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Revised wording about sends.

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Evaluation is done before communication starts.

R=gri
DELTA=19  (4 added, 1 deleted, 14 changed)
OCL=16357
CL=16416
This commit is contained in:
Rob Pike 2008-10-03 11:18:45 -07:00
parent ed139c1e37
commit 569a107e0d
1 changed files with 17 additions and 14 deletions
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31
doc/go_spec.txt
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@ -1756,8 +1756,10 @@ a channel and a value (expression):
ch <- 3 ch <- 3
In this form, the send operation is an (expression) statement that In this form, the send operation is an (expression) statement that
blocks until the send can proceed, at which point the value is sends the value on the channel. Both the channel and the expression
transmitted on the channel. are evaluated before communication begins. Communication blocks
until the send can proceed, at which point the value is transmitted
on the channel.
If the send operation appears in an expression context, the value If the send operation appears in an expression context, the value
of the expression is a boolean and the operation is non-blocking. of the expression is a boolean and the operation is non-blocking.
@ -1775,6 +1777,7 @@ success of the operation. If the program does not test the value,
the operation blocks until it succeeds. the operation blocks until it succeeds.
TODO: Adjust the above depending on how we rule on the ok semantics. TODO: Adjust the above depending on how we rule on the ok semantics.
For instance, does the sent expression get evaluated if ok is false?
The receive operation uses the prefix unary operator "<-". The receive operation uses the prefix unary operator "<-".
The value of the expression is the value received: The value of the expression is the value received:
@ -2123,20 +2126,20 @@ cases all referring to communication operations.
SendExpr = Expression "<-" Expression . SendExpr = Expression "<-" Expression .
RecvExpr = [ PrimaryExpr ( "=" | ":=" ) ] "<-" Expression . RecvExpr = [ PrimaryExpr ( "=" | ":=" ) ] "<-" Expression .
First, for all the send and receive expressions in the select For all the send and receive expressions in the select
statement, the channel expression is evaluated. If any of the statement, the channel expression is evaluated. Any values
resulting channels can proceed, one is chosen and the corresponding that appear on the right hand side of send expressions are also
communication (including the value to be sent, if any) and statements evaluated. If any of the resulting channels can proceed, one is
are evaluated. Otherwise, if there is a default case, that executes; 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 if not, the statement blocks until one of the communications can
complete. The channels are not re-evaluated. A channel pointer complete. The channels and send expressions are not re-evaluated.
may be nil, which is equivalent to that case not being present in A channel pointer may be nil, which is equivalent to that case not
the select statement. being present in the select statement.
Note that since all the channels are evaluated, any side effects in Since all the channels and send expressions are evaluated, any side
that evaluation will occur for all the channels in the select. On the effects in that evaluation will occur for all the communications
other hand, for sends, only the communication that proceeds has in the select.
its right-hand-side expression evaluated.
If the channel sends or receives an interface type, its If the channel sends or receives an interface type, its
communication can proceed only if the type of the communication communication can proceed only if the type of the communication