[GHC] #8961: Require PatternSynonyms language extension to *use* pattern synonyms

[GHC]

#8961: Require PatternSynonyms language extension to *use* pattern synonyms
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        Reporter:  cactus            |            Owner:  cactus
            Type:  bug               |           Status:  closed
        Priority:  normal            |        Milestone:  7.8.1
       Component:  Compiler (Type    |          Version:  7.8.1-rc2
  checker)                           |         Keywords:
      Resolution:  fixed             |     Architecture:  Unknown/Multiple
Operating System:  Unknown/Multiple  |       Difficulty:  Easy (less than 1
 Type of failure:  None/Unknown      |  hour)
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by tibbe):

 I think it's very attractive to allow the change to be transparent to the
 user, as Isaac said. Imagine we want to change `String` to be an opaque
 type with a different representation. We could use a pattern synonym (and
 expression synonym?) to allow users who use `(:)` to be none the wiser. I
 admit that I haven't thought about all the implications though. Would it
 really be transparent to the user if a real constructor or a synonym is
 used?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8961#comment:10>
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