[rust.git] / doc / po / tutorial-macros.md.pot

# SOME DESCRIPTIVE TITLE
# Copyright (C) YEAR The Rust Project Developers
# This file is distributed under the same license as the Rust package.
# FIRST AUTHOR <EMAIL@ADDRESS>, YEAR.
#
#, fuzzy
msgid ""
msgstr ""
"Project-Id-Version: Rust 0.8-pre\n"
"POT-Creation-Date: 2013-07-22 23:37+0900\n"
"PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
"Language-Team: LANGUAGE <LL@li.org>\n"
"Language: \n"
"MIME-Version: 1.0\n"
"Content-Type: text/plain; charset=UTF-8\n"
"Content-Transfer-Encoding: 8bit\n"

#. type: Plain text
#: doc/rust.md:4 doc/rustpkg.md:4 doc/tutorial.md:4
#: doc/tutorial-borrowed-ptr.md:4 doc/tutorial-ffi.md:4
#: doc/tutorial-macros.md:4 doc/tutorial-tasks.md:4
msgid "# Introduction"
msgstr ""

#. type: Plain text
#: doc/rust.md:2136 doc/rust.md:2223 doc/tutorial-macros.md:323
msgid "~~~~"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:2
msgid "% Rust Macros Tutorial"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:12
msgid ""
"Functions are the primary tool that programmers can use to build "
"abstractions.  Sometimes, however, programmers want to abstract over compile-"
"time syntax rather than run-time values.  Macros provide syntactic "
"abstraction.  For an example of how this can be useful, consider the "
"following two code fragments, which both pattern-match on their input and "
"both return early in one case, doing nothing otherwise:"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:30
#, no-wrap
msgid ""
"~~~~\n"
"# enum t { special_a(uint), special_b(uint) };\n"
"# fn f() -> uint {\n"
"# let input_1 = special_a(0);\n"
"# let input_2 = special_a(0);\n"
"match input_1 {\n"
"    special_a(x) => { return x; }\n"
"    _ => {}\n"
"}\n"
"// ...\n"
"match input_2 {\n"
"    special_b(x) => { return x; }\n"
"    _ => {}\n"
"}\n"
"# return 0u;\n"
"# }\n"
"~~~~\n"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:38
msgid ""
"This code could become tiresome if repeated many times.  However, no "
"function can capture its functionality to make it possible to abstract the "
"repetition away.  Rust's macro system, however, can eliminate the "
"repetition. Macros are lightweight custom syntax extensions, themselves "
"defined using the `macro_rules!` syntax extension. The following "
"`early_return` macro captures the pattern in the above code:"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:59
#, no-wrap
msgid ""
"~~~~\n"
"# enum t { special_a(uint), special_b(uint) };\n"
"# fn f() -> uint {\n"
"# let input_1 = special_a(0);\n"
"# let input_2 = special_a(0);\n"
"macro_rules! early_return(\n"
"    ($inp:expr $sp:ident) => ( // invoke it like `(input_5 special_e)`\n"
"        match $inp {\n"
"            $sp(x) => { return x; }\n"
"            _ => {}\n"
"        }\n"
"    );\n"
")\n"
"// ...\n"
"early_return!(input_1 special_a);\n"
"// ...\n"
"early_return!(input_2 special_b);\n"
"# return 0;\n"
"# }\n"
"~~~~\n"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:65
msgid ""
"Macros are defined in pattern-matching style: in the above example, the text "
"`($inp:expr $sp:ident)` that appears on the left-hand side of the `=>` is "
"the *macro invocation syntax*, a pattern denoting how to write a call to the "
"macro. The text on the right-hand side of the `=>`, beginning with `match "
"$inp`, is the *macro transcription syntax*: what the macro expands to."
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:67
msgid "# Invocation syntax"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:71
msgid ""
"The macro invocation syntax specifies the syntax for the arguments to the "
"macro. It appears on the left-hand side of the `=>` in a macro definition. "
"It conforms to the following rules:"
msgstr ""

#. type: Bullet: '1. '
#: doc/tutorial-macros.md:76
msgid "It must be surrounded by parentheses."
msgstr ""

