[GHC] #12188: Pattern variables bound by PatternGuards are not accessible in where clause

[GHC]

#12188: Pattern variables bound by PatternGuards are not accessible in where clause
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        Reporter:  heisenbug         |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:  invalid           |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by heisenbug):

 Replying to [comment:4 simonpj]:
 > Richard is right.  In Haskell
 >
 > * The patterns of the function definition scope over guards, where
 clause, and body
 > * The where clause scopes over the guards, and body.
 > * The guards scope over the body only
 >
 > Once could define the language differently, but that's the way it is in
 Haskell.

 Oh, pity. Surely those rules were set in stone when pattern guards weren't
 a thing yet. Thanks for the `let` tip, I'll look into it and see how it
 helps in my case.

 >
 > You can include let-binding in the guards, if you want:
 > {{{
 > foo :: Int -> Bool
 > foo a | Just b <- return a
 >       , let c = a == b
 >       = c
 > }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12188#comment:5>
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