[GHC] #9953: Pattern synonyms don't work with GADTs

[GHC]

#9953: Pattern synonyms don't work with GADTs
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        Reporter:  simonpj           |                   Owner:  cactus
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:  7.10.1
       Component:  Compiler (Type    |                 Version:  7.10.1-rc1
  checker)                           |                Keywords:
      Resolution:                    |  PatternSynonyms
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by simonpj):

 Well Richard, you are proposing that you can write `P1`'s type signature
 just like `T1`'s.  Fair enough, I can see some merit; indeed it was my
 initial position.  But then you '''must''' write `P2`'s type signature in
 some other way.  What other way do you propose??

 Since we already ''have'' two different ways to write the signature, we
 are at liberty to decide that those two different ways define two
 different behaviours; and that is what I am proposing.  Yes, we could
 invent a third way, but that introduces a whole new raft of complications.
 (E.g. can you use that third way in a GADT declaration?  For an ordinary
 function?  etc)

 I'm totally open to considering alternative.  But until we have one, let's
 get on with implementing the proposal we have.  That is often
 illuminating; and if we come up with a better idea meanwhile we can retro-
 fit it easily enough.

 For data families, I think it should all work fine.  But indeed we should
 check.

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9953#comment:13>
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