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{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE UnicodeSyntax #-}
{-# LANGUAGE LinearTypes #-}
module Foo where
data Either a b = Left a | Right b
either :: (a -> c) -> (b -> c) -> Either a b -> c
either f g (Left a) = f a
either f g (Right b) = g b
class Iden p where
iden :: p a a
instance Iden (->) where
iden x = x
class Cat p where
comp :: p b c -> p a b -> p a c
instance Cat (->) where
comp f g = \x -> f (g x)
class ArrowChoice a where
(+++) :: a b c -> a b' c' -> a (Either b b') (Either c c')
(|||) :: a b d -> a c d -> a (Either b c) d
instance ArrowChoice (->) where
-- This doesn't work as |p| is inferred to be |-o| because of |Left|.
-- Then GHC complains that |f| is not the same type before it realises
-- that the overall type must be (->)
-- f +++ g = (Left `comp` f) ||| (Right `comp` g)
f +++ g = (comp @(->) Left f) ||| (comp @(->) Right g)
(|||) = either
-- This shouldn't work
foo :: a ⊸ a
foo = iden
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