mirror
/
go
mirror of https://github.com/golang/go.git
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

spec: permit "for range x" (no index variables) 

This is a fully backward-compatible language change.

There are not a lot of cases in the std library, but
there are some. Arguably this makes the syntax a bit
more regular - any trailing index variable that is _
can be left away, and there's some analogy to type
switches where the temporary can be left away.

Implementation-wise the change should be trivial as
it can be done completely syntactically. For instance,
the respective change in go/parser is a dozen lines
(see https://golang.org/cl/112970044 ).

Fixes #6102.

LGTM=iant, r, rsc
R=r, rsc, iant, ken
CC=golang-codereviews
https://golang.org/cl/104680043
This commit is contained in:
Robert Griesemer 2014-07-14 15:08:09 -07:00
parent 8b836fa872
commit 20ae6d9bc5
1 changed files with 19 additions and 15 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

34
doc/go_spec.html
View File

@ -1,6 +1,6 @@
<!--{ <!--{
"Title": "The Go Programming Language Specification", "Title": "The Go Programming Language Specification",
"Subtitle": "Version of June 24, 2014", "Subtitle": "Version of July 14, 2014",
"Path": "/ref/spec" "Path": "/ref/spec"
}--> }-->
@ -4714,41 +4714,42 @@ for { S() } is the same as for true { S() }
A "for" statement with a "range" clause A "for" statement with a "range" clause
iterates through all entries of an array, slice, string or map, iterates through all entries of an array, slice, string or map,
or values received on a channel. For each entry it assigns <i>iteration values</i> or values received on a channel. For each entry it assigns <i>iteration values</i>
to corresponding <i>iteration variables</i> and then executes the block. to corresponding <i>iteration variables</i> if present and then executes the block.
</p> </p>
<pre class="ebnf"> <pre class="ebnf">
RangeClause = ( ExpressionList "=" | IdentifierList ":=" ) "range" Expression . RangeClause = [ ExpressionList "=" | IdentifierList ":=" ] "range" Expression .
</pre> </pre>
<p> <p>
The expression on the right in the "range" clause is called the <i>range expression</i>, The expression on the right in the "range" clause is called the <i>range expression</i>,
which may be an array, pointer to an array, slice, string, map, or channel permitting which may be an array, pointer to an array, slice, string, map, or channel permitting
<a href="#Receive_operator">receive operations</a>. <a href="#Receive_operator">receive operations</a>.
As with an assignment, the operands on the left must be As with an assignment, if present the operands on the left must be
<a href="#Address_operators">addressable</a> or map index expressions; they <a href="#Address_operators">addressable</a> or map index expressions; they
denote the iteration variables. If the range expression is a channel, only denote the iteration variables. If the range expression is a channel, at most
one iteration variable is permitted, otherwise there may be one or two. In the latter case, one iteration variable is permitted, otherwise there may be up to two.
if the second iteration variable is the <a href="#Blank_identifier">blank identifier</a>, If the last iteration variable is the <a href="#Blank_identifier">blank identifier</a>,
the range clause is equivalent to the same clause with only the first variable present. the range clause is equivalent to the same clause without that identifier.
</p> </p>
<p> <p>
The range expression is evaluated once before beginning the loop, The range expression is evaluated once before beginning the loop,
with one exception. If the range expression is an array or a pointer to an array with one exception: if the range expression is an array or a pointer to an array
and only the first iteration value is present, only the range expression's and at most one iteration variable is present, only the range expression's
length is evaluated; if that length is constant length is evaluated; if that length is constant,
<a href="#Length_and_capacity">by definition</a>, <a href="#Length_and_capacity">by definition</a>
the range expression itself will not be evaluated. the range expression itself will not be evaluated.
</p> </p>
<p> <p>
Function calls on the left are evaluated once per iteration. Function calls on the left are evaluated once per iteration.
For each iteration, iteration values are produced as follows: For each iteration, iteration values are produced as follows
if the respective iteration variables are present:
</p> </p>
<pre class="grammar"> <pre class="grammar">
Range expression 1st value 2nd value (if 2nd variable is present) Range expression 1st value 2nd value
array or slice a [n]E, *[n]E, or []E index i int a[i] E array or slice a [n]E, *[n]E, or []E index i int a[i] E
string s string type index i int see below rune string s string type index i int see below rune
@ -4760,7 +4761,7 @@ channel c chan E, &lt;-chan E element e E
<li> <li>
For an array, pointer to array, or slice value <code>a</code>, the index iteration For an array, pointer to array, or slice value <code>a</code>, the index iteration
values are produced in increasing order, starting at element index 0. values are produced in increasing order, starting at element index 0.
If only the first iteration variable is present, the range loop produces If at most one iteration variable is present, the range loop produces
iteration values from 0 up to <code>len(a)-1</code> and does not index into the array iteration values from 0 up to <code>len(a)-1</code> and does not index into the array
or slice itself. For a <code>nil</code> slice, the number of iterations is 0. or slice itself. For a <code>nil</code> slice, the number of iterations is 0.
</li> </li>
@ -4841,6 +4842,9 @@ var ch chan Work = producer()
for w := range ch { for w := range ch {
doWork(w) doWork(w)
} }
// empty a channel
for range ch {}
</pre> </pre>