[swift-evolution] SE-0187: Introduce Sequence.filterMap(_:)

[BJ Homer]

> On Nov 9, 2017, at 9:37 AM, Sam Warner via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I accept the motivation for this change, although I would mention that:
> - in 2.5 years on a ~200k lines of Swift project
> - we've seen a plenty of instances of `flatMap` used where `map` would have been sufficient, but
> - we've never burned time on tracking down the sort of compiler issue described by the author in the Motivation. As I say, the argument is fair, but I am questioning how frequently this problem occurs.
> 
> I can't speak to the previous attempts to solve this problem, but I'd add my voice to anyone advocating solving this by having a compiler warning when `flatMap` is used redundantly(?).
> 
> If this proposal were to be accepted, I'd also question the choice of `filterMap` as a name. This name is presumably shorthand for `filterNonesAndMap`, which conveniently ignores the following:
> - `flatMap` may be used on a sequence-of-sequences to flatten our to a single-dimensional sequence.
> - `flatMap` may be used on other 'box' types, e.g. on an optional: `Optional<String>("foo").flatMap { .some($0 + "bar") }` or on the common `Result` type.
> 
> Any re-naming of one `flatMap` should take these other uses into account, as they share conceptual details.
> 
> Thanks,
> 
> Sam

This proposal only proposes renaming the “filterNonesAndMap” variant (though that name is misleading, it would be “filterNonesAfterMapping” if anything); the other uses of flatMap will remain unchanged.

-BJ