#. type: Bullet: '2. '
#: doc/tutorial-macros.md:76
msgid "`$` has special meaning (described below)."
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:76
#, no-wrap
msgid ""
"3. The `()`s, `[]`s, and `{}`s it contains must balance. For example, `([)` is\n"
"forbidden.\n"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:78
msgid "Otherwise, the invocation syntax is free-form."
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:83
#, no-wrap
msgid ""
"To take as an argument a fragment of Rust code, write `$` followed by a name\n"
" (for use on the right-hand side), followed by a `:`, followed by a *fragment\n"
" specifier*. The fragment specifier denotes the sort of fragment to match. The\n"
" most common fragment specifiers are:\n"
msgstr ""

#. type: Bullet: '* '
#: doc/tutorial-macros.md:92
msgid ""
"`ident` (an identifier, referring to a variable or item. Examples: `f`, `x`, "
"`foo`.)"
msgstr ""

#. type: Bullet: '* '
#: doc/tutorial-macros.md:92
msgid ""
"`expr` (an expression. Examples: `2 + 2`; `if true then { 1 } else { 2 }`; "
"`f(42)`.)"
msgstr ""

#. type: Bullet: '* '
#: doc/tutorial-macros.md:92
msgid "`ty` (a type. Examples: `int`, `~[(char, ~str)]`, `&T`.)"
msgstr ""

#. type: Bullet: '* '
#: doc/tutorial-macros.md:92
msgid ""
"`pat` (a pattern, usually appearing in a `match` or on the left-hand side of "
"a declaration. Examples: `Some(t)`; `(17, 'a')`; `_`.)"
msgstr ""

