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spec: channel operations are restricted by the channel direction

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Also:
- 'for' statements with a range clause do not accept send-only
   channels
- '_, _ = range ch' is not equivalent to "_ = range ch" if ch
   is a channel (rewriting the latter to the former leads to
   an invalid range clause).

These clarifications document the status quo.

R=rsc, r, iant, ken
CC=golang-dev
https://golang.org/cl/6874053
This commit is contained in:
Robert Griesemer 2012-12-03 14:23:41 -08:00
parent 3167c12eb2
commit cc3f21cefe
1 changed files with 12 additions and 9 deletions
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21
doc/go_spec.html
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@ -1,6 +1,6 @@
<!--{
"Title": "The Go Programming Language Specification",
"Subtitle": "Version of November 29, 2012",
"Subtitle": "Version of December 3, 2012",
"Path": "/ref/spec"
}-->
@ -3249,8 +3249,9 @@ will cause a <a href="#Run_time_panics">run-time panic</a>.
<p>
For an operand <code>ch</code> of <a href="#Channel_types">channel type</a>,
the value of the receive operation <code>&lt;-ch</code> is the value received
from the channel <code>ch</code>. The type of the value is the element type of
the channel. The expression blocks until a value is available.
from the channel <code>ch</code>. The channel direction must permit receive operations,
and the type of the receive operation is the element type of the channel.
The expression blocks until a value is available.
Receiving from a <code>nil</code> channel blocks forever.
Receiving from a <a href="#Close">closed</a> channel always succeeds,
immediately returning the element type's <a href="#The_zero_value">zero
@ -3873,8 +3874,9 @@ len("foo") // illegal if len is the built-in function
<p>
A send statement sends a value on a channel.
The channel expression must be of <a href="#Channel_types">channel type</a>
and the type of the value must be <a href="#Assignability">assignable</a>
The channel expression must be of <a href="#Channel_types">channel type</a>,
the channel direction must permit send operations,
and the type of the value to be sent must be <a href="#Assignability">assignable</a>
to the channel's element type.
</p>
@ -4319,12 +4321,13 @@ RangeClause = Expression [ "," Expression ] ( "=" | ":=" ) "range" Expression .
<p>
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.
which may be an array, pointer to an array, slice, string, map, or channel permitting
<a href="#Receive_operator">receive operations</a>.
As with an assignment, the operands on the left must be
<a href="#Address_operators">addressable</a> or map index expressions; they
denote the iteration variables. If the range expression is a channel, only
one iteration variable is permitted, otherwise there may be one or two.
If the second iteration variable is the <a href="#Blank_identifier">blank identifier</a>,
one iteration variable is permitted, otherwise there may be one or two. In the latter case,
if the second 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.
</p>
@ -4342,7 +4345,7 @@ Range expression 1st value 2nd value (if 2nd v
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
map m map[K]V key k K m[k] V
channel c chan E element e E
channel c chan E, <-chan E element e E
</pre>
<ol>