Source m o = ConduitM () o m (); why is Source m o not ConduitM Void o m ()?

I can't think of a really good answer to this, but here's a mediocre answer: you can always "step" a ConduitM that is blocked on trivial input. So the promise of a Source is not that it never blocks, but rather, that it only blocks in such a way that it is trivial to unblock.

You may like the Producer type synonym better:

type Producer m o = forall i. ConduitM i o m ()

When you have a Producer m o, it can be instantiated to ConduitM Void o m (), because you can select i = Void.

Now for your main question...

So the thing about ConduitM composition is that the "upstream result" must be (). If you peel away the ConduitM layer of abstraction and take a look at Data.Conduit.Internal.Pipe, you'll find the operator you're looking for:

http://hackage.haskell.org/package/conduit-1.2.6.1/docs/src/Data-Conduit-Internal-Pipe.html#awaitE

awaitE :: Pipe l i o u m (Either u i)

I'm not quite sure how to surface this into the ConduitM level of abstraction.

-- Dan Burton

On Fri, Feb 12, 2016 at 12:40 PM, David Turner <dct25-561bs@mythic-beasts.com> wrote:

Hi,

I've got a conduit thing that yields infinitely many values and never exits, which I've given the type ConduitM () o m Void - a bit like Source m o = ConduitM () o m () except that it can't exit due to the Void.

(One side-question: why is Source m o not ConduitM Void o m ()?)

I would now like to get the first item it yields; I'm currently using Data.Conduit.List.head but of course this returns a Maybe o in case the upstream thing exits. Is there a way to do this without that Maybe? I can't see anything obvious, but nor can I think of a terribly good reason why not.

One thing that I was pondering was a kind of fuse operator with a type like ...

ConduitM a b m r1 -> ConduitM b c m r2 -> ConduitM a c m (Either r1 r2)

... which returns the result of whichever thing exits first. Does such a thing exist? Does it even make sense? If it existed, I think I could use it here as it'd specialise to

ConduitM () o m Void -> ConduitM o Void m o -> ConduitM () Void m (Either Void o)

and of course (Either Void o) is isomorphic to o so I'd be home and dry.

Having written this, I'm now also struggling to work out what the thing of type ConduitM o Void m o would be. Maybe I'm going about this all the wrong way, or maybe I'm just confused?

Any help greatly appreciated!

Cheers,

David

_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe