HList.hs

module HList
       ( H
       , repH
       , absH
       ) where

type H a = [a] -> [a]

{-# INLINE repH #-}
repH :: [a] -> H a
repH xs = (xs ++)

{-# INLINE absH #-}
absH :: H a -> [a]
absH f = f []

-- Should be in a "List" module
{-# RULES "++ []"  forall xs .  xs ++ [] = xs #-}
{-# RULES "++ strict"           (++) undefined = undefined #-}

-- The "Algebra" for repH
{-# RULES "repH ++" forall xs ys   . repH (xs ++ ys) = repH xs . repH ys #-}
{-# RULES "repH []" 	             repH [] = id  	       	         #-}
{-# RULES "repH (:)" forall x xs   . repH (x:xs) = ((:) x) . repH xs     #-}