add filterer-lens

[AlexKnauth]

> (lens-view (filterer-lens number?) '(1 a 2 b c 3 d e))
'(1 2 3)
> (lens-set (filterer-lens number?) '(1 a 2 b c 3 d e) '(4 5 6 7))
'(4 5 6 7 a b c d e)

[AlexKnauth]

P.S. I'm not sure: will this filterer-lens always follow the lens laws? (assuming a well-behaved predicate)

[AlexKnauth]

No it won't, because it messes with the order, (lens-set (filterer-lens number?) '(a 1 2 3) (lens-view (filterer-lens number?) '(a 1 2 3))) = '(1 2 3 a), but I meant when order doesn't matter, does it follow them?

[jackfirth]

What if only operated on sets? Then I think it would obey the laws. We'd need set lenses though.

[AlexKnauth]

I'm already working on a version for sets.

[jackfirth]

Given that on lists filter lenses breaks the lens laws, I think we should only make lens constructors for filter/filter-not/partition on sets. It seems counter-intuitive to do it with lists, as with sets lens let you change a subset matching a property but on lists that doesn't feel like it would be a sensible use case.

[jackfirth]

Re: partition-lens. Not sure if that would be ugly to use, since lenses can't have multi-value views.

[AlexKnauth]

Ok I'll remove the list ones then.

[AlexKnauth]

The set version is now in #161 .