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
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@joachim-breitner.de http://www.joachim-breitner.de/
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