We've already accepted a similar addition to Data.List, so I see no harm in having an implementation for Data.Sequence. -Edward On Fri, Jun 10, 2016 at 12:45 PM, David Feuer wrote:

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" 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-unconstra...

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.

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries