gistfile1.txt

{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}
import Type.Reflection

data Foo = Int Int | Bool Bool

wrap :: Typeable b => b -> Maybe Foo
wrap b
	| Just HRefl <- eqTypeRep t (typeRep @Int) = Just (Int b)
	| Just HRefl <- eqTypeRep t (typeRep @Bool) = Just (Bool b)
	| otherwise = Nothing
	where
	t = typeOf b

onFoo :: (Typeable a, Typeable b) => (a -> b) -> Foo -> Maybe Foo
onFoo f foo = case foo of
	Int i | Just HRefl <- eqTypeRep t (typeRep @Int) -> wrap (f i)
	Bool b | Just HRefl <- eqTypeRep t (typeRep @Bool) -> wrap (f b)
	_ -> Nothing
	where
	Fun t _ = typeOf f