Pattern synonyms and GADTs in GHC 8.0.1

[Simon Peyton Jones]

GHC 8.0 swaps the order of provided vs required contexts in a pattern synonym signature.  (7.10 was advertised as experimental).  Thus:

pattern AddP :: () => (Num a, Eq a) => Exp a -> Exp a -> Exp a


Then it's fine

Simon

|  -----Original Message-----
|  From: Glasgow-haskell-users [mailto:glasgow-haskell-users-
|  bounces at haskell.org] On Behalf Of Emil Axelsson
|  Sent: 26 May 2016 16:27
|  To: glasgow-haskell-users <glasgow-haskell-users at haskell.org>
|  Subject: Pattern synonyms and GADTs in GHC 8.0.1
|  
|  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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