Add sortOn function to Data.List

The following passed for two weeks on the libraries mailing list without any dissenting votes.

The request is to add the following function to Data.List:

-- | Sort a list by comparing the results of a key function applied to each
--   element.  @sortOn f@ is equivalent to @sortBy . comparing f@, but has the
--   performance advantage of only evaluating @f@ once for each element in the
--   input list.  This is called the decorate-sort-undecorate paradigm, or
--   Schwartzian transform.
sortOn :: Ord b => (a -> b) -> [a] -> [a]
sortOn f = map snd . sortBy (comparing fst)
                   . map (\x -> let y = f x in y `seq` (y, x))

Trac metadata

Trac field	Value
Version	7.8.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
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CC	hvr
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