[GHC] #9241: Add a"same" function to Data.Eq

[GHC]

#9241: Add a"same" function to Data.Eq
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       Reporter:  mhwombat          |             Owner:
           Type:  feature request   |            Status:  new
       Priority:  low               |         Milestone:
      Component:  Prelude           |           Version:  7.8.2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 Add a"same" function to Data.Eq that parallels the "comparing" function in
 Data.Ord. For example:

 -- |
 -- > same p x y = (p x) == (p y)
 --
 -- Useful combinator for use in conjunction with the @xxxBy@ family
 -- of functions from "Data.List", for example:
 --
 -- >   ... groupBy (same fst) ...
 same :: (Eq a) => (b -> a) -> b -> b -> Bool
 same p x y = (p x) == (p y)

 For a closer parallel, I suppose "same" could return "Equality" (similar
 to "Ordering"), and the functions "nubBy", "deleteBy", "deleteFirstsBy",
 "unionBy", "intersectBy" and "groupBy" should take (a -> a -> Equality) (a
 -> a -> Bool). But I don't know if there's any benefit to that.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9241>
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