[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 sgraf):

 Just hit this with the following program missing `-XBangPatterns` from
 #11284:

 {{{
 module T11284 where

 import Data.Char (isSpace)
 import Data.List (foldl')
 import GHC.Exts (build)
 import qualified  Data.Text as T
 import qualified  Data.Text.Array as A

 longestWord :: T.Text -> Int
 longestWord t = foldl' max 0 $ map T.length $ fusedWords t

 fusedWords :: T.Text -> [T.Text]
 fusedWords t0 = build $ \cons nil ->
   let go !t
         | T.null t  = nil
         | otherwise = let (w, rest) = T.span (not . isSpace) t
                       in cons w (go $ T.dropWhile isSpace rest)
   in go t0

 -- For reference
 data Text = Text
     {-# UNPACK #-} !A.Array          -- payload (Word16 elements)
     {-# UNPACK #-} !Int              -- offset (units of Word16, not Char)
     {-# UNPACK #-} !Int              -- length (units of Word16, not Char)
 }}}

 Complains with

 {{{
 T11284.hs:18:6: error: Variable not in scope: go :: T.Text -> b
    |
 18 |   in go t0
 }}}

 I'd say a warning when there's no space in a binary operator definition
 for `(!)` before its second parameter is the way to go.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13600#comment:12>
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