GADTs

{-# LANGUAGE GADTs #-}

module Main where

data PR t a = PR (Maybe ([t], a))
data P  t a = P  ([t] -> PR t a)

a :: P Char (T A)
a = undefined

b :: P Char (T B)
b = undefined

data A = A
data B = B
data E = E String

data T a where
    A' :: T A
    B' :: T B
    E' :: E -> T E

type H t = [Char] -> (t, [Char])
type TT t = [Char] -> [t]

tt :: H t -> [P Char t] -> TT t
tt = undefined

h :: H (T a)
h []          = (E' (E "E"), [])
h (head:rest) = (E' (E ("E")), rest)

tt' :: [Char] -> [T a]
tt' = tt h [a, b]

main :: IO ()
main = putStrLn "Hello"