Extended patterns

[gallais]

Gathering here the proposal to extend the pattern-matching language
(for rational patterns, see #43) as well as interesting examples using
the introduced features.

• intersection patterns p@q matches if p matches and q matches (right-to-left shadowing)

• union patterns p+q matches if either p matches or if it does not and q matches (usually using | but here it would clash with Cell...)

zip([[]|xs] + [xs|[]]) -> xs
zip([[x|xs] | [y|ys]]) -> [[x|y] | zip([xs|ys])

This example is a bit artificial and requires the function to be uncurried... :/

• negation pattern !p matches if p does not match (no bindings)

flatten([])                  -> []
flatten([x@!([]+[_|_]) |xs]) -> [x|flatten(xs)]
flatten([x|xs])              -> append(x,flatten(xs))

[pigworker]

Yes to intersection! Yes to negation! Note that !_ becomes the grumpy pattern.

Union patterns are a tad scarier. It's fun to imagine the type system for them, or even how to scope them. Perhaps p+q binds only those pattern variables which occur in both?

I wonder whether intersection should be & rather than @, or indeed whether /\ and \/ might have some part to play.