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{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleContexts #-}
module Bug where
class C a b | a -> b where
op :: a -> b -> ()
f x = op True x
{- We could accept this, quantifying over a C Bool b constraint. But this is a
bit silly, actually, because the b is fixed by the fundep. We don't know what
it's fix to, but it's definitely fixed. So, in the end, we choose not to
Henry Ford polymorphism ("it works for any b as long as b is ???") and not
to quantify. Users can quantify manually if they want.
-}
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