gistfile1.txt

{-# LANGUAGE MultiParamTypeClasses,
             FunctionalDependencies,
             FlexibleInstances #-}

class Option a b | a -> b where
    toOption :: a -> Maybe b

instance Option (Maybe a) a where
    toOption = id

instance Option Int Int where
    toOption = Just

instance Option String String where
    toOption = Just

-- GHCi
-- *Main> :r
-- [1 of 1] Compiling Main             ( Dynamic.hs, interpreted )
-- Ok, modules loaded: Main.
-- *Main> toOption (Just 42)
-- Just 42
-- *Main> toOption "test"
-- Just "test"
-- *Main> toOption (42 :: Int)
-- Just 42
-- *Main>