I have a problem where a pattern synonym doesn't provide the expected
type refinement in GHC 8.0.1.
{-# LANGUAGE GADTs #-}
{-# LANGUAGE PatternSynonyms #-}
data Exp a
where
Num :: (Eq a, Num a) => a -> Exp a
Add :: (Eq a, Num a) => Exp a -> Exp a -> Exp a
pattern NumP a = Num a
pattern AddP :: (Num a, Eq a) => Exp a -> Exp a -> Exp a
pattern AddP a b = Add a b
simplifyP :: Exp a -> Exp a
simplifyP (AddP a (NumP 0)) = a
simplifyP a = a
This gives the error
• No instance for (Eq a) arising from a pattern
Possible fix:
add (Eq a) to the context of
the type signature for:
simplifyP :: Exp a -> Exp a
• In the pattern: AddP a (NumP 0)
In an equation for ‘simplifyP’: simplifyP (AddP a (NumP 0)) = a
If I remove the type signature for `AddP`, the code goes through.
Unfortunately, in my real code I need the type signature in order to
resolve overloading.
GHC 7.10 didn't have this problem.
Is this a bug?
/ Emil
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