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{-# LANGUAGE MultiParamTypeClasses, TypeFamilies, FlexibleContexts #-}
module Main where
class (There a ~ b, BackAgain b ~ a) => Foo a b where
type There a
type BackAgain b
there :: a -> b
back :: b -> a
tickle :: b -> b
instance Foo Char Int where
type There Char = Int
type BackAgain Int = Char
there = fromEnum
back = toEnum
tickle = (+1)
test :: (Foo a b) => a -> a
test = back . tickle . there
main :: IO ()
main = print $ test 'F'
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