Add a FilterMap operator

[Kixiron]

This'd be a nice ergonomic tweak, an operator in the style of Iterator::filter_map() would be really great. Currently there's a somewhat strange workaround using .flat_map(), e.g.

stream.flat_map(|foo| {
    if foo.bar() {
        Some(foo.baz)
    } else {
        None
    }
})

and while this would functionally be nearly identical, the FilterMap operator could likely specialize somewhat since it only has to handle a single output value

[frankmcsherry]

I think filter_map is just flat_map, isn't it? The latter takes something that implements IntoIterator<Item=T> and Option<T> implements that trait.

[ryzhyk]

Heh, sorry about my (deleted) posts, I'm used to seeing your messages in the DDlog repo :)

But yeah, I use flat_map to express filter_map in DD.

[Kixiron]

Implementation-wise yes, filter_map and flat_map are equivalent. However, they do have the semantic difference of "flattening an expanding collection" vs. "removing some items from a collection". It's a very slight thing that tripped me up until I discovered the flat_map pattern, it works but it feels like a slight misuse of impl Iterator for Option, almost like writing for x in Some(thing) {}