[Haskell-cafe] Continuations and coroutines

[Yves Parès]

It helps me understand better, but would you have some simple code that
would do that ?


2010/6/19 Paul Johnson <paul at cogito.org.uk>

> On 19/06/10 10:36, Yves Parčs wrote:
>
>> Hello,
>>
>> I saw on the haskell wikibook that coroutines could be implemented by
>> using continuations :
>> http://en.wikibooks.org/wiki/Haskell/Continuation_passing_style#Example:_coroutines(unhappily, the section is empty)
>> Since I'm actually learning the wonders of continuations, I just wonder :
>> how ?
>>
>>
>
> Coroutines depend on the ability to suspend and resume execution.  A
> continuation acts as the "resume point" in the current function.  The
> "callCC" function in the continuation monad takes a function that expects
> the continuation as an argument (which is how you get access to it).  So you
> say something like:
>
> >  yield = callCC $ \continuation -> ....
>
> Then you would typically store the continuation somewhere and call some
> other previously stored continuation to switch contexts.
>
> Continuations can be used to pass data back into the continuation: you call
> the continuation with an argument, and that argument becomes the return
> value of the "callCC".  In this case you probably just want to use ().
>
> You typically have a queue for continuations, so the new continuation goes
> on the back of the queue and then you call the head of the queue.  Obvious
> modifications for priority, simulated time, real time or whatever else you
> are trying to schedule.  This implies some kind of monadic state to store
> the queue in, so you will probably make your monad of type "ContT (State
> Queue)"
>
> If you want a thread to wait, say on a semaphore, then you have a queue of
> continuations in the semaphore data structure.
>
> Is this any help?
>
> Paul.
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