Re: [GHC] #14395: Redefining pattern synonym in GHCi triggers "‘p’ is untouchable" error

[GHC]

#14395: Redefining pattern synonym in GHCi triggers "‘p’ is untouchable" error
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |             Keywords:
      Resolution:                    |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 The error message is indeed confusing, but I think what is happening is
 this

 *  You load `Bug.hs`

 * But the interactive context does not have `-XPatternSynonyms`

 * So when you say
 {{{
 pattern Bar = Nothing
 }}}
   GHCi doesn't see `pattern` as a keyword; it sees it as a perfectly
 ordinary identifier. So its' as if you had written
 {{{
 f Bar = Nothing
 }}}
   to define function `f`.

 * That's why it says, for example
 {{{
                    in an equation for ‘pattern’
 }}}
   Quite right!  This is a function definition for function `pattern`.

 It would be cool if it said "Perhaps you meant to use `PatternSynonuyms`"
 but I don't really see where to put such a test.

 Anyway I think it's a case of: not a bug, just a confusing error message.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14395#comment:1>
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