sourceToSinkTransducer :: SourceTransducer x y -> SinkTransducer x y
I can't find any way to implement the sourceToSinkTransducer function
Indeed that cannot be done because it breaks referential transparency.
Ok, thanks for clarifying the reason.
Food for thought:
You need some sort of side effects (I'm not quite sure which), so it'd be interesting to try
newtype Source m x = Source (m (x, Source x)) newtype Sink m x = Sink (x -> m (Sink x))
where m is an appropriate monad. Will the continuation monad work?
I'm not sure. I'm trying to find the minimal data types that will let me implement the higher-level combinators. So far, the best I've found is the CPS and the following declarations: data Source i r = Source {get :: Sink i r -> r} data Monoid r => Sink i r = Sink {put :: i -> Source i r -> r, close :: r} newtype Filter x y = Filter { transduce :: forall r. Monoid r => Source x r -> Sink y r -> r } newtype Splitter i = Splitter { split :: forall r. Monoid r => Source i r -> Sink i r -> Sink i r -> r } The 'close' function and result type 'r' are there because the streams are usually finite. The 'Source' type should really be declared as data Source i = Source {get :: forall r. Sink i r -> r} because sources should not depend the result type, but that caused a lot of problems with type checking: the two 'r's in Source and Sink declaration are not unified. I don't see a way to express my intention. Anyway, thanks again for the explanation of why it has to be this complicated.