RuntimeTypeContains.hs

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications    #-}

import           Data.Proxy
import           Data.Typeable

contains :: forall a b. (Typeable a, Typeable b) => Bool
contains = go typeB
  where typeA = typeRep (Proxy :: Proxy a)
        typeB = typeRep (Proxy :: Proxy b)
        go x = x == typeA || any go (typeRepArgs x)

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