#. type: Bullet: '* '
#: doc/tutorial-macros.md:92
msgid ""
"`block` (a sequence of actions. Example: `{ log(error, \"hi\"); return 12; }"
"`)"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:95
msgid ""
"The parser interprets any token that's not preceded by a `$` literally. "
"Rust's usual rules of tokenization apply,"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:98
msgid ""
"So `($x:ident -> (($e:expr)))`, though excessively fancy, would designate a "
"macro that could be invoked like: `my_macro!(i->(( 2+2 )))`."
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:100
msgid "## Invocation location"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:105
msgid ""
"A macro invocation may take the place of (and therefore expand to)  an "
"expression, an item, or a statement.  The Rust parser will parse the macro "
"invocation as a \"placeholder\" for whichever of those three nonterminals is "
"appropriate for the location."
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:112
msgid ""
"At expansion time, the output of the macro will be parsed as whichever of "
"the three nonterminals it stands in for. This means that a single macro "
"might, for example, expand to an item or an expression, depending on its "
"arguments (and cause a syntax error if it is called with the wrong argument "
"for its location). Although this behavior sounds excessively dynamic, it is "
"known to be useful under some circumstances."
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:115
msgid "# Transcription syntax"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:119
msgid ""
"The right-hand side of the `=>` follows the same rules as the left-hand "
"side, except that a `$` need only be followed by the name of the syntactic "
"fragment to transcribe into the macro expansion; its type need not be "
"repeated."
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:126
msgid ""
"The right-hand side must be enclosed by delimiters, which the transcriber "
"ignores.  Therefore `() => ((1,2,3))` is a macro that expands to a tuple "
"expression, `() => (let $x=$val)` is a macro that expands to a statement, "
"and `() => (1,2,3)` is a macro that expands to a syntax error (since the "
"transcriber interprets the parentheses on the right-hand-size as delimiters, "
"and `1,2,3` is not a valid Rust expression on its own)."
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:133
msgid ""
"Except for permissibility of `$name` (and `$(...)*`, discussed below), the "
"right-hand side of a macro definition is ordinary Rust syntax. In "
"particular, macro invocations (including invocations of the macro currently "
"being defined)  are permitted in expression, statement, and item locations. "
"However, nothing else about the code is examined or executed by the macro "
"system; execution still has to wait until run-time."
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:135
msgid "## Interpolation location"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:139
msgid ""
"The interpolation `$argument_name` may appear in any location consistent "
"with its fragment specifier (i.e., if it is specified as `ident`, it may be "
"used anywhere an identifier is permitted)."
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:141
msgid "# Multiplicity"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:143
msgid "## Invocation"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:150
msgid ""
"Going back to the motivating example, recall that `early_return` expanded "
"into a `match` that would `return` if the `match`'s scrutinee matched the "
"\"special case\" identifier provided as the second argument to "
"`early_return`, and do nothing otherwise. Now suppose that we wanted to "
"write a version of `early_return` that could handle a variable number of "
"\"special\" cases."
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:156
msgid ""
"The syntax `$(...)*` on the left-hand side of the `=>` in a macro definition "
"accepts zero or more occurrences of its contents. It works much like the `*` "
"operator in regular expressions. It also supports a separator token (a comma-"
"separated list could be written `$(...),*`), and `+` instead of `*` to mean "
"\"at least one\"."
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:179
#, no-wrap
msgid ""
"~~~~\n"
"# enum t { special_a(uint),special_b(uint),special_c(uint),special_d(uint)};\n"
"# fn f() -> uint {\n"
"# let input_1 = special_a(0);\n"
"# let input_2 = special_a(0);\n"
"macro_rules! early_return(\n"
"    ($inp:expr, [ $($sp:ident)|+ ]) => (\n"
"        match $inp {\n"
"            $(\n"
"                $sp(x) => { return x; }\n"
"            )+\n"
"            _ => {}\n"
"        }\n"
"    );\n"
")\n"
"// ...\n"
"early_return!(input_1, [special_a|special_c|special_d]);\n"
"// ...\n"
"early_return!(input_2, [special_b]);\n"
"# return 0;\n"
"# }\n"
"~~~~\n"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:181
msgid "### Transcription"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:191
msgid ""
"As the above example demonstrates, `$(...)*` is also valid on the right-hand "
"side of a macro definition. The behavior of `*` in transcription, especially "
