Hi, Am Samstag, den 02.12.2017, 12:59 -0800 schrieb Conal Elliott:
Thanks for the reply, Ed.
I'd assume that `x` didn't occur in either `u` or `v`
This is exactly the issue I'm wondering about. Since rewrite rules admit lambdas and only first-order matching, I'm wondering whether they're interpreted as you did (and I'd tend to), namely that `x` doesn't occur freely in `u`or `v`, in which case lambdas don't seem useful in rules (and yet were implemented for some reason)
even with these restrictions, they are still useful for rules like map (\x -> x) = id
With a wee bit of higher-order matching, one might make `u` and `v` functions and instead write:
foo (\ x -> fmap (u x) (v x)) = bar u v
In that case I'd expect `u` and `v` to be synthesized rather than literally matched. For instance, `foo (\ (a,b) -> fmap (+ a) [b,b,b])` would match with `u = \ (a,b) -> (+ a)` and `v = \ (a,b) -> [b,b,b]`.
That would be nice and interesting, but we don’t do that now, unfortunately. And of course higher-order pattern matching is quite a can of worms. I implemented it in http://incredible.pm/ but that was a much simpler setting than a complex typed language like Core. Implementing some form of higher-order pattern matching might actually be doable, but would it be reliable? When does it become undecidable? Joachim -- Joachim Breitner mail@joachim-breitner.de http://www.joachim-breitner.de/