[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:2 goldfire]:
 > Right now, GHC works the other way, accepting this:
 >
 > {{{#!hs
 > foo :: Int -> Int
 > foo x
 >   | should_add = x + 1
 >   | otherwise  = x - 1
 >   where
 >     should_add = x > 0
 > }}}
 >
 > I've not checked to see whether this is specified in the Report, but my
 guess is that it is.
 >
 > I'm closing as invalid, but do reopen if I've misunderstood.

 I am not sure whether we are on the same page. In your example
 `should_add` is a boolean guard, it does not establish a new binding.

 In my example `b` is a pattern variable bound by the pattern guard. Your
 example does not contain such a thing. Your `should_add` use is morally
 equivalent to my `return` (and `a`) use.

 Do you still think this ticket is invalid?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12188#comment:3>
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