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haskellhaskell-pipes

Running a consumer inside a pipe

I need to compose a consumer and a pipe so that the output of the consumer would feed the input of the pipe.

I guess this could be solved with a combinator like this:

Consumer i m r -> (r -> Producer o m r') -> Pipe i o m r'

or this:

Consumer i m i' -> Pipe i' o m r -> Pipe i o m r

or a lifting function like the following:

Consumer i m r -> Pipe i o m r

or like this:

Consumer i m o -> Pipe i o m r

I tried doing consumer >~ pipe without a success. So how to approach this?

Solution

  • Something similar to your signature Consumer i m o -> Pipe i o m r could be done like this:

    {-# LANGUAGE RankNTypes #-}
import Pipes

foo :: Monad m => Consumer' i m o -> Pipe i o m ()
foo consumer = consumer >>= yield

    I have used the Consumer' polymorphic type synonym which, since it is not really closed "downstream", it can be used as a Pipe that never actually yields. To make it yield the return value of the consumer, we simply use a monadic bind.

    As for your Consumer i m r -> Pipe i o m r signature, it's simply the identity using the polymorphic type synonym:

    iden :: Monad m => Consumer' i m r -> Pipe i o m r
iden consumer = consumer