"in cases where multiple `*`s are nested, and multiple different names are "
"involved, can seem somewhat magical and intuitive at first. The system that "
"interprets them is called \"Macro By Example\". The two rules to keep in "
"mind are (1) the behavior of `$(...)*` is to walk through one \"layer\" of "
"repetitions for all of the `$name`s it contains in lockstep, and (2) each `"
"$name` must be under at least as many `$(...)*`s as it was matched against.  "
"If it is under more, it'll be repeated, as appropriate."
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:193
msgid "## Parsing limitations"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:197
msgid ""
"For technical reasons, there are two limitations to the treatment of syntax "
"fragments by the macro parser:"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:210
#, no-wrap
msgid ""
"1. The parser will always parse as much as possible of a Rust syntactic\n"
"fragment. For example, if the comma were omitted from the syntax of\n"
"`early_return!` above, `input_1 [` would've been interpreted as the beginning\n"
"of an array index. In fact, invoking the macro would have been impossible.\n"
"2. The parser must have eliminated all ambiguity by the time it reaches a\n"
"`$name:fragment_specifier` declaration. This limitation can result in parse\n"
"errors when declarations occur at the beginning of, or immediately after,\n"
"a `$(...)*`. For example, the grammar `$($t:ty)* $e:expr` will always fail to\n"
"parse because the parser would be forced to choose between parsing `t` and\n"
"parsing `e`. Changing the invocation syntax to require a distinctive token in\n"
"front can solve the problem. In the above example, `$(T $t:ty)* E $e:exp`\n"
"solves the problem.\n"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:212
msgid "# Macro argument pattern matching"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:214
msgid "Now consider code like the following:"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:216
msgid "## Motivation"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:236
#, no-wrap
msgid ""
"~~~~\n"
"# enum t1 { good_1(t2, uint), bad_1 };\n"
"# pub struct t2 { body: t3 }\n"
"# enum t3 { good_2(uint), bad_2};\n"
"# fn f(x: t1) -> uint {\n"
"match x {\n"
"    good_1(g1, val) => {\n"
"        match g1.body {\n"
"            good_2(result) => {\n"
"                // complicated stuff goes here\n"
"                return result + val;\n"
"            },\n"
"            _ => fail!(\"Didn't get good_2\")\n"
"        }\n"
"    }\n"
"    _ => return 0 // default value\n"
"}\n"
"# }\n"
"~~~~\n"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:241
msgid ""
"All the complicated stuff is deeply indented, and the error-handling code is "
"separated from matches that fail. We'd like to write a macro that performs a "
"match, but with a syntax that suits the problem better. The following macro "
"can solve the problem:"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:263
#, no-wrap
msgid ""
"~~~~\n"
"macro_rules! biased_match (\n"
"    // special case: `let (x) = ...` is illegal, so use `let x = ...` instead\n"
"    ( ($e:expr) ~ ($p:pat) else $err:stmt ;\n"
"      binds $bind_res:ident\n"
"    ) => (\n"
"        let $bind_res = match $e {\n"
"            $p => ( $bind_res ),\n"
"            _ => { $err }\n"
"        };\n"
"    );\n"
"    // more than one name; use a tuple\n"
"    ( ($e:expr) ~ ($p:pat) else $err:stmt ;\n"
"      binds $( $bind_res:ident ),*\n"
"    ) => (\n"
"        let ( $( $bind_res ),* ) = match $e {\n"
"            $p => ( $( $bind_res ),* ),\n"
"            _ => { $err }\n"
"        };\n"
"    )\n"
")\n"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:277
#, no-wrap
msgid ""
"# enum t1 { good_1(t2, uint), bad_1 };\n"
"# pub struct t2 { body: t3 }\n"
"# enum t3 { good_2(uint), bad_2};\n"
"# fn f(x: t1) -> uint {\n"
"biased_match!((x)       ~ (good_1(g1, val)) else { return 0 };\n"
"              binds g1, val )\n"
"biased_match!((g1.body) ~ (good_2(result) )\n"
"                  else { fail!(\"Didn't get good_2\") };\n"
"              binds result )\n"
"// complicated stuff goes here\n"
"return result + val;\n"
"# }\n"
"~~~~\n"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:288
#, no-wrap
msgid ""
"This solves the indentation problem. But if we have a lot of chained matches\n"
"like this, we might prefer to write a single macro invocation. The input\n"
"pattern we want is clear:\n"
"~~~~\n"
"# macro_rules! b(\n"
"    ( $( ($e:expr) ~ ($p:pat) else $err:stmt ; )*\n"
"      binds $( $bind_res:ident ),*\n"
"    )\n"
"# => (0))\n"
"~~~~\n"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:291
msgid ""
"However, it's not possible to directly expand to nested match statements. "
"But there is a solution."
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:293
msgid "## The recursive approach to macro writing"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:297
msgid ""
"A macro may accept multiple different input grammars. The first one to "
"successfully match the actual argument to a macro invocation is the one that "
"\"wins\"."
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:301
msgid ""
