[ghc-steering-committee] #555: Higher Order Patterns in Rewrite Rules, rec accept

[Arnaud Spiwack]

Hello all, I'm back for real this time,

The proposal (link https://github.com/ghc-proposals/ghc-proposals/pull/555
) is quite reasonable. I think that, in general, higher order patterns are
an uncontroversial extension to rewrite rules (it could be extended further
but if I remember correctly, there are choices there).

I've got a question though: is the following rewrite rule something I can
write in Haskell today?

```
{- RULES … forall f g. map (\x -> f (g x)) = map f . map g
```

In this rule `f (g x)` is not a pattern (because `g` is not rigid). And it
is not clear to me how purely syntactic unification and pattern unification
interact.

I think the automatic addition of lambdas is likely to be fairly innocuous
(despite the fact that it may change `undefined` into, say, `\x y ->
undefined y x`): it happens under lambdas already, so it's unlikely to
effectively change the behaviour of program unbeknownst to the rule author.
I'm sure someone will eventually manage to be bitten by it. But I think
that it's an ok compromise.

/Arnaud

On Thu, 19 Jan 2023 at 15:51, Chris Dornan <chris at chrisdornan.com> wrote:

> I am sorry for not getting back sooner Joachim,
>
> I agree, rewrite rules are cool and this is clearly a useful
> generalisation.
>
> I vote in favour.
>
> A small suggestion when crafting instructions to us -- do not under any
> circumstances, ever, give us the option of doing nothing! You will be taken
> up on it (which is a sad reflection os us, not you). :-)
>
> Chris
>
> On 2023-01-19, at 09:10, Simon Peyton Jones <simon.peytonjones at gmail.com>
> wrote:
>
> Dear GHC Steering Committee
>
> Joachim wrote to us ten days ago recommending acceptance of #555.  No one
> has responded.
>
> Would you like to respond, please?  (I think this is an easy one.)
>
> Thanks
>
> Simon
>
> On Tue, 10 Jan 2023 at 11:17, Joachim Breitner <mail at joachim-breitner.de>
> wrote:
>
>> Dear Committee,
>>
>> Jaro Reinders and Simon PJ propose to allow  Higher Order Patterns in
>> Rewrite
>> Rules.
>>
>> The idea, by way of an example,
>>
>>    {-# RULES forall f.  foo  (\y. f y + f y) = bar f #-}
>>
>> will now not only match "foo (\y. negate y + negate y)" (with f set to
>> negate)
>> but also "foo (\y. y*y + y*y)" (with f set to (\x. x*x)).
>>
>> Here "f y" is a higher-order pattern, which are restricted to a
>> _pattern_ variable followed by a list of _local_ variable, indicating
>> which variable the matched expression may depend on (previously, only
>> closed expressions could be matched).
>>
>> An implementation is sitting ready at
>> https://gitlab.haskell.org/ghc/ghc/-/merge_requests/9343
>>
>> The design was carefully crafted to be backward-compatible and not
>> introduce spurious etwa-expansion where there was non before.
>>
>> It is not guarded by a LANGUAGE pragma (but RULES themselves are not).
>> Library authors who care about backward compat will have to deal with
>> CPP pragmas.
>>
>>
>> I’m a big fan of rewrite rules, and the proposal is straight forward
>> and provides a feature that I'd maybe optimistically already assumed to
>> be there already. Therefore, I’m recommending acceptance.
>>
>> If you disagree please speak up within two weeks, or speed up the
>> process by indicating agreement earlier.
>>
>> Cheers,
>> Joachim
>>
>> --
>> Joachim Breitner
>>   mail at joachim-breitner.de
>>   http://www.joachim-breitner.de/
>>
>> _______________________________________________
>> ghc-steering-committee mailing list
>> ghc-steering-committee at haskell.org
>> https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee
>>
> _______________________________________________
> ghc-steering-committee mailing list
> ghc-steering-committee at haskell.org
> https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee
>
>
> _______________________________________________
> ghc-steering-committee mailing list
> ghc-steering-committee at haskell.org
> https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-steering-committee/attachments/20230119/8c57c15a/attachment-0001.html>