Proposal: Add `restriction` to Data.Map and Data.IntMap

[Edward Kmett]

Given the fragility of rewrite rules (and the fact that they don't happen
at all at the default optimization level) that seems too delicate of a
solution to endorse.

That doesn't prevent us from adding the rewrite rules to try to help
existing code that already uses filter (`S.member` set), in addition to
these combinators, though. That idiom is already rather common given the
lack of an alternative to date and improving its performance wouldn't hurt.

-Edward

On Mon, Jul 25, 2016 at 6:05 PM, Joachim Breitner <mail at joachim-breitner.de>
wrote:

> Hi,
>
> Am Montag, den 25.07.2016, 14:59 -0400 schrieb David Feuer:
> > Both `restrictKeys` and `withoutKeys` are special cases of
> > `filterWithKey` (although they should be considerably more efficient
> > than implementations using that function).
>
> I’m risking to derailing this discussion, but how viable would it be to
> *not* add a new exported name for this, but recommend (e.g. in the
> docs) to use
>
>     filter (`S.elem` set) map
>
> and then rely on rewrite rules to get the desired performance effect
> (by rewriting this expression to an non-exported restrictKeys)?
>
> I guess the answer is: Not viable enough, because rewrite rules are not
> applied reliably enough, and because you cannot easily have the effect
> of a partial "map `restrictKeys`". But nevertheless it is an
> interesting idea to imagine a programming language where APIs can be
> minimal, modular and composable without performance penalties.
>
> Greetings,
> Joachim
>
>
>
>
> --
> Joachim “nomeata” Breitner
>   mail at joachim-breitner.de • https://www.joachim-breitner.de/
>   XMPP: nomeata at joachim-breitner.de • OpenPGP-Key: 0xF0FBF51F
>   Debian Developer: nomeata at debian.org
>
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