[GHC] #13042: Allow type annotations / visible type application in pattern synonyms

29 Dec 2016


      #13042: Allow type annotations / visible type application in pattern synonyms
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
  TypeApplications, PatternSynonyms  |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 data Lift f a = Pure a | Other (f a)
 data V0 a
 }}}

 To make a pattern synonym for `a -> Lift V0 a` I would like to be able to
 write either

 {{{#!hs
 pattern A a = Pure @V0 a
 }}}

 or

 {{{#!hs
 pattern B a = (Pure :: _ -> _ V0 _) a
 }}}

 to be equivalent to

 {{{#!hs
 pattern C :: a -> Lift V0 a
 pattern C a = Pure a
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13042
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