Source m o = ConduitM https://hackage.haskell.org/package/conduit-1.2.6/docs/Data-Conduit.html#t:C... () o m (); why is Source m o not ConduitM https://hackage.haskell.org/package/conduit-1.2.6/docs/Data-Conduit.html#t:C... 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-Int... 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 https://hackage.haskell.org/package/conduit-1.2.6/docs/Data-Conduit.html#t:C... () o m Void - a bit like Source m o = ConduitM https://hackage.haskell.org/package/conduit-1.2.6/docs/Data-Conduit.html#t:C... () o m () except that it can't exit due to the Void.
(One side-question: why is Source m o not ConduitM https://hackage.haskell.org/package/conduit-1.2.6/docs/Data-Conduit.html#t:C... 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 https://hackage.haskell.org/package/conduit-1.2.6/docs/Data-Conduit.html#t:C... a b m r1 -> ConduitM https://hackage.haskell.org/package/conduit-1.2.6/docs/Data-Conduit.html#t:C... b c m r2 -> ConduitM https://hackage.haskell.org/package/conduit-1.2.6/docs/Data-Conduit.html#t:C... 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 https://hackage.haskell.org/package/conduit-1.2.6/docs/Data-Conduit.html#t:C... () o m Void -> ConduitM https://hackage.haskell.org/package/conduit-1.2.6/docs/Data-Conduit.html#t:C... o Void m o -> ConduitM https://hackage.haskell.org/package/conduit-1.2.6/docs/Data-Conduit.html#t:C... () 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 https://hackage.haskell.org/package/conduit-1.2.6/docs/Data-Conduit.html#t:C... 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
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