Request: Add sortOn to Data.Sequence

[David Feuer]

I think thIs is an interesting idea, generally. There seem to be two
situations that may call for separate functions. If the "key" type can be
stored in an unboxed mutable array, then it seems likely desirable to sort
using such an array. Otherwise, we can use the existing sortBy as suggested.
On Jun 10, 2016 12:30 PM, "Alan Isaac" <alan.isaac at gmail.com> wrote:

REQUEST
-------

The request is to add `sortOn` to Data.Sequence.
`sortOn f` would be equivalent to `sortBy . comparing f`,
but would offer a performance advantage by using a Schwartzian
transform to only evaluate `f` once for each element
in the input sequence.


BACKGROUND
----------

Adding `sortOn` to `Data.List` was proposed at least as early as 2008:
http://www.haskell.org/pipermail/libraries/2008-October/010797.html
It was added in 2014, in response to this feature request:
https://ghc.haskell.org/trac/ghc/ticket/9004


RATIONALES
----------

- convenience
- performance: faster than `sortBy`
- safety: encourages (although does not require) reliance on existing
instances of Ord
- interface consistency (specifically, when comparing to Data.List)

Comment: it is certainly possible to pick at each of these rationales.
See the discussion here:
http://stackoverflow.com/questions/37691898/why-is-sortby-so-oddly-unconstrained-and-why-no-general-sorton/37735121


SCOPE
-----

Introduces no new dependencies except possibly on `Data.Ord.comparing`.


SOLUTIONS
-----------

The implementation in Data.List is::

        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))

An analogous implementation is possible for Data.Sequence.
However, this request does not propose a specific implementation.


WHAT MIGHT BREAK
----------------

No anticipated breakage.

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