[GHC] #9330: Introduce shortcut for "on (==)" much like there's a shortcut for "on compare".

[GHC]

#9330: Introduce shortcut for "on (==)" much like there's a shortcut for "on
compare".
-------------------------------------+-------------------------------------
        Reporter:  frerich           |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  libraries/base    |                 Version:  7.8.2
        Keywords:                    |  Differential Revisions:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
       Test Case:                    |              Difficulty:  Unknown
        Blocking:                    |              Blocked By:
                                     |         Related Tickets:
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 A common use case for `on` (from `Data.Function`) is to use it with
 `compare`, e.g. {{{compare `on` snd}}}. In fact, this pattern is so common
 that there's a convenient `comparing` function which provides a shortcut
 for this use case such that one can write

 {{{
 #!haskell
 sortBy (comparing snd)
 }}}

 instead of

 {{{
 #!haskell
 sortBy (compare `on` snd)
 }}}

 I think another common use case is to use `on` together with `(==)` as in

 {{{
 #!haskell
 groupBy ((==) `on` snd)
 }}}

 In a similiar vein as with `comparing`, I think it would be nice if there
 was a function which encapsulates this use case, like

 {{{
 #!haskell
 equating :: Eq b => (a -> b) -> a -> a -> Bool
 equating = on (==)
 }}}

 such that one can write

 {{{
 #!haskell
 groupBy (equating snd)
 }}}

 The (IMHO fairly nice) name `equating` is not my idea but was suggested by
 //ClaudiusMaximus// of #haskell fame.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9330>
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