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{-# LANGUAGE ScopedTypeVariables, TypeFamilies #-}
module T2203b where
class Foo a where
type TheFoo a
foo :: TheFoo a -> a
foo' :: a -> Int
class Bar b where
bar :: b -> Int
instance (b ~ TheFoo a, Foo a) => Bar (Either a b) where
bar (Left a) = foo' a
bar (Right b) = foo' (foo b :: a)
instance Foo Int where
type TheFoo Int = Int
foo = id
foo' = id
val :: Either Int Int
val = Left 5
res :: Int
res = bar val
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