Den 2017-12-02 kl. 21:56, skrev Joachim Breitner:
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?
Miller's pattern unification is quite useful for rewriting, and the
above example falls within that (all meta-variables are applied only to
object variables). Rewriting based on pattern unification is decidable
and efficient; see this paper:
http://www.cse.chalmers.se/~emax/documents/axelsson2015lightweight.pdf
However, that paper assumes a normalized representation, so I'm not sure
how well it would work in a compiler like GHC.
/ Emil
Joachim
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