Function to add to Data.List

[Henning Thielemann]

Am 06.04.2014 00:27, schrieb Niklas Haas:
> On Sat, 05 Apr 2014 17:53:04 -0400, John Wiegley <johnw at fpcomplete.com> wrote:
>> I heard a talk that mentioned this transform today at Hac NYC, and was
>> surprised it wasn't already in Data.List:
>>
>>      -- | Sort a list using a key on each element.  This implements the
>>      --   decorate-sort-undecorate paradigm, also called a Schwarzian transform.
>>      sortByKey :: Ord b => (a -> b) -> [a] -> [a]
>>      sortByKey f = map snd . sortBy (comparing fst) . map (\x -> (f x, x))
>>
>> I would like to propose adding it.
>>
>> John
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>
> This already exists in the form of
>
> sortBy . comparing
>
> or
>
> sortBy (comparing f)

Not exactly, because sortByKey memorizes the results of 'f' and (sortBy 
. comparing) recomputes 'f' all the time.