copattern hs

copatternStream.hs

{-# LANGUAGE GADTs, RankNTypes   #-}

main = putStrLn "hello!"
-- StreamModel is what we ordinarily write as "Stream" in haskell, 
-- though is not ideal for a variety of reasons.
data StreamModel a = StreamModel { head :: a , tail :: StreamModel a}

data StreamTag a  {- param list -} res {- result type!  -} where 
   HeadTag :: StreamTag a a 
   TailTag :: StreamTag a (Stream a)

newtype Stream a = Stream  { unStream  :: (forall res . StreamTag a res -> res) }

headStream:: Stream a -> a 
headStream (Stream f) = f HeadTag 

tailStream :: Stream a -> Stream a 
tailStream (Stream f) = f TailTag

-- this is just zipWith  for Stream, 
rawRawZipWith :: (a -> b ->c  ) -> Stream a -> Stream b -> Stream c 
rawRawZipWith f sta stb = Stream $ \ x -> 
      case  x of 
        HeadTag -> f (headStream sta) (headStream stb)
        TailTag -> rawRawZipWith f (tailStream sta) (tailStream stb)

 --- edit1: heres how to go from Stream (the coinductive definition)
 ---  to StreamModel (the usual stream definition in haskell)

stream2StreamModel :: Stream a -> StreamModel a 
stream2StreamModel strm =  StreamModel (headStream strm) (tailStream strm)
--  edit2 : clarified that the StreamModel type is only included for simplicity :) 

discussion at https://www.reddit.com/r/haskell/comments/4aju8f/simple_example_of_emulating_copattern_matching_in/