[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):

 Replying to [comment:25 cactus]:

 > > 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
 >
 > What if you could just export `pattern P` (where `P` is the name of the
 data constructor) without having to define `AbsP`? Would that be taking
 things too far?

 That would treat pattern synonyms and data constructors differently, which
 I am keen to avoid.  Under what you propose:
  * If P is a data constructor then `pattern P` would export P uni-
 directionally (pattern only)
  * If P is a (bi-directonal) pattern synonym then `pattern P` would export
 P bi-directionally.
 I don't like that... a data constructor is precisely a (degenerate)
 bidirectional pattern.

 I suppose you could put the directionality in the export list:
 {{{
 module M(
   pattern P,    -- Uni-directional
   constructor Q   -- Bidirectional
  ) where

 pattern P x = [x]
 pattern Q y = [y,y]
 }}}
 But I don't want to go this way.  If you export P you get all of what P
 is.  Otherwise we'll get into exporting it unidirectionally from one
 module, bidirectionally from another, then importing both of those, and
 combining the directinoality of those imports. Too complicated.

 Maybe `datacon P`, or `view P`, rather than `pattern P` would address
 Ian's worry?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5144#comment:27>
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