[swift-evolution] [Accepted and Focused Re-review] SE-0187: Introduce Sequence.filterMap(_:)

Thu Nov 16 11:06:40 CST 2017

I’ll put my vote in for `filterMap`!

I’d like to call out some fun properties and generalizations that
`filterMap` has that might give a little more credence to why it should be
called this.

There is a very natural way of bridging the worlds of optionals and
booleans.   In fact, considering that `Bool` is the unique type with two
elements, we could even define `typealias Bool = Void?`, where `Void()` is
true and `nil` is false! Though I am definitely not advocating that!

Given that perspective, there is then also a natural way of lifting
predicates `(A) -> Bool` to the world of functions that `filterMap`
understands:

```
func optionalBool<A>(_ p: @escaping (A) -> Bool) -> (A) -> A? {
  return { p($0) ? .some($0) : nil }
}
```

Given such a function, then `filter` on arrays is nothing but
`filterMap(optionalBool(p))`!

The really exciting part is then to take note that the highly contested and
dreaded `Either<A, B>` is just a generalization of `Optional<B>`, in that
we model the absence of `B` by providing a value of type `A`. Given that
observation, what does the filter/filterMap story look like for `Either`s?
Why that’s none other than `partition` and `partitionMap`! These are
functions that allow you to partition an array into two subsets. The first
one doesn’t change the wrapped value of the array (much like `filter`) but
the second allows providing an additional mapping function. Here’s how they
might look:

```
func _partitionMap<A, B>(_ p: @escaping (Element) -> Either<A, B>) ->
(left: [A], right: [B]) {
  var result = (left: [A](), right: [B]())
  for x in self {
    switch p(x) {
    case let .left(a):
      result.left.append(a)
    case let .right(b):
      result.right.append(b)
    }
  }
  return result
}

func _partition(_ p: @escaping (Element) -> Bool) -> (`true`: Array,
`false`: Array) {
  // Can easily define `_partition` in terms of `_partitionMap`!
  return _partitionMap(eitherBool(p))
}
```

The things that are nice about this:

* Builds on a few foundational, atomic ideas.
* Shows that there are future generalizations that we might not even be
thinking about right now
* Allows one to see the shadows of simpler constructions (e.g.
filter/partition) from the more complex constructions (e.g.
filterMap/partitionMap)

I will conclude that I 100% am not advocating for us to bring in `Either`
and `optionalBool` and `eitherBool` and all that wildness. I simply want to
show that there are some wonderful ideas lurking behind these names, and so
it’d be nice to not hide them with overly specific and overly descriptive
names.

If anyone wants to play with these functions I have put them in a gist:

https://gist.github.com/mbrandonw/7aab415312379022e60f3a9a107b6792

On Nov 15, 2017, at 12:55 PM, John McCall via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> Hello, Swift Community!
>
> The initial review of "SE-0187: Introduce Sequence.filterMap(_:)" ran
> through yesterday, November 14th, 2017.  The proposal is available here:
>
>
> https://github.com/apple/swift-evolution/blob/master/proposals/0187-introduce-filtermap.md
>
>
> There was a significant amount of discussion, and people came down with
> reasonable arguments both for and against the proposal.  After reviewing
> that feedback, the core team feels that the central question is whether
> Swift benefits from overloading flatMap in this way.  There is a reasonable
> argument that an Optional is a sort of container, and therefore it makes
> sense to "flatten" that container into a surrounding container.  But Swift
> has resisted applying that interpretation in its library design; for
> example, you cannot directly iterate an Optional or append its contents to
> an Array.  In general, we feel that using different operations for working
> with Optionals tends to make code easier to both write and understand,
> especially given the existence of implicit optional promotion, which we
> cannot eliminate or easily suppress based on the context.  On reflection,
> we think it was a mistake to use the same name in the first place, and
> there is no better time to fix a mistake than now.
>
> While we accept that this will cause some amount of "code churn" for
> developers when they adopt Swift 5, the required change is a simple rename
> that should be painless to automatically migrate.  Of course, sample code
> on the internet will become obsolete, but fix-its will easily update that
> code if pasted into a project, and the samples themselves (once corrected)
> should become clearer and easier to teach after this change, as is
> generally true when overloading is removed.
>
> Accordingly, SE-0187 is *accepted*, at least as far as not calling the
> operation "flatMap".  We are re-opening the review until next Monday,
> November 20th, 2017, in order to have a focused discussion about the new
> name.  Names that seemed to gain some traction in the first review include:
>
>   - filterMap, which has precedent in existing functional languages, as
> well as some popular Swift libraries, but which some people view as
> confusing
>
>   - compactMap, which builds off the precedent of "compact" in Ruby
>
> But please feel free to suggest a name other than these.
>
> *Reviews*
>
> Reviews are an important part of the Swift evolution process.  All reviews
> should be sent to the swift-evolution mailing list at
>
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
>
> or, if you would like to keep your feedback private, directly to me as the
> review manager.  When replying, please try to keep the proposal link at the
> top of the message:
>
> Proposal link:
>
>
> https://github.com/apple/swift-evolution/blob/master/proposals/0187-introduce-filtermap.md
>
> Reply text
>
> Other replies
>
> *What goes into a review?*
>
> The goal of the review process is to improve the proposal under review
> through constructive criticism and, eventually, determine the direction of
> Swift.
>
> When writing your review, here are some questions you might want to answer
> in your review:
>
> • What is your evaluation of the proposal?
> • Is the problem being addressed significant enough to warrant a change to
> Swift?
> • Does this proposal fit well with the feel and direction of Swift?
> • If you have used other languages or libraries with a similar feature,
> how do you feel that this proposal compares to those?
> • How much effort did you put into your review? A glance, a quick reading,
> or an in-depth study?
>
> More information about the Swift evolution process is available at:
>
> https://github.com/apple/swift-evolution/blob/master/process.md
>
>
> As always, thank you for contributing to the evolution of Swift.
>
> John McCall
> Review Manager
>
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