statement block is simply a way of restricting its scope to a narrow region containing all of its uses; it is otherwise identical in meaning to declaring the item outside the statement block. Note: there is no implicit capture of the function's dynamic environment when declaring a function-local item. @node Ref.Stmt.Decl.Slot @subsubsection Ref.Stmt.Decl.Slot @c * Ref.Stmt.Decl.Slot:: Statement declaring an slot. @cindex Local slot @cindex Variable, see @i{Local slot} @cindex Type inference @cindex Automatic slot A @code{slot declaration statement} has one one of two forms: @itemize @item @code{let} @var{type} @var{slot} @var{optional-init}; @item @code{auto} @var{slot} @var{optional-init}; @end itemize Where @var{type} is a type expression, @var{slot} is the name of the slot being declared, and @var{optional-init} is either the empty string or an equals sign (@code{=}) followed by a primitive expression. Both forms introduce a new slot into the containing block scope. The new slot is visible across the entire scope, but is initialized only at the point following the declaration statement. The latter (@code{auto}) form of slot declaration causes the compiler to infer the static type of the slot through unification with the types of values assigned to the slot in the remaining code in the block scope. Inference only occurs on frame-local slots, not argument slots. Function, iterator and object signatures must always declared types for all argument slots. @xref{Ref.Mem.Slot}. @node Ref.Stmt.Copy @subsection Ref.Stmt.Copy @c * Ref.Stmt.Copy:: Statement for copying a value. @cindex Copy statement @cindex Assignment operator, see @i{Copy statement} A @dfn{copy statement} consists of an @emph{lval} followed by an equals-sign (@code{=}) and a primitive expression. @xref{Ref.Expr}. Executing a copy statement causes the value denoted by the expression -- either a value or a primitive combination of values -- to be copied into the memory location denoted by the @emph{lval}. A copy may entail the adjustment of reference counts, execution of destructors, or similar adjustments in order to respect the path through the memory graph implied by the @code{lval}, as well as any existing value held in the memory being written-to. All such adjustment is automatic and implied by the @code{=} operator. An example of three different copy statements: @example x = y; x.y = z; x.y = z + 2; @end example @node Ref.Stmt.Spawn @subsection Ref.Stmt.Spawn @c * Ref.Stmt.Spawn:: Statements creating new tasks. @cindex Spawn statement A @code{spawn} statement consists of keyword @code{spawn}, followed by an optional literal string naming the new task and then a normal @emph{call} statement (@pxref{Ref.Stmt.Call}). A @code{spawn} statement causes the runtime to construct a new task executing the called function with the given name. The called function is referred to as the @dfn{entry function} for the spawned task, and its arguments are copied from the spawning task to the spawned task before the spawned task begins execution. If no explicit name is present, the task is implicitly named with the string of the call statement. Functions taking alias-slot arguments, or returning non-nil values, cannot be spawned. Iterators cannot be spawned. The result of a @code{spawn} statement is a @code{task} value. An example of a @code{spawn} statement: @example fn helper(chan[u8] out) @{ // do some work. out <| result; @} let port[u8] out; let task p = spawn helper(chan(out)); let task p2 = spawn "my_helper" helper(chan(out)); // let task run, do other things. auto result <- out; @end example @node Ref.Stmt.Send @subsection Ref.Stmt.Send @c * Ref.Stmt.Send:: Statements for sending a value into a channel. @cindex Send statement @cindex Communication Sending a value through a channel can be done via two different statements. Both statements take an @emph{lval}, denoting a channel, and a value to send into the channel. The action of @emph{sending} varies depending on the @dfn{send operator} employed. The @emph{asynchronous send} operator @code{<+} adds a value to the channel's queue, without blocking. If the queue is full, it is extended, taking memory from the task's domain. If the task memory budget is exhausted, a signal is sent to the task. The @emph{semi-synchronous send} operator @code{<|} adds a value to the channel's queue @emph{only if} the queue has room; if the queue is full, the operation @emph{blocks} the sender until the queue has room. An example of an asynchronous send: @example chan[str] c = @dots{}; c <+ "hello, world"; @end example An example of a semi-synchronous send: @example chan[str] c = @dots{}; c <| "hello, world"; @end example @node Ref.Stmt.Recv @subsection Ref.Stmt.Recv @c * Ref.Stmt.Recv:: Statement for receiving a value from a channel. @cindex Receive statement @cindex Communication The @dfn{receive statement} takes an @var{lval} to receive into and an