From 569a107e0dd54657b3cf337a72757703ffa6fea8 Mon Sep 17 00:00:00 2001 From: Rob Pike Date: Fri, 3 Oct 2008 11:18:45 -0700 Subject: [PATCH] Revised wording about sends. Evaluation is done before communication starts. R=gri DELTA=19 (4 added, 1 deleted, 14 changed) OCL=16357 CL=16416 --- doc/go_spec.txt | 31 +++++++++++++++++-------------- 1 file changed, 17 insertions(+), 14 deletions(-) diff --git a/doc/go_spec.txt b/doc/go_spec.txt index c43024a00d3..ec905624f83 100644 --- a/doc/go_spec.txt +++ b/doc/go_spec.txt @@ -1756,8 +1756,10 @@ a channel and a value (expression): ch <- 3 In this form, the send operation is an (expression) statement that -blocks until the send can proceed, at which point the value is -transmitted on the channel. +sends the value on the channel. Both the channel and the expression +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 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. 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 value of the expression is the value received: @@ -2123,20 +2126,20 @@ cases all referring to communication operations. SendExpr = Expression "<-" Expression . RecvExpr = [ PrimaryExpr ( "=" | ":=" ) ] "<-" Expression . -First, for all the send and receive expressions in the select -statement, the channel expression is evaluated. If any of the -resulting channels can proceed, one is chosen and the corresponding -communication (including the value to be sent, if any) and statements -are evaluated. Otherwise, if there is a default case, that executes; +For all the send and receive expressions in the select +statement, the channel expression is evaluated. Any values +that appear on the right hand side of send expressions are also +evaluated. If any of the resulting channels 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 are not re-evaluated. A channel pointer -may be nil, which is equivalent to that case not being present in -the select statement. +complete. The channels and send expressions are not re-evaluated. +A channel pointer may be nil, which is equivalent to that case not +being present in the select statement. -Note that since all the channels are evaluated, any side effects in -that evaluation will occur for all the channels in the select. On the -other hand, for sends, only the communication that proceeds has -its right-hand-side expression evaluated. +Since all the channels and send expressions are evaluated, any side +effects in that evaluation will occur for all the communications +in the select. If the channel sends or receives an interface type, its communication can proceed only if the type of the communication