[GHC] #10653: PatternSynonyms should be imported/exported as part of the wildcard notation

[GHC]

#10653: PatternSynonyms should be imported/exported as part of the wildcard
notation
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        Reporter:  gridaphobe        |                   Owner:  mpickering
            Type:  feature request   |                  Status:  new
        Priority:  high              |               Milestone:  8.0.1
       Component:  Compiler          |                 Version:  7.11
      Resolution:                    |                Keywords:  pattern
                                     |  synonyms
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by simonpj):

 Thanks for the wiki page.  But before implementing anything, can you make
 it a proposed ''specification'' rather than a general idea illustrated by
 an example or two?

 Notably:
  * When, precisely, is it OK to export `T( P )`, where `P` is a pattern
 synonym?
  * What, precisely, is exported by `T(..)`?

 To give you the idea, the
 [https://www.haskell.org/onlinereport/haskell2010/haskellch5.html#x11-1000005.2
 current spec is here].

 I think it's important that one can answer these questions without doing
 type checking.  (In implementation terms, the renamer has to answer these
 questions, and typechecking has not happened yet.)

 I think your answers will say something like this. Given
 {{{
 pattern P xs = C p1 p2
 }}}
 where `C` is a data constructor, then `T( P )` is a valid export item iff
 (a) `C` is a data constructor from data type `T`, and (b) `P` is in scope.
 It does not matter whether or not `C` is in scope (correct?).

 But it gets more complicated if `C` is itself a pattern synonym; or,
 worse, if it is a view pattern.  Maybe it's driven off the pattern
 signature, if there is one?

 You might find it helpful first to define the notion of "a pattern synonym
 `P` ''belongs to'' a data type `T`".

 Anyway, a specification is badly needed!

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10653#comment:20>
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