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

[Simon Peyton Jones]

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/
>
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