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{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
UndecidableInstances #-}
-- !!! An instance decl with a context containing a free type variable
-- The interest here is that there's a "b" in the instance decl
-- context that isn't mentioned in the instance head.
-- Hence UndecidableInstances
module ShouldCompile where
class HasConverter a b | a -> b where
convert :: a -> b
data Foo a = MkFoo a
instance (HasConverter a b,Show b) => Show (Foo a) where
show (MkFoo value) = show (convert value)
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