[GHC] #14478: Abstract pattern synonyms (for hsig and hs-boot)

[GHC]

#14478: Abstract pattern synonyms (for hsig and hs-boot)
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           Reporter:  ezyang         |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
  (Type checker)                     |
           Keywords:  backpack       |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Most declaration forms (data types, values, type families, etc) support
 forward declaration in hs-boot/hsig files. However, pattern synonyms do
 not. This seems like a major omission!

 Some problems to solve:

 * The obvious syntax `pattern Foo :: pat_ty` is insufficient to specify
 whether or not a pattern is bidirectional or unidirectional. How should
 this be represented syntactically?
 * What is the interaction with bundling?  Should it be possible to export
 a bundle of abstract pattern synonyms, with the intent that this means an
 implementation must also have bundled them together
 * See also #12717; abstract pattern synonym should be implementable with a
 plain old constructor

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14478>
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