[GHC] #5144: Pattern synonyms

[GHC]

#5144: Pattern synonyms
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        Reporter:  simonpj           |            Owner:  cactus
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:  _|_
       Component:  Compiler          |          Version:
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by simonpj):

 We must treat data constructors uniformly with pattern synonyms.  For
 example, it should definitely be OK to have
 {{{
 module M( pattern P, foo, bar ) where
   -- The "pattern P" exports data constructor P

    data T = P | Q

    foo :: T
    foo = P

    bar :: T -> Int
    bar P = 1
    bar Q = 2

 module A where
 import M
 -- T is not in scope
 w1 = bar P   -- Uses P as an expression
 w2 = case foo of
        P -> 1   -- Uses P as a pattern
 }}}
 We have at various times discussed how to allow a module to export a data
 constructor that can be used as a ''pattern'' but not as a ''constructor''
 (in expressions), because the data type is abstract and you should only
 use a smart constructor.  With pattern synonyms you can (almost) do that:
 {{{
 module M( T, pattern AbsP, pattern Q ) where
   data T = P Int | Q
   pattern AbsP = P
 }}}
 But I really wanted `AbsP` to be uni-directional and according to the
 current spec [wiki:PatternSynonyms] it is "implicitly bidirectional".  I
 suggest we allow the user to control whether a pattern synonym is
 bidirectional or not.  I'm not sure what syntax to use for that.  We want
 to be able to say
  * Uni-directional pattern synonym
  * Bi-directional pattern synonym
  * Bi-directional pattern synonym with explicit user code for the reverse
 direction

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5144#comment:23>
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