expression denoting a port, and applies the @emph{receive operator} (@code{<-}) to the pair, copying a value out of the port and into the @var{lval}. The statement causes the receiving task to enter the @emph{blocked reading} state until a task is sending a value to the port, at which point the runtime pseudo-randomly selects a sending task and copies a value from the head of one of the task queues to the receiving location in memory, and un-blocks the receiving task. @xref{Ref.Run.Comm}. An example of a @emph{receive}: @example port[str] p = @dots{}; let str s <- p; @end example @node Ref.Stmt.Call @subsection Ref.Stmt.Call @c * Ref.Stmt.Call:: Statement for calling a function. @cindex Call statement @cindex Function calls A @dfn{call statement} invokes a function, providing a tuple of input slots and an alias slot to serve as the function's output, bound to the @var{lval} on the right hand side of the call. If the function eventually returns, then the statement completes. A call statement statically requires that the precondition declared in the callee's signature is satisfied by the statement prestate. In this way, typestates propagate through function boundaries. @xref{Ref.Stmt.Stat}. An example of a call statement: @example let int x = add(1, 2); @end example @node Ref.Stmt.Bind @subsection Ref.Stmt.Bind @c * Ref.Stmt.Bind:: Statement for binding arguments to functions. @cindex Bind statement @cindex Closures @cindex Currying A @dfn{bind statement} constructs a new function from an existing function.@footnote{The @code{bind} statement is analogous to the @code{bind} expression in the Sather language.} The new function has zero or more of its arguments @emph{bound} into a new, hidden boxed tuple that holds the bindings. For each concrete argument passed in the @code{bind} statement, the corresponding parameter in the existing function is @emph{omitted} as a parameter of the new function. For each argument passed the placeholder symbol @code{_} in the @code{bind} statement, the corresponding parameter of the existing function is @emph{retained} as a parameter of the new function. Any subsequent invocation of the new function with residual arguments causes invocation of the existing function with the combination of bound arguments and residual arguments that was specified during the binding. An example of a @code{bind} statement: @example fn add(int x, int y) -> int @{ ret x + y; @} type single_param_fn = fn(int) -> int; let single_param_fn add4 = bind add(4, _); let single_param_fn add5 = bind add(_, 5); check (add(4,5) == add4(5)); check (add(4,5) == add5(4)); @end example A @code{bind} statement generally stores a copy of the bound arguments in the hidden, boxed tuple, owned by the resulting first-class function. For each bound slot in the bound function's signature, space is allocated in the hidden tuple and populated with a copy of the bound value. The @code{bind} statement is a lightweight mechanism for simulating the more elaborate construct of @emph{lexical closures} that exist in other languages. Rust has no support for lexical closures, but many realistic uses of them can be achieved with @code{bind} statements. @node Ref.Stmt.Ret @subsection Ref.Stmt.Ret @c * Ref.Stmt.Ret:: Statement for stopping and producing a value. @cindex Return statement Executing a @code{ret} statement@footnote{A @code{ret} statement is analogous to a @code{return} statement in the C family.} copies a value into the output slot of the current function, destroys the current function activation frame, and transfers control to the caller frame. An example of a @code{ret} statement: @example fn max(int a, int b) -> int @{ if (a > b) @{ ret a; @} ret b; @} @end example @node Ref.Stmt.Be @subsection Ref.Stmt.Be @c * Ref.Stmt.Be:: Statement for stopping and executing a tail call. @cindex Be statement @cindex Tail calls Executing a @code{be} statement @footnote{A @code{be} statement in is analogous to a @code{become} statement in Newsqueak or Alef.} destroys the current function activation frame and replaces it with an activation frame for the called function. In other words, @code{be} executes a tail-call. The syntactic form of a @code{be} statement is therefore limited to @emph{tail position}: its argument must be a @emph{call expression}, and it must be the last statement in a block. An example of a @code{be} statement: @example fn print_loop(int n) @{ if (n <= 0) @{ ret; @} else @{ print_int(n); be print_loop(n-1); @} @} @end example The above example executes in constant space, replacing each frame with a new copy of itself. @node Ref.Stmt.Put @subsection Ref.Stmt.Put @c * Ref.Stmt.Put:: Statement for pausing and producing a value. @cindex Put statement @cindex Iterators Executing a @code{put} statement copies a value into the output slot of the current iterator, suspends execution of the current iterator, and transfers control to the current put-recipient frame. A @code{put} statement is only valid within an iterator. @footnote{A @code{put} statement is analogous to a @code{yield} statement in the CLU, and Sather languages, or in more recent languages providing a ``generator'' facility, such as Python, Javascript or C#. Like the generators of CLU and Sather but @emph{unlike} these later languages, Rust's iterators reside on the stack and obey a strict stack discipline.