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

[GHC]

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

 It's not just a parsing thing.  That definition of `go` really might be
 defining `(!)`, as if you'd written
 {{{
   let (!) go t
          | T.null t = ...
          | otherwise = ...
 }}}
 We could simply discourage infix definitions of `(!)` (which is used for
 array indexing anyway), by issuing a warning and asking the user to write
 their definition prefix (as I have done above) to avoid the potential
 ambiguity with bang-patterns.

 Or, maybe only warn if there is no space between the `!` and the argument
 (comment:2).

 I dunno. This is user-facing, so a GHC proposal (a very small one!) might
 be indicated.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13600#comment:14>
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