[GHC] #12187: Clarify the scoping of existentials for pattern synonym signatures

[GHC]

#12187: Clarify the scoping of existentials for pattern synonym signatures
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        Reporter:  mpickering        |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12108            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 I don't think it's complicated.
 {{{
 qualifier ::= forall tv1 .. tvn.
            |  forall tv1 .. tvn. ctxt =>
            | ctxt =>

 body ::= btype -> body
       |  type   -- without an arrow at the top

 pat_ty ::= qualifier qualifier body   -- Univ and ext
          | qualifier body             -- Univ only
          | body
 }}}
 A qualifier has a `forall` or `=>` or both.  A pattern type has zero, one,
 or two qualifiers.

 Needless to say I'm open to better ideas.  But we need SOME way to specify
 it.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12187#comment:9>
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