} The current put-recipient will eventually resume the suspended iterator containing the @code{put} statement, either continuing execution after the @code{put} statement, or terminating its execution and destroying the iterator frame. @node Ref.Stmt.Fail @subsection Ref.Stmt.Fail @c * Ref.Stmt.Fail:: Statement for causing task failure. @cindex Fail statement @cindex Failure @cindex Unwinding Executing a @code{fail} statement causes a task to enter the @emph{failing} state. In the @emph{failing} state, a task unwinds its stack, destroying all frames and freeing all resources until it reaches its entry frame, at which point it halts execution in the @emph{dead} state. @node Ref.Stmt.Log @subsection Ref.Stmt.Log @c * Ref.Stmt.Log:: Statement for logging values to diagnostic buffers. @cindex Log statement @cindex Logging Executing a @code{log} statement may, depending on runtime configuration, cause a value to be appended to an internal diagnostic logging buffer provided by the runtime or emitted to a system console. Log statements are enabled or disabled dynamically at run-time on a per-task and per-item basis. @xref{Ref.Run.Log}. Executing a @code{log} statement is not considered an @code{io} effect in the effect system. In other words, a pure function remains pure even if it contains a log statement. @example @end example @node Ref.Stmt.Note @subsection Ref.Stmt.Note @c * Ref.Stmt.Note:: Statement for logging values during failure. @cindex Note statement @cindex Logging @cindex Unwinding @cindex Failure A @code{note} statement has no effect during normal execution. The purpose of a @code{note} statement is to provide additional diagnostic information to the logging subsystem during task failure. @xref{Ref.Stmt.Log}. Using @code{note} statements, normal diagnostic logging can be kept relatively sparse, while still providing verbose diagnostic ``back-traces'' when a task fails. When a task is failing, control frames @emph{unwind} from the innermost frame to the outermost, and from the innermost lexical block within an unwinding frame to the outermost. When unwinding a lexical block, the runtime processes all the @code{note} statements in the block sequentially, from the first statement of the block to the last. During processing, a @code{note} statement has equivalent meaning to a @code{log} statement: it causes the runtime to append the argument of the @code{note} to the internal logging diagnostic buffer. An example of a @code{note} statement: @example fn read_file_lines(&str path) -> vec[str] @{ note path; vec[str] r; file f = open_read(path); for each (str &s = lines(f)) @{ vec.append(r,s); @} ret r; @} @end example In this example, if the task fails while attempting to open or read a file, the runtime will log the path name that was being read. If the function completes normally, the runtime will not log the path. A value that is marked by a @code{note} statement is @emph{not} copied aside when control passes through the @code{note}. In other words, if a @code{note} statement notes a particular @var{lval}, and code after the @code{note} mutates that slot, and then a subsequent failure occurs, the @emph{mutated} value will be logged during unwinding, @emph{not} the original value that was denoted by the @var{lval} at the moment control passed through the @code{note} statement. @node Ref.Stmt.While @subsection Ref.Stmt.While @c * Ref.Stmt.While:: Statement for simple conditional looping. @cindex While statement @cindex Loops @cindex Control-flow A @code{while} statement is a loop construct. A @code{while} loop may be either a simple @code{while} or a @code{do}-@code{while} loop. In the case of a simple @code{while}, the loop begins by evaluating the boolean loop conditional expression. If the loop conditional expression evaluates to @code{true}, the loop body block executes and control returns to the loop conditional expression. If the loop conditional expression evaluates to @code{false}, the @code{while} statement completes. In the case of a @code{do}-@code{while}, the loop begins with an execution of the loop body. After the loop body executes, it evaluates the loop conditional expression. If it evaluates to @code{true}, control returns to the beginning of the loop body. If it evaluates to @code{false}, control exits the loop. An example of a simple @code{while} statement: @example while (i < 10) @{ print("hello\n"); i = i + 1; @} @end example An example of a @code{do}-@code{while} statement: @example do @{ print("hello\n"); i = i + 1; @} while (i < 10); @end example @node Ref.Stmt.Break @subsection Ref.Stmt.Break @c * Ref.Stmt.Break:: Statement for terminating a loop. @cindex Break statement @cindex Loops @cindex Control-flow Executing a @code{break} statement immediately terminates the innermost loop enclosing it. It is only permitted in the body of a loop. @node Ref.Stmt.Cont @subsection Ref.Stmt.Cont @c * Ref.Stmt.Cont:: Statement for terminating a single loop iteration. @cindex Continue statement @cindex Loops @cindex Control-flow Executing a @code{cont} statement immediately terminates the current iteration of the innermost loop enclosing it, returning control to the loop @emph{head}. In the case of a @code{while} loop, the head is the conditional expression controlling the loop. In the case of a @code{for} or @code{for each} loop, the head is the iterator or vector-slice increment controlling the loop. A @code{cont} statement is only permitted in the body of a loop. @node Ref.Stmt.For @subsection Ref.Stmt.For @c * Ref.Stmt.For:: Statement for looping over strings and vectors. @cindex For statement @cindex Loops @cindex Control-flow A @dfn{for loop} is controlled by a vector or string. The for loop bounds-checks the underlying sequence @emph{once} when initiating the loop, then repeatedly copies each value of the underlying sequence into the element variable, executing the loop body once per copy. To perform a for loop on a sub-range of a vector or string, form a temporary slice over the sub-range and run the loop over the slice. Example of 4 for loops, all identical: @example let vec[foo] v = vec(a, b, c); for (&foo e in v.(0, _vec.len(v))) @{ bar(e); @} for (&foo e in v.(0,)) @{ bar(e); @} for (&foo e in v.(,)) @{ bar(e); @} for (&foo e in v) @{ bar(e); @} @end example @node Ref.Stmt.Foreach @subsection Ref.Stmt.Foreach @c * Ref.Stmt.Foreach:: Statement for general conditional looping. @cindex Foreach statement @cindex Loops @cindex Control-flow An @dfn{foreach loop} is denoted by the @code{for each} keywords, and is controlled by an iterator. The loop executes once for each value @code{put} by the iterator. When the iterator returns or fails, the loop terminates. Example of a foreach loop: @example let str txt; let vec[str] lines; for each (&str s = _str.split(txt, "\n")) @{ vec.push(lines, s); @} @end example @node Ref.Stmt.If @subsection Ref.Stmt.If @c * Ref.Stmt.If:: Statement for simple conditional branching. @cindex If statement @cindex Control-flow An @code{if} statement is a conditional branch in program control. The form of an @code{if} statement is a parenthesized condition expression, followed by a consequent block, and an optional trailing @code{else} block. The condition expression must have type @code{bool}. If the condition expression evaluates to @code{true}, the consequent block is executed and any @code{else} block is skipped. If the condition expression evaluates to @code{false}, the consequent block is skipped and any @code{else} block is executed. @node Ref.Stmt.Alt @subsection Ref.Stmt.Alt @c * Ref.Stmt.Alt:: Statement for complex conditional branching. @cindex Alt statement @cindex Control-flow @cindex Switch statement, see @i{Alt statement} An @code{alt} statement is a multi-directional branch in program control. There are three kinds of @code{alt} statement: communication @code{alt} statements, pattern @code{alt} statements, and @code{alt type} statements. The form of each kind of @code{alt} is similar: an initial @emph{head} that describes the criteria for branching, followed by a sequence of zero or more @emph{arms}, each of which describes a @emph{case} and provides a @emph{block} of statements associated with the case. When an @code{alt} is executed, control enters the head, determines which of the cases to branch to, branches to the block associated with the chosen case, and then proceeds to the statement following the @code{alt} when the case block completes. @menu * Ref.Stmt.Alt.Comm:: Statement for branching on communication events. * Ref.Stmt.Alt.Pat:: Statement for branching on pattern matches. * Ref.Stmt.Alt.Type:: Statement for branching on types. @end menu @node Ref.Stmt.Alt.Comm @subsubsection Ref.Stmt.Alt.Comm @c * Ref.Stmt.Alt.Comm:: Statement for branching on communication events. @cindex Communication alt statement @cindex Control-flow @cindex Communication @cindex Multiplexing The simplest form of @code{alt} statement is the a @emph{communication} @code{alt}. The cases of a communication @code{alt}'s arms are send and receive statements. @xref{Ref.Task.Comm}. To execute a communication @code{alt}, the runtime checks all of the ports and channels involved in the arms of the statement to see if any @code{case} can execute without blocking. If no @code{case} can execute, the task blocks, and the runtime unblocks the task when a @code{case} @emph{can} execute. The runtime then makes a