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{-# LANGUAGE ScopedTypeVariables #-}
-- !!! Error messages with scoped type variables
module Foo where
data Set a = Set a
unionSetB :: Eq a => Set a -> Set a -> Set a
unionSetB (s1 :: Set a) s2 = unionSets s1 s2
where
unionSets :: Eq a => Set a -> Set a -> Set a
unionSets a b = a
{- In GHC 4.04 this gave the terrible message:
None of the type variable(s) in the constraint `Eq a'
appears in the type `Set a -> Set a -> Set a'
In the type signature for `unionSets'
-}
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