[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 simonpj):

 I was sceptical too.  GHC's general convention is to require an extension
 at the ''definition'' site of something, but not its ''use'' site. For
 example, you can use a kind-polymorphic type constructor without
 `PolyKinds`; and you can use overlapping instances without
 `OverlappingInstances`.

 Really the only justification for not following this convention is if it's
 going to be hard for the user to understand what's going on.  For example,
 a uni-directional pattern synonym can be used in a pattern but not an
 expression, and that would be puzzling if you had no idea that `K` is a
 pattern synonym.  Also pattern matching on a pattern synonym might be much
 more expensive than pattern matching on a data constructor.  And that kind
 of thing was the justification for #2905.

 FWIW, my own instinct is ''not'' to require `PatternSynonyms` at the use
 site; but I didn't feel strongly about it to reverse the choice.  And it's
 easier to lift the restriction later than to add it.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8961#comment:11>
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