pseudo-random choice from among the set of @code{case} statements that can execute, executes the statement of the @code{case} and branches to the block of that arm. An example of a communication @code{alt} statement: @example let chan c[int] = foo(); let port p[str] = bar(); let int x = 0; let vec[str] strs; alt @{ case (str s <- p) @{ vec.append(strs, s); @} case (c <| x) @{ x++; @} @} @end example @node Ref.Stmt.Alt.Pat @subsubsection Ref.Stmt.Alt.Pat @c * Ref.Stmt.Alt.Pat:: Statement for branching on pattern matches. @cindex Pattern alt statement @cindex Control-flow A pattern @code{alt} statement branches on a @emph{pattern}. The exact form of matching that occurs depends on the pattern. Patterns consist of some combination of literals, tag constructors, variable binding specifications and placeholders (@code{_}). A pattern @code{alt} has a parenthesized @emph{head expression}, which is the value to compare to the patterns. The type of the patterns must equal the type of the head expression. To execute a pattern @code{alt} statement, first the head expression is evaluated, then its value is sequentially compared to the patterns in the arms until a match is found. The first arm with a matching @code{case} pattern is chosen as the branch target of the @code{alt}, any variables bound by the pattern are assigned to local @emph{auto} slots in the arm's block, and control enters the block. An example of a pattern @code{alt} statement: @example type list[X] = tag(nil(), cons(X, @@list[X])); let list[int] x = cons(10, cons(11, nil())); alt (x) @{ case (cons(a, cons(b, _))) @{ process_pair(a,b); @} case (cons(v=10, _)) @{ process_ten(v); @} case (_) @{ fail; @} @} @end example @node Ref.Stmt.Alt.Type @subsubsection Ref.Stmt.Alt.Type @c * Ref.Stmt.Alt.Type:: Statement for branching on type. @cindex Type alt statement @cindex Control-flow An @code{alt type} statement is similar to a pattern @code{alt}, but branches on the @emph{type} of its head expression, rather than the value. The head expression of an @code{alt type} statement must be of type @code{any}, and the arms of the statement are slot patterns rather than value patterns. Control branches to the arm with a @code{case} that matches the @emph{actual type} of the value in the @code{any}. An example of an @code{alt type} statement: @example let any x = foo(); alt type (x) @{ case (int i) @{ ret i; @} case (list[int] li) @{ ret int_list_sum(li); @} case (list[X] lx) @{ ret list_len(lx); @} case (_) @{ ret 0; @} @} @end example @node Ref.Stmt.Prove @subsection Ref.Stmt.Prove @c * Ref.Stmt.Prove:: Statement for static assertion of typestate. @cindex Prove statement @cindex Typestate system A @code{prove} statement has no run-time effect. Its purpose is to statically check (and document) that its argument constraint holds at its statement entry point. If its argument typestate does not hold, under the typestate algorithm, the program containing it will fail to compile. @node Ref.Stmt.Check @subsection Ref.Stmt.Check @c * Ref.Stmt.Check:: Statement for dynamic assertion of typestate. @cindex Check statement @cindex Typestate system A @code{check} statement connects dynamic assertions made at run-time to the static typestate system. A @code{check} statement takes a constraint to check at run-time. If the constraint holds at run-time, control passes through the @code{check} and on to the next statement in the enclosing block. If the condition fails to hold at run-time, the @code{check} statement behaves as a @code{fail} statement. The typestate algorithm is built around @code{check} statements, and in particular the fact that control @emph{will not pass} a check statement with a condition that fails to hold. The typestate algorithm can therefore assume that the (static) postcondition of a @code{check} statement includes the checked constraint itself. From there, the typestate algorithm can perform dataflow calculations on subsequent statements, propagating conditions forward and statically comparing implied states and their specifications. @xref{Ref.Stmt.Stat}. @example fn even(&int x) -> bool @{ ret x & 1 == 0; @} fn print_even(int x) : even(x) @{ print(x); @} fn test() @{ let int y = 8; // Cannot call print_even(y) here. check even(y); // Can call print_even(y) here, since even(y) now holds. print_even(y); @} @end example @node Ref.Stmt.IfCheck @subsection Ref.Stmt.IfCheck @c * Ref.Stmt.IfCheck:: Statement for dynamic testing of typestate. @cindex If check statement @cindex Typestate system @cindex Control-flow An @code{if check} statement combines a @code{if} statement and a @code{check} statement in an indivisible unit that can be used to build more complex conditional control-flow than the @code{check} statement affords. In fact, @code{if check} is a ``more primitive'' statement @code{check}; instances of the latter can be rewritten as instances of the former. The following two examples