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

[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 at joachim-breitner.de
  http://www.joachim-breitner.de/