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Go specification: Lock down some details about channels and select:

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- nil channel in regular send or receive panics
- empty select blocks forever

R=rsc, gri, iant, ken2
CC=golang-dev
https://golang.org/cl/1825043
This commit is contained in:
Rob Pike 2010-07-13 16:23:54 -07:00
parent 2e3dc2cc35
commit 041d11623f
1 changed files with 21 additions and 10 deletions
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31
doc/go_spec.html
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@ -17,7 +17,6 @@ TODO
[ ] specify iteration direction for range clause
[ ] review language on implicit dereferencing
[ ] clarify what it means for two functions to be "the same" when comparing them
[ ] need to specify what happends when sending/receiving from a nil channel
-->
@ -3063,6 +3062,12 @@ to <code>false</code> and <code>x</code> is set to the
zero value for its type (§<a href="#The_zero_value">The zero value</a>).
</p>
<p>
Except in a communications clause of a <a href="#Select_statements">select statement</a>,
sending or receiving from a <code>nil</code> channel causes a
<a href="#Run_time_panics">run-time panic</a>.
</p>
<!---
<p>
<span class="alert">TODO: Probably in a separate section, communication semantics
@ -4048,18 +4053,22 @@ RecvExpr = [ Expression ( "=" | ":=" ) ] "&lt;-" Expression .
<p>
For all the send and receive expressions in the "select"
statement, the channel expressions are evaluated, along with
any expressions that appear on the right hand side of send expressions,
in top-to-bottom order.
If any of the resulting operations can proceed, one is
chosen and the corresponding communication and statements are
evaluated. Otherwise, if there is a default case, that executes;
if not, the statement blocks until one of the communications can
complete. The channels and send expressions are not re-evaluated.
statement, the channel expressions are evaluated in top-to-bottom order, along with
any expressions that appear on the right hand side of send expressions.
A channel pointer may be <code>nil</code>,
which is equivalent to that case not
being present in the select statement
except, if a send, its expression is still evaluated.
If any of the resulting operations can proceed, one of those is
chosen and the corresponding communication and statements are
evaluated. Otherwise, if there is a default case, that executes;
if there is no default case, the statement blocks until one of the communications can
complete.
If there are no cases with non-<code>nil</code> channels,
the statement blocks forever.
Even if the statement blocks,
the channel and send expressions are evaluated only once,
upon entering the select statement.
</p>
<p>
Since all the channels and send expressions are evaluated, any side
@ -4067,7 +4076,7 @@ effects in that evaluation will occur for all the communications
in the "select" statement.
</p>
<p>
If multiple cases can proceed, a uniform fair choice is made to decide
If multiple cases can proceed, a pseudo-random fair choice is made to decide
which single communication will execute.
<p>
The receive case may declare a new variable using a
@ -4092,6 +4101,8 @@ for { // send random sequence of bits to c
case c &lt;- 1:
}
}
select { } // block forever
</pre>