[GHC] #13000: minor doc bug about list fusion in GHC user's guide

[GHC]

#13000: minor doc bug about list fusion in GHC user's guide
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           Reporter:  George         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  low            |         Milestone:
          Component:  Documentation  |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Documentation
  Unknown/Multiple                   |  bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 First, the Users Guide (8.0.1.20161117) in section 10.32.6,List fusion,
 should mention length as a good consumer.

 Secondly, it says: "If you want to write your own good consumers or
 producers, look at the Prelude
 definitions of the above functions to see how to do so." However if you go
 to https://hackage.haskell.org/package/base-4.9.0.0/docs/Prelude.html and
 look at the source code for length you end up at
 https://hackage.haskell.org/package/base-4.9.0.0/docs/src/Data.Foldable.html#length
 which is not a good consumer. I think the User's Guide should be changed
 to replace "Prelude" with
 "https://hackage.haskell.org/package/base-4.9.0.0/docs/src/GHC.List.html".

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13000>
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