[GHC] #10928: Refine pattern synonym signatures

[GHC]

#10928: Refine pattern synonym signatures
-------------------------------------+-------------------------------------
        Reporter:  mpickering        |                Owner:
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by goldfire):

 While we're changing the parser, we should make sure that explicit
 quantification is allowed. For example,

 {{{
 pattern P :: forall a b. ... => forall c d. ... => ....
 }}}

 should introduce `a` and `b` as universals, and `c` and `d` as
 existentials. The existentials should scope over the provided constraints
 and the arguments, but not the result. The universals scope over the whole
 shebang.

 These should also be made available as scoped type variables in the
 pattern definition. There are four places where these variables might be
 in scope, labeled below:

 {{{
 pattern P = (1)
 pattern P <- (2)
 pattern P (3) ...
  ... where P = (4)
 }}}

 The universals should be in scope everywhere. The existentials should be
 in scope only in `(3)` and `(4)`, I believe, but I'm really quite unsure.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10928#comment:26>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler