[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 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.

 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:4>
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