Injective.hs

   {-# LANGUAGE MultiParamTypeClasses #-}

   module Injective
     ( Injective (..),
       Foo (..),
     )
   where

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

   data Foo = Foo | Bar | Buz

   instance Injective Bool Foo where
     to True = Foo
     to False = Bar
     from Foo = Just True
     from Bar = Just False
     from Buz = Nothing