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[release-branch.go1.2] spec: clarify rules for blank identifiers

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««« CL 14415043 / 4daa80747394
spec: clarify rules for blank identifiers

This documents the status quo more precisely.
Not a language change.

Fixes #6006.

R=r, rsc, iant, ken
CC=golang-dev
https://golang.org/cl/14415043

»»»

R=golang-dev
CC=golang-dev
https://golang.org/cl/25790043
This commit is contained in:
Andrew Gerrand 2013-11-13 14:40:41 +11:00 committed by Russ Cox
parent cd618b24a6
commit 61af4f905b
1 changed files with 58 additions and 30 deletions
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88
doc/go_spec.html
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@ -1,6 +1,6 @@
<!--{
"Title": "The Go Programming Language Specification",
"Subtitle": "Version of Oct 16, 2013",
"Subtitle": "Version of Nov 13, 2013",
"Path": "/ref/spec"
}-->
@ -1456,10 +1456,6 @@ by a value of type <code>T</code>.
</li>
</ul>
<p>
Any value may be assigned to the <a href="#Blank_identifier">blank identifier</a>.
</p>
<h2 id="Blocks">Blocks</h2>
@ -1516,6 +1512,11 @@ No identifier may be declared twice in the same block, and
no identifier may be declared in both the file and package block.
</p>
<p>
The <a href="#Blank_identifier">blank identifier</a> may be used like any other identifier
in a declaration, but it does not introduce a binding and thus is not declared.
</p>
<pre class="ebnf">
Declaration = ConstDecl | TypeDecl | VarDecl .
TopLevelDecl = Declaration | FunctionDecl | MethodDecl .
@ -1585,8 +1586,10 @@ the body of any nested function.
<h3 id="Blank_identifier">Blank identifier</h3>
<p>
The <i>blank identifier</i>, represented by the underscore character <code>_</code>, may be used in a declaration like
any other identifier but the declaration does not introduce a new <a href="#Declarations_and_scope">binding</a>.
The <i>blank identifier</i> is represented by the underscore character <code>_</code>.
It serves as an anonymous placeholder instead of a regular (non-blank)
identifier and has special meaning in <a href="#Declarations_and_scope">declarations</a>,
as an <a href="#Operands">operand</a>, and in <a href="#Assignments">assignments</a>.
</p>
@ -2077,8 +2080,8 @@ operators and functions to operands.
<p>
Operands denote the elementary values in an expression. An operand may be a
literal, a (possibly <a href="#Qualified_identifiers">qualified</a>) identifier
denoting a
literal, a (possibly <a href="#Qualified_identifiers">qualified</a>)
non-<a href="#Blank_identifier">blank</a> identifier denoting a
<a href="#Constant_declarations">constant</a>,
<a href="#Variable_declarations">variable</a>, or
<a href="#Function_declarations">function</a>,
@ -2086,6 +2089,11 @@ a <a href="#Method_expressions">method expression</a> yielding a function,
or a parenthesized expression.
</p>
<p>
The <a href="#Blank_identifier">blank identifier</a> may appear as an
operand only on the left-hand side of an <a href="#Assignments">assignment</a>.
</p>
<pre class="ebnf">
Operand = Literal | OperandName | MethodExpr | "(" Expression ")" .
Literal = BasicLit | CompositeLit | FunctionLit .
@ -4255,7 +4263,8 @@ assign_op = [ add_op | mul_op ] "=" .
<p>
Each left-hand side operand must be <a href="#Address_operators">addressable</a>,
a map index expression, or the <a href="#Blank_identifier">blank identifier</a>.
a map index expression, or (for <code>=</code> assignments only) the
<a href="#Blank_identifier">blank identifier</a>.
Operands may be parenthesized.
</p>
@ -4268,12 +4277,13 @@ a[i] = 23
<p>
An <i>assignment operation</i> <code>x</code> <i>op</i><code>=</code>
<code>y</code> where <i>op</i> is a binary arithmetic operation is equivalent
<code>y</code> where <i>op</i> is a binary arithmetic operation equivalent
to <code>x</code> <code>=</code> <code>x</code> <i>op</i>
<code>y</code> but evaluates <code>x</code>
only once. The <i>op</i><code>=</code> construct is a single token.
In assignment operations, both the left- and right-hand expression lists
must contain exactly one single-valued expression.
must contain exactly one single-valued expression, and the left-hand
expression must not be the blank identifier.
</p>
<pre>
@ -4298,21 +4308,26 @@ x, y = f()
<p>
assigns the first value to <code>x</code> and the second to <code>y</code>.
The <a href="#Blank_identifier">blank identifier</a> provides a
way to ignore values returned by a multi-valued expression:
</p>
<pre>
x, _ = f() // ignore second value returned by f()
</pre>
<p>
In the second form, the number of operands on the left must equal the number
of expressions on the right, each of which must be single-valued, and the
<i>n</i>th expression on the right is assigned to the <i>n</i>th
operand on the left.
operand on the left:
</p>
<pre>
one, two, three = '一', '二', '三'
</pre>
<p>
The <a href="#Blank_identifier">blank identifier</a> provides a way to
ignore right-hand side values in an assignment:
</p>
<pre>
_ = x // evaluate x but ignore it
x, _ = f() // evaluate f() but ignore second result value
</pre>
<p>
The assignment proceeds in two phases.
First, the operands of <a href="#Index_expressions">index expressions</a>
@ -4350,16 +4365,29 @@ for i, x[i] = range x { // set i, x[2] = 0, x[0]
</pre>
<p>
In assignments, each value must be
<a href="#Assignability">assignable</a> to the type of the
operand to which it is assigned. If an untyped <a href="#Constants">constant</a>
is assigned to a variable of interface type, the constant is <a href="#Conversions">converted</a>
to type <code>bool</code>, <code>rune</code>, <code>int</code>, <code>float64</code>,
<code>complex128</code> or <code>string</code>
respectively, depending on whether the value is a
boolean, rune, integer, floating-point, complex, or string constant.
In assignments, each value must be <a href="#Assignability">assignable</a>
to the type of the operand to which it is assigned, with the following special cases:
</p>
<ol>
<li><p>
If an untyped <a href="#Constants">constant</a>
is assigned to a variable of interface type or the blank identifier,
the constant is first <a href="#Conversions">converted</a> to type
<code>bool</code>, <code>rune</code>, <code>int</code>, <code>float64</code>,
<code>complex128</code> or <code>string</code> respectively, depending on
whether the value is a boolean, rune, integer, floating-point, complex, or
string constant.
</p></li>
<li><p>
<!-- Note that the result of a comparison is an untyped bool that may not be constant. -->
If a left-hand side is the blank identifier, any typed or non-constant
value except for the predeclared identifier
<a href="#Predeclared_identifiers"><code>nil</code></a>
may be assigned to it.
</p></li>
</ol>
<h3 id="If_statements">If statements</h3>