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

[GHC]

#13600: surprising error message with bang pattern
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        Reporter:  andrewufrank      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  Poor/confusing    |  (amd64)
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by mpickering):

 You need to turn on the `BangPatterns` extension, otherwise you are
 defining an infix operator `!`. The error message is then accurate as you
 have given a type signature for `f4` but then defined `!`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13600#comment:1>
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