HFilter.hs

{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
module HFilter where

data a :* b = a :* b
  deriving (Show, Eq)
infixr 5 :*

hlist :: Int :* String :* Int :* Maybe Float :* ()
hlist = 1 :* "hello" :* 2 :* Just 3 :* ()

class TypeFilter h a h' where
    hfilter :: a -> h -> h'

instance TypeFilter () a () where
    hfilter _ = id

instance TypeFilter h a h' => TypeFilter (t :* h) a h' where
    hfilter a' (_ :* h) = hfilter a' h

instance TypeFilter h a h' => TypeFilter (a :* h) a (a :* h') where
    hfilter a' (a :* h) = a :* hfilter a' h