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Add select statement
SVN=114147
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@ -1163,7 +1163,7 @@ Statements control execution.
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[ LabelDecl ] ( StructuredStat | UnstructuredStat ) .
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StructuredStat =
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Block | IfStat | SwitchStat | ForStat | RangeStat .
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Block | IfStat | SwitchStat | SelectStat | ForStat | RangeStat .
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UnstructuredStat =
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Declaration | SimpleVarDecl |
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@ -1399,6 +1399,65 @@ If the expression is omitted, it is equivalent to "true".
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}
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Select statements
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----
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A select statement chooses which of a set of possible communications
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will proceed. It looks similar to a switch statement but with the
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cases all referring to communication operations.
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SelectStat = "select" "{" { CommClause } "}" .
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CommClause = CommCase { Statement } .
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CommCase = ( "default" | ( "case" ( SendCase | RecvCase) ) ) ":" .
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SendCase = Send .
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RecvCase = [ identifier '=' ] RecvExpression .
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RecvExpression = '<' Expression .
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The select statement evaluates all the channel (pointers) involved.
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If any of the channels can proceed, the corresponding communication
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and statements are evaluated. Otherwise, if there is a default case,
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that executes; if not, the statement blocks until one of the
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communications can complete. A channel pointer may be nil, which is
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equivalent to that case not being present in the select statement.
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If the channel sends or receives "any" or an interface type, its
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communication can proceed only if the type of the communication
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clause matches that of the dynamic value to be exchanged.
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If multiple cases can proceed, a uniform fair choice is made regarding
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which single communication will execute.
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var c, c1, c2 *chan int;
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select {
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case i1 = <c1:
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printf("received %d from c1\n", i1);
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case >c2 = i2:
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printf("sent %d to c2\n", i2);
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default:
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printf("no communication\n");
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}
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for { // send random sequence of bits to c
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select {
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case >c = 0: // note: no statement, no fallthrough, no folding of cases
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case >c = 1:
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}
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}
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var ca *chan any;
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var i int;
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var f float;
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select {
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case i = <ca:
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printf("received int %d from ca\n", i);
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case f = <ca:
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printf("received float %f from ca\n", f);
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}
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TODO: do we allow case i := <c: ?
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TODO: need to precise about all the details but this is not the right doc for that
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For statements
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----
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@ -1591,7 +1650,6 @@ TODO
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----
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- TODO: type switch?
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- TODO: select
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- TODO: words about slices
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- TODO: what is nil? do we type-test by a nil conversion or something else?
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