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{-# LANGUAGE MultiParamTypeClasses #-}
-- !!! Multi-param type classes test: ambiguity bug
-- GHC actually accepts this program because
-- q :: Collection c a => a -> Bool
-- and there is no a priori reason to suppose that
-- q would be ambiguous in every context. For example,
-- it could be fine if we had
-- instance c Int where ...
-- Of course, it'd be hard to fill in the "..." in this particular
-- case, but that relies on observations about the form of the types
-- of the class methods, surely beyond what a compiler should do.
-- That's why GHC accepts it
module ShouldFail where
class Collection c a where
empty :: c a
add :: a -> c a -> c a
isempty :: c a -> Bool
singleton x = add x empty
q x = isempty (singleton x)
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