Haskell pipeline

pipeline.hs

{-# LANGUAGE GADTs #-}

import Data.Char (digitToInt)

data Nil
data Cons a b

data Pipeline ts a c where
    (:>) :: (a -> b) -> Pipeline ts b c -> Pipeline (Cons (a -> b) ts) a c
    Id :: Pipeline Nil t t

infixr 9 :>

pHead :: Pipeline (Cons (a -> b) fs) a c -> (a -> b)
pHead (f :> _) = f

pTail :: Pipeline (Cons (a -> b) fs) a c -> Pipeline fs b c
pTail (_ :> fs) = fs

testP = digitToInt :> (+ 1) :> show :> Id