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Add select statement

SVN=114147
This commit is contained in:
Rob Pike 2008-03-27 21:42:25 -07:00
parent e118551c29
commit 8c1408dd8e

View File

@ -1163,7 +1163,7 @@ Statements control execution.
[ LabelDecl ] ( StructuredStat | UnstructuredStat ) .
StructuredStat =
Block | IfStat | SwitchStat | ForStat | RangeStat .
Block | IfStat | SwitchStat | SelectStat | ForStat | RangeStat .
UnstructuredStat =
Declaration | SimpleVarDecl |
@ -1399,6 +1399,65 @@ If the expression is omitted, it is equivalent to "true".
}
Select statements
----
A select statement chooses which of a set of possible communications
will proceed. It looks similar to a switch statement but with the
cases all referring to communication operations.
SelectStat = "select" "{" { CommClause } "}" .
CommClause = CommCase { Statement } .
CommCase = ( "default" | ( "case" ( SendCase | RecvCase) ) ) ":" .
SendCase = Send .
RecvCase = [ identifier '=' ] RecvExpression .
RecvExpression = '<' Expression .
The select statement evaluates all the channel (pointers) involved.
If any of the channels can proceed, 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. A channel pointer may be nil, which is
equivalent to that case not being present in the select statement.
If the channel sends or receives "any" or an interface type, its
communication can proceed only if the type of the communication
clause matches that of the dynamic value to be exchanged.
If multiple cases can proceed, a uniform fair choice is made regarding
which single communication will execute.
var c, c1, c2 *chan int;
select {
case i1 = <c1:
printf("received %d from c1\n", i1);
case >c2 = i2:
printf("sent %d to c2\n", i2);
default:
printf("no communication\n");
}
for { // send random sequence of bits to c
select {
case >c = 0: // note: no statement, no fallthrough, no folding of cases
case >c = 1:
}
}
var ca *chan any;
var i int;
var f float;
select {
case i = <ca:
printf("received int %d from ca\n", i);
case f = <ca:
printf("received float %f from ca\n", f);
}
TODO: do we allow case i := <c: ?
TODO: need to precise about all the details but this is not the right doc for that
For statements
----
@ -1591,7 +1650,6 @@ TODO
----
- TODO: type switch?
- TODO: select
- TODO: words about slices
- TODO: what is nil? do we type-test by a nil conversion or something else?