[go-nuts] Select statement execution, rationale

On Sun, Nov 23, 2014 at 10:52 AM, Nick Patavalis wrote:

Hello gophers,

Reading from the language spec:

For all the cases in the statement, the channel operands of receive
operations and the channel **and right-hand-side expressions of send
statements** are evaluated exactly once, in source order, upon entering
the "select" statement. The result is a set of channels to receive from or
send to, and the corresponding values to send.


I admit, the emphasized part caught me by surprise. Why? I would expect
the right-hand-side part of send statements to be evaluated *only* when the
case selected. This would allow someone to do something like:

case out <- q.Pop():


Now you have to mess with temporary variables and the code gets uglier.

I understand that it is how it is, but I'm wondering, is there any reason
it was chosen to be so?

/npat

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On Sunday, November 23, 2014 9:13:28 PM UTC+2, Caleb Spare wrote:
Quoting Jan Mercl from the
​golang-dev
thread:

How could the case be selected without evaluating the RHS? Without
​The RHS of send statements doesn't affect whether the send can proceed;
only the LHS.

On Sun, Nov 23, 2014 at 8:12 PM, Caleb Spare wrote:
The RHS of send statements doesn't affect whether the send can proceed; only
the LHS.

On Sun, Nov 23, 2014 at 11:18 AM, Jan Mercl wrote:
You're right. I don't know why I was thinking that the RHS is a
channel receive, sorry.

-j

On Sunday, November 23, 2014 9:22:14 PM UTC+2, Caleb Spare wrote:
Nick -- while it would make the code nicer, your version of select would
not allow for as efficient implementations of select+channel operations.
The channel in question would have to be locked while computing the RHS
value. Here's an old golang-nuts thread about it:

https://groups.google.com/d/topic/golang-nuts/qZO4kldyWak/discussion