types.hs

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE MultiParamTypeClasses #-}

class IsSubtypeOf a b where
  to :: a -> b
  from :: b -> Maybe a

instance IsSubtypeOf Int Int where
  to = id
  from = Just

instance IsSubtypeOf Bool Int where
  to True = 1
  to False = 0

  from 0 = Just False
  from 1 = Just True
  from _ = Nothing

add :: (IsSubtypeOf a Int, IsSubtypeOf b Int) => a -> b -> Int
add a b = to a + to b

main :: IO ()
main = do
  print $ add True (3::Int)
  print $ add True False
  print $ add (3::Int) (7::Int)