[Haskell-cafe] Continuations and coroutines

Mario Blažević mblazevic at stilo.com
Thu Jun 24 12:19:00 EDT 2010

Yves Parès wrote:
> It helps me understand better, but would you have some simple code that 
> would do that ?

	You can look at the definition of the coroutine monad transformer in 
the monad-coroutine package as well:


	The heart of the library is in the data type

> newtype Coroutine s m r = Coroutine {
>    resume :: m (Either (s (Coroutine s m r)) r)
> }

where s is an arbitrary functor (like Yield, for example), m is an 
arbitrary monad, and r is the coroutine's final result type.

	You can also read the "Trampolined Style" and "The essence of 
multitasking" papers:


> 2010/6/19 Paul Johnson <paul at cogito.org.uk <mailto: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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Mario Blazevic
mblazevic at stilo.com
Stilo International

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