TypeLevelContains.hs

{-# LANGUAGE AllowAmbiguousTypes  #-}
{-# LANGUAGE DataKinds            #-}
{-# LANGUAGE PolyKinds            #-}
{-# LANGUAGE TypeFamilies         #-}
{-# LANGUAGE TypeOperators        #-}
{-# LANGUAGE UndecidableInstances #-}

import           Data.Kind      (Type)
import           Data.Type.Bool


type family Contains (a :: Type) (b :: k) :: Bool where
  Contains a a     = 'True
  Contains a (f x) = Contains a f || Contains a x
  Contains _ _     = 'False


class ToBool (b :: Bool) where
  toBool :: Bool

instance ToBool 'True  where toBool = True
instance ToBool 'False where toBool = False


main :: IO ()
main = do
  print $ toBool @(Contains Int         (Either (Maybe [IO Int]) String)
  print $ toBool @(Contains [IO Int]    (Either (Maybe [IO Int]) String)
  print $ toBool @(Contains (Maybe Int) (Either (Maybe [IO Int]) String)