[go-nuts] Channel select, blocking and close

On Wed, Jan 2, 2013 at 3:40 AM, Lukáš Zapletal wrote:

Hello,

I think I am missing how close and select work together.

http://play.golang.org/p/EwmpUrVL_7

I created two int channels a and b with buffer set to 3 and sent six
numbers in total there, channel a is closed then, b is left unclosed. Then
in a loop of ten cycles I read from channels.

No matter if I close a channel or not, channels keeps giving me "empty
value" (zero in case of integer). I would expect this behavior:

- if both a and b are not closed, select blocks
- if a is closed then it should print empty values from a
- the same for b
- if both are closed, default block is executed

Why select does not block?

On Tue, Jan 1, 2013 at 2:44 PM, minux wrote:


because a closed channel is always ready for read (return the empty
value), you can do this
http://play.golang.org/p/Phx56qVuQy
if you don't want to always receiving empty values from a closed channel.

--

always succeeds, immediately returning the element type's zero value<http://golang.org/ref/spec#The_zero_value>
.
On Tuesday, January 1, 2013 11:40:45 AM UTC-8, Lukáš Zapletal wrote:

Hello,

I think I am missing how close and select work together.

http://play.golang.org/p/EwmpUrVL_7

I created two int channels a and b with buffer set to 3 and sent six
numbers in total there, channel a is closed then, b is left unclosed. Then
in a loop of ten cycles I read from channels.

No matter if I close a channel or not, channels keeps giving me "empty
value" (zero in case of integer). I would expect this behavior:

- if both a and b are not closed, select blocks
- if a is closed then it should print empty values from a
- the same for b
- if both are closed, default block is executed

Why select does not block?

On Tuesday, January 1, 2013 8:47:39 PM UTC+1, Thorsten von Eicken wrote:

Doesn't seem very intuitive at first, but if select didn't return
something for channel a then how could you be notified that it got closed?
From http://golang.org/ref/spec#Receive_operator :

Receiving from a nil channel blocks forever. Receiving from a closed<http://golang.org/ref/spec#Close> channel

On Wed, Jan 2, 2013 at 3:53 AM, Lukáš Zapletal wrote:

I vote for writing this here: http://tour.golang.org/#65

On Tuesday, January 1, 2013 9:43:27 PM UTC+1, minux wrote:


On Wed, Jan 2, 2013 at 3:53 AM, Lukáš Zapletal <luk...@zapletalovi.com<javascript:>
There are already 2 notes on that single page....
I'm afraid adding too many details there will overwhelm the learner.

The Go language specification is always a must-read for Go users.

On Wed, Jan 2, 2013 at 8:00 AM, Lukáš Zapletal wrote:

I am afraid close and select does not work well together. Once I close a
channel, blocking is gone so I am not able to leverage select effectively
and I am ending up in a loops. I guess I need to refactor my program again,
I guess close is not good in my case.