[GHC] #13600: surprising error message with bang pattern

[GHC]

#13600: surprising error message with bang pattern
-------------------------------------+-------------------------------------
        Reporter:  andrewufrank      |                Owner:  v0d1ch
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.0.2
      Resolution:  fixed             |             Keywords:  BangPatterns,
                                     |  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Poor/confusing    |            Test Case:  T13600a,
  error message                      |  T13600b
      Blocked By:                    |             Blocking:
 Related Tickets:  #15166, #15458    |  Differential Rev(s):  Phab:D5040
       Wiki Page:                    |
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Comment (by simonpj):

 My apologies -- I should have looked at the Phab earlier.

 Would it be possible to add a Note to explain what is happening in the
 code, perhaps near `Passer.warnSpaceAfterBang`.

 I believe that the specification is this:
 --------
 GHC warns if it sees

 * A infix definition of `(!)`
 * that lacks a space before the `!` operator

 That is, something like
 {{{
 x !p = e
 }}}
 where `p` is a pattern.

 If `BangPatterns` are on, this example would be treated as a definition of
 the function `x` applied the to the bang-pattern `!p`.

 If there is a space after the bang, thus
 {{{
 x ! p = e
 }}}
 then the definition is treated as a definition of `(!)` (without
 `BangPatterns`) or as `x (!p) = e` (with `BangPatterns`), with no warning
 in either case.
 ------------
 Is that the correct specification?  If so, it'd be good to explain this in
 the user manual, and in the Note in the code.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13600#comment:20>
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