[GHC] #12360: Extend support for binding implicit parameters

[GHC]

#12360: Extend support for binding implicit parameters
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        Reporter:  MichaelBurge      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Iceland_jack: I could not parse your comments.

 But currently GHC does allow this:
 {{{
 data X where
   MkX :: (?x :: Int) => X

 f :: X -> Int -> Int
 f MkX y = ?x + y    -- The pattern match on MkX binds an
                     -- implicit parameter ?x

 g = let ?x = 7 in f MkX 5
   -- Here the MkX needs a ?x constraint, which it gets from
   -- the let-binding.  So g = 12
 }}}
 Function `f` looks a bit odd because it has a use of `?x` but it is far
 from clear where it is bound: you have to look at the captured constraints
 for the pattern-matched constructors.

 But there is nothing technically complicated or unsound about this.

 Pattern synonyms are just sugar on top of this.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12360#comment:6>
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