On Wed, Nov 2, 2016, at 10:35 PM, Gustavo Niemeyer wrote:
> Hello there,
>
> On Wed, Nov 2, 2016 at 11:09 AM, Ian Davis <m...@iandavis.com> wrote:
>> __
>>
>> On Wed, Nov 2, 2016, at 12:56 PM, 'Axel Wagner' via golang-
>> nuts wrote:
>>> AIUI: A child or grandchild function is not supposed to signal that.
>>> They can return an error and let the parent cancel, or they can
>>> create their own child context WithCancel and cancel that. Context
>>> doesn't replace exceptions.
>>
>>
>> OK I see that. I think I don't understand the purpose of the Err
>> method if the function that creates the context is the one that
>> cancels it or closes the Done channel. It seems that the only purpose
>> would be to detect a Timeout.
>
> Err allows you to do this:
>
> select {
> case <-ctx.Done():
> return ctx.Err()
> ...
> }
In this situation, other than a timeout, what would be closing the Done
channel? The goroutine that created the context already knows the error
since it's the only one that is supposed to call cancel.
>
>>
>>> You should consider context a way to implement dynamic scoping[1]. A
>>> context is a thing valid for the current stack frame and downwards
>>> and should never affect the stack upwards. Which is also why there's
>>> the "put values in there, not pointers" rule; if you have a pointer,
>>> it might modify an upstack context.
>>>
>>> The advantage of that (and context in general) is, that you don't
>>> need to worry too much about synchronization and everything; as a
>>> context is fundamentally immutable (-ish), it's fine to pass it to
>>> separate goroutines, because they can only do stuff with it
>>> downstack.
>>
>> Yes, this is what I am trying to do. I was wondering whether the
>> returned cancelFunc is something that could be passed downstream to
>> signal parents that work should be aborted.
>
> No, the purpose of Context is the opposite: fired tasks are free to do
> whatever they please until it's time to stop, because the boss says
> so. Once that happens, error propagation happens as usual to tell what
> went wrong.
>
> The http://gopkg.in/tomb.v2 package works closer to the model you
> describe.
Thanks. I've used tomb before and I've played with the context/tomb
integration. I guess I need both.
Links:
1. https://en.wikipedia.org/wiki/Scope_(computer_science)#Dynamic_scoping
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