[Haskell-cafe] Stateful iteratees

[Ertugrul Soeylemez]

Gregory Collins <greg at gregorycollins.net> wrote:

> On Thu, Apr 7, 2011 at 7:35 PM, Ertugrul Soeylemez <es at ertes.de> wrote:
>
> > > Why can't you use #1 and do this when you call "run_"?
> >
> > Because that runs the iteratee and leaves me with a StateT.  Even
> > though I use a CPS-based StateT, I doubt that it can be converted
> > back to Iteratee easily.
> >
> > With the first variant, I would need a function like this:
> >
> >    runMyApp :: Iteratee a (StateT MyConfig m) b -> Iteratee a m b
>
> Let me rephrase my question: why do you need a function like this?

Because I have multiple, independent libraries, which use a stateful
iteratee.  They work perfectly well, as long as you don't try to combine
them.  I have found a solution now, which seems to solve the problems.
I have described it in my last post.


> Anyways, something like this is definitely doable (using enumerator
> formulation, sorry), but your type needs to be this:
>
>     runStateIteratee :: Monad m => Iteratee a (StateT s m) r -> s ->
>                         Iteratee a m (r, s)

Well, yes, but that's only part of the problem.  After doing this, I
need to put the iteratee on top of another monad transformer, while
remembering the state to (again) change the underlying state type.  All
in all it would become very complicated.  The solution I'm working on
seems to solve this more nicely and keeps the iteratees clean.

But it's good to know that such a transformation is possible.  It may
help out later.  Thanks.


Greets,
Ertugrul


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