[GHC] #4370: Bring back monad comprehensions

[GHC]

#4370: Bring back monad comprehensions
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    Reporter:  simonpj           |        Owner:  nsch        
        Type:  feature request   |       Status:  new         
    Priority:  normal            |    Milestone:  7.2.1       
   Component:  Compiler          |      Version:  6.12.3      
    Keywords:                    |     Testcase:              
   Blockedby:                    |   Difficulty:              
          Os:  Unknown/Multiple  |     Blocking:              
Architecture:  Unknown/Multiple  |      Failure:  None/Unknown
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Comment(by nsch):

 You should be able to apply these two patches to ghc.git and the base
 (darcs) package.

 The base patch is pretty much final, the compiler patch is almost done
 (some comments still use the old `zipM` name instead of the new `mzip`
 etc), but you should be able to see if you can reproduce the error we were
 talking about.

 However, we didn't manage to test this with the latest HEAD because of
 #4990 and #4991, but it worked fine with the version a couple of days ago…

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4370#comment:42>
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