[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
           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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 the following code

 {{{
 f3 :: [Int] -> IO Int
 f3 x = return (sum x)

 f4 :: [Int] -> IO Int
 f4 !x = return (sum x)
 }}}

 gives `The type signature for ‘f4’ lacks an accompanying binding` for the
 second function (the two functions are exactly the same except for the
 added bang). I do not understand bang patterns well, but would expect a
 more instructive error message. without the type definition, the second
 function f4 compiles with the bang).

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