[GHC] #9538: unwords does not participate in list fusion

[GHC]

#9538: unwords does not participate in list fusion
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       Reporter:  dfeuer             |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.2
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 I'm not quite sure how far this can go, but `unwords` should almost
 certainly be a good consumer or a good consumer. Currently it is neither.
 Making it a good consumer is trivial, given `concatMap` that fuses:

 {{{#!hs
 unwords = tail . concatMap (' ':)
 }}}

 Alternatively, we should be able to make it a good producer without too
 much trouble, I think. I have my doubts about making it both.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9538>
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