leftover-preserving.hs

{-# LANGUAGE RankNTypes #-}
import Debug.Trace
import           Control.Applicative
import           Control.Lens               (view, from, zoom, iso, Iso')
import           Control.Monad.State.Strict (evalState)
import           Pipes
import           Pipes.Core                 as Pc
import qualified Pipes.Parse                as Pp
import qualified Pipes.Prelude              as P

newtype A = A Int
    deriving Show
newtype B = B Int
    deriving Show

atob (A i) = traceShow ("atob", i) (B i)
btoa (B i) = traceShow ("btoa", i) (A i)

ab :: Iso' A B
ab = iso atob btoa

piso :: Monad m => Iso' a b -> Iso' (Producer a m r) (Producer b m r)
piso i = iso (P.map (view i) <-<) (>-> P.map (view $ from i))

main :: IO ()
main = do
  let src = P.map A <-< each [1..10]
  let parser = (,,,) <$> zoom (piso ab) Pp.peek
                     <*> zoom (Pp.splitAt 3) Pp.drawAll
                     <*> zoom (Pp.splitAt 3 . piso ab) Pp.drawAll
                     <*> Pp.drawAll
  let res = evalState parser src
  print res

output.txt

("atob",1)
("btoa",1)
("atob",2)
("btoa",2)
("atob",3)
("btoa",3)
("atob",4)
("btoa",4)
("atob",4)
("atob",5)
("btoa",5)
("atob",5)
("atob",6)
("btoa",6)
("atob",6)
("atob",7)
("btoa",7)
("atob",8)
("btoa",8)
("atob",9)
("btoa",9)
("atob",10)
("btoa",10)
(Just (B 1),[A 1,A 2,A 3],[B 4,B 5,B 6],[A 7,A 8,A 9,A 10])

Sure this does not work at times when you do not have an isomorphism for the type your are streaming.

The basic idea is that I have an isomorphism, and I'd like to map all input values to convert from type A to type B.

This sentence in your stackoverflow question is where I, I would bet the same is true for J. Abrahamson, made the assumption that A is isomorphic to B in your problem case.

In the cases you care about it is not an isomorphism of the types you are streaming but from isomorphism between two streams. In which case it seems like you need a different approach then the one taken here or on stackoverflow(http://stackoverflow.com/a/21650705/128583).