[GHC] #3524: Add mfilter to Control.Monad

[GHC]

#3524: Add mfilter to Control.Monad
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Reporter:  JonFairbairn      |          Owner:                  
    Type:  proposal          |         Status:  new             
Priority:  normal            |      Component:  libraries/base  
 Version:  6.10.4            |       Severity:  normal          
Keywords:                    |       Testcase:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
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 In the comments of #3453, I contemplated a filter function on MonadPlus.
 The consensus seems to be that this function is an appropriate addition.

 mfilter is just List.filter generalised to any MonadPlus.

 It must have been embodied in Monad comprehensions when we had those, so
 I'm sure it's simply an oversight that it hasn't been included here
 already.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3524>
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