are equivalent: @sp 1 Example using @code{check}: @example check even(x); print_even(x); @end example @sp 1 Equivalent example using @code{if check}: @example if check even(x) @{ print_even(x); @} else @{ fail; @} @end example @page @node Ref.Run @section Ref.Run @c * Ref.Run:: Organization of runtime services. @cindex Runtime library The Rust @dfn{runtime} is a relatively compact collection of C and Rust code that provides fundamental services and datatypes to all Rust tasks at run-time. It is smaller and simpler than many modern language runtimes. It is tightly integrated into the language's execution model of memory, tasks, communication, reflection, logging and signal handling. @menu * Ref.Run.Mem:: Runtime memory management service. * Ref.Run.Type:: Runtime built-in type services. * Ref.Run.Comm:: Runtime communication service. * Ref.Run.Refl:: Runtime reflection system. * Ref.Run.Log:: Runtime logging system. * Ref.Run.Sig:: Runtime signal handler. @end menu @node Ref.Run.Mem @subsection Ref.Run.Mem @c * Ref.Run.Mem:: Runtime memory management service. @cindex Memory allocation The runtime memory-management system is based on a @emph{service-provider interface}, through which the runtime requests blocks of memory from its environment and releases them back to its environment when they are no longer in use. The default implementation of the service-provider interface consists of the C runtime functions @code{malloc} and @code{free}. The runtime memory-management system in turn supplies Rust tasks with facilities for allocating, extending and releasing stacks, as well as allocating and freeing boxed values. @node Ref.Run.Type @subsection Ref.Run.Type @c * Ref.Run.Mem:: Runtime built-in type services. @cindex Built-in types The runtime provides C and Rust code to manage several built-in types: @itemize @item @code{vec}, the type of vectors. @item @code{str}, the type of UTF-8 strings. @item @code{big}, the type of arbitrary-precision integers. @item @code{chan}, the type of communication channels. @item @code{port}, the type of communication ports. @item @code{task}, the type of tasks. @end itemize Support for other built-in types such as simple types, tuples, records, and tags is open-coded by the Rust compiler. @node Ref.Run.Comm @subsection Ref.Run.Comm @c * Ref.Run.Comm:: Runtime communication service. @cindex Communication @cindex Process @cindex Thread The runtime provides code to manage inter-task communication. This includes the system of task-lifecycle state transitions depending on the contents of queues, as well as code to copy values between queues and their recipients and to serialize values for transmission over operating-system inter-process communication facilities. @node Ref.Run.Refl @subsection Ref.Run.Refl @c * Ref.Run.Refl:: Runtime reflection system. @cindex Reflection @cindex DWARF The runtime reflection system is driven by the DWARF tables emitted into a crate at compile-time. Reflecting on a slot or item allocates a Rust data structure corresponding to the DWARF DIE for that slot or item. @node Ref.Run.Log @subsection Ref.Run.Log @c * Ref.Run.Log:: Runtime logging system. @cindex Logging The runtime contains a system for directing logging statements to a logging console and/or internal logging buffers. @xref{Ref.Stmt.Log}. Logging statements can be enabled or disabled via a two-dimensional filtering process: @itemize @sp 1 @item By Item Each @emph{item} (module, function, iterator, object, type) in Rust has a static name-path within its crate module, and can have logging enabled or disabled on a name-path-prefix basis. @sp 1 @item By Task Each @emph{task} in a running Rust program has a unique ownership-path through the task ownership tree, and can have logging enabled or disabled on an ownership-path-prefix basis. @end itemize Logging is integrated into the language for efficiency reasons, as well as the need to filter logs based on these two built-in dimensions. @node Ref.Run.Sig @subsection Ref.Run.Sig @c * Ref.Run.Sig:: Runtime signal handler. @cindex Signals The runtime signal-handling system is driven by a signal-dispatch table and a signal queue associated with each task. Sending a signal to a task inserts the signal into the task's signal queue and marks the task as having a pending signal. At the next scheduling opportunity, the runtime processes signals in the task's queue using its dispatch table. The signal queue memory is charged to the task's domain; if the queue grows too big, the task will fail. @c ############################################################ @c end main body of nodes @c ############################################################ @page @node Index @chapter Index @printindex cp @bye @c Local Variables: @c mode: texinfo @c fill-column: 78; @c indent-tabs-mode: nil @c buffer-file-coding-system: utf-8-unix @c compile-command: "make -k 2>&1 | sed -e 's/\\/x\\//x:\\//g'"; @c End: