[GHC] #4983: Warning about redundant import is wrong when hiding identifiers in order to avoid export ambiguities

[GHC]

#4983: Warning about redundant import is wrong when hiding identifiers in order to
avoid export ambiguities
---------------------------------+------------------------------------------
    Reporter:  Lemming           |       Owner:                                   
        Type:  bug               |      Status:  new                              
    Priority:  normal            |   Component:  Compiler                         
     Version:  7.0.1             |    Keywords:                                   
    Testcase:                    |   Blockedby:                                   
          Os:  Unknown/Multiple  |    Blocking:                                   
Architecture:  Unknown/Multiple  |     Failure:  Incorrect warning at compile-time
---------------------------------+------------------------------------------
 Although warnings about redundant imports are almost perfect since
 GHC-6.12, I have a corner case, where the warning about a redundant import
 is wrong.
 Consider the module
 {{{
 module Data.Monoid.HT (cycle, ) where

 import Data.Monoid (Monoid, mappend, )
 import Data.Function (fix, )

 import Prelude hiding (cycle, )

 {- |
 Generalization of 'Data.List.cycle' to any monoid.
 -}
 cycle :: Monoid m => m -> m
 cycle x =
    fix (mappend x)
 }}}

 GHC says:
 {{{
 src/Data/Monoid/HT.hs:6:1:
     Warning: The import of `Prelude' is redundant
                except perhaps to import instances from `Prelude'
              To import instances alone, use: import Prelude()
 }}}

 I have to hide 'cycle' from Prelude if I want to export it unqualified.

 I have several work-arounds: Export 'cycle' with qualification, use no
 export list at all, import from Prelude by enumeration of needed
 functions. I think this problem has low priority. I report it just for the
 case that someone claims that Haskell's module system is simple and could
 be more complicated. ;-)

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4983>
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