I was hoping to use Haskell's laziness to implement some of the stuff from "Continuation and Transducer Composition" (http://www-static.cc.gatech.edu/~shivers/papers/fcoro.pdf) a bit more elegantly, without manually coding the coroutine logic in continuation-passing style. Unfortunately, I'm beginning to think there is no workaround. Here's the challenge in a simplified form: newtype Source x = Source (x, Source x) newtype Sink x = Sink (x -> Sink x) type SourceTransducer x y = Source x -> Source y type SinkTransducer x y = Sink y -> Sink x sourceToSinkTransducer :: SourceTransducer x y -> SinkTransducer x y I can't find any way to implement the sourceToSinkTransducer function and its inverse. This is disappointing, because SourceTransducer and SinkTransducer are obviously isomorphic and there are languages like Oz that can accomplish this. Tell me I'm missing something obvious?