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) or they're interpreted as allowing `x` in `u` and `v`, and substitution captures. I'm still puzzled. 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]`. -- Conal On Sat, Dec 2, 2017 at 12:20 PM, Edward Kmett <ekm...@gmail.com> wrote: > I don't knw of a formal writeup anywhere. > > But does that actually mean what you are trying to write? > > With the effective placement of "forall" quantifiers on the outside for u > and v I'd assume that x didn't occur in either u or v. Effectively you have > some scope like forall u v. exists x. ... > > Under that view, the warnings are accurate, and the rewrite is pretty > purely syntactic. > > I don't see how we could write using our current vocabulary that which you > want. > > On Sun, Dec 3, 2017 at 4:50 AM, Conal Elliott <co...@conal.net> wrote: > >> Is there a written explanation and/or examples of rewrite rules involving >> a LHS lambda? Since rule matching is first-order, I'm wondering how terms >> with lambda are matched on the LHS and substituted into on the RHS. For >> instance, I want to restructure a lambda term as follows: >> >> > foo (\ x -> fmap u v) = bar (\ x -> u) (\ x -> v) >> >> My intent is that the terms `u` and `v` may contain `x` and that whatever >> variable name is actually used in a term being rewritten is preserved so as >> to re-capture its occurrences on the RHS. >> >> When I write this sort of rule, I get warnings about `x` being defined >> but not used. >> >> Thanks, -- Conal >> >> _______________________________________________ >> Glasgow-haskell-users mailing list >> Glasgow-haskell-users@haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users >> >> >
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