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@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@joachim-breitner.de • https://www.joachim-breitner.de/
XMPP: nomeata@joachim-breitner.de • OpenPGP-Key: 0xF0FBF51F
Debian Developer: nomeata@debian.org

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