"In the case of the example above, we want to write a recursive macro to "
"process the semicolon-terminated lines, one-by-one. So, we want the "
"following input patterns:"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:308
#, no-wrap
msgid ""
"~~~~\n"
"# macro_rules! b(\n"
"    ( binds $( $bind_res:ident ),* )\n"
"# => (0))\n"
"~~~~\n"
"...and:\n"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:317
#, no-wrap
msgid ""
"~~~~\n"
"# macro_rules! b(\n"
"    (    ($e     :expr) ~ ($p     :pat) else $err     :stmt ;\n"
"      $( ($e_rest:expr) ~ ($p_rest:pat) else $err_rest:stmt ; )*\n"
"      binds  $( $bind_res:ident ),*\n"
"    )\n"
"# => (0))\n"
"~~~~\n"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:321
msgid ""
"The resulting macro looks like this. Note that the separation into "
"`biased_match!` and `biased_match_rec!` occurs only because we have an outer "
"piece of syntax (the `let`) which we only want to transcribe once."
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:342
#, no-wrap
msgid ""
"macro_rules! biased_match_rec (\n"
"    // Handle the first layer\n"
"    (   ($e     :expr) ~ ($p     :pat) else $err     :stmt ;\n"
"     $( ($e_rest:expr) ~ ($p_rest:pat) else $err_rest:stmt ; )*\n"
"     binds $( $bind_res:ident ),*\n"
"    ) => (\n"
"        match $e {\n"
"            $p => {\n"
"                // Recursively handle the next layer\n"
"                biased_match_rec!($( ($e_rest) ~ ($p_rest) else $err_rest ; )*\n"
"                                  binds $( $bind_res ),*\n"
"                )\n"
"            }\n"
"            _ => { $err }\n"
"        }\n"
"    );\n"
"    ( binds $( $bind_res:ident ),* ) => ( ($( $bind_res ),*) )\n"
")\n"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:364
#, no-wrap
msgid ""
"// Wrap the whole thing in a `let`.\n"
"macro_rules! biased_match (\n"
"    // special case: `let (x) = ...` is illegal, so use `let x = ...` instead\n"
"    ( $( ($e:expr) ~ ($p:pat) else $err:stmt ; )*\n"
"      binds $bind_res:ident\n"
"    ) => (\n"
"        let ( $( $bind_res ),* ) = biased_match_rec!(\n"
"            $( ($e) ~ ($p) else $err ; )*\n"
"            binds $bind_res\n"
"        );\n"
"    );\n"
"    // more than one name: use a tuple\n"
"    ( $( ($e:expr) ~ ($p:pat) else $err:stmt ; )*\n"
"      binds  $( $bind_res:ident ),*\n"
"    ) => (\n"
"        let ( $( $bind_res ),* ) = biased_match_rec!(\n"
"            $( ($e) ~ ($p) else $err ; )*\n"
"            binds $( $bind_res ),*\n"
"        );\n"
"    )\n"
")\n"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:378
#, no-wrap
msgid ""
"# enum t1 { good_1(t2, uint), bad_1 };\n"
"# pub struct t2 { body: t3 }\n"
"# enum t3 { good_2(uint), bad_2};\n"
"# fn f(x: t1) -> uint {\n"
"biased_match!(\n"
"    (x)       ~ (good_1(g1, val)) else { return 0 };\n"
"    (g1.body) ~ (good_2(result) ) else { fail!(\"Didn't get good_2\") };\n"
"    binds val, result )\n"
"// complicated stuff goes here\n"
"return result + val;\n"
"# }\n"
"~~~~\n"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:382
msgid ""
"This technique applies to many cases where transcribing a result all at once "
"is not possible.  The resulting code resembles ordinary functional "
"programming in some respects, but has some important differences from "
"functional programming."
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:390
msgid ""
"The first difference is important, but also easy to forget: the "
"transcription (right-hand) side of a `macro_rules!` rule is literal syntax, "
"which can only be executed at run-time. If a piece of transcription syntax "
"does not itself appear inside another macro invocation, it will become part "
"of the final program. If it is inside a macro invocation (for example, the "
"recursive invocation of `biased_match_rec!`), it does have the opportunity "
"to affect transcription, but only through the process of attempted pattern "
"matching."
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:398
msgid ""
"The second, related, difference is that the evaluation order of macros feels "
"\"backwards\" compared to ordinary programming. Given an invocation `m1!(m2!"
"())`, the expander first expands `m1!`, giving it as input the literal "
"syntax `m2!()`. If it transcribes its argument unchanged into an appropriate "
"position (in particular, not as an argument to yet another macro "
"invocation), the expander will then proceed to evaluate `m2!()` (along with "
"any other macro invocations `m1!(m2!())` produced)."
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:400
msgid "# A final note"
msgstr ""

#. type: Plain text
#: doc/tutorial-macros.md:407
msgid ""
"Macros, as currently implemented, are not for the faint of heart. Even "
"ordinary syntax errors can be more difficult to debug when they occur inside "
"a macro, and errors caused by parse problems in generated code can be very "
"tricky. Invoking the `log_syntax!` macro can help elucidate intermediate "
"states, invoking `trace_macros!(true)` will automatically print those "
"intermediate states out, and passing the flag `--pretty expanded` as a "
"command-line argument to the compiler will show the result of expansion."
msgstr ""