[swift-evolution] [draft] Introduce Sequence.filteredMap(_:)

[Nobuo Saito]

Hi there,

In my opinion, `map` vs `flatMap` confusing comes from implicitly casting
`T` to `T?`.
I believe it make many confusing and problems that compiler allow this
implicitly casting.
1 way of solve this topic that show warning if use implicitly casting `T`
to `T?`. Of course it should be explict casting can avoid this warning.
Such as `x as Int?`, `Int?.some(x)`, `intClosure as (() -> Int?)`.

2017-10-24 7:15 GMT+09:00 Max Moiseev via swift-evolution <
swift-evolution at swift.org>:

> Hi swift-evolution!
>
> I would like to propose the following change to the standard library:
>
> deprecate `Sequence.flatMap<U>(_: (Element) -> U?) -> [U]` and make this
> functionality available under a new name `Sequence.filteredMap(_:)`.
>
> The draft is available at https://gist.github.com/moi
> seev/2f36376c8ef4c2b1273cff0bfd9c3b95 and is included below for your
> convenience.
>
> Max
>
> Introduce Sequence.filteredMap(_:)
>
>    - Proposal: SE-NNNN <https://gist.github.com/moiseev/NNNN-filename.md>
>    - Authors: Max Moiseev <https://github.com/moiseev>
>    - Review Manager: TBD
>    - Status: Awaiting implementation
>
>
> <https://gist.github.com/moiseev/2f36376c8ef4c2b1273cff0bfd9c3b95#introduction>
> Introduction
>
> We propose to deprecate the controversial version of a Sequence.flatMap method
> and provide the same functionality under a different, and potentially more
> descriptive, name.
>
> <https://gist.github.com/moiseev/2f36376c8ef4c2b1273cff0bfd9c3b95#motivation>
> Motivation
>
> The Swift standard library currently defines 3 distinct overloads for
> flatMap:
>
> Sequence.flatMap<S>(_: (Element) -> S) -> [S.Element]
>     where S : SequenceOptional.flatMap<U>(_: (Wrapped) -> U?) -> U?Sequence.flatMap<U>(_: (Element) -> U?) -> [U]
>
> The last one, despite being useful in certain situations, can be (and
> often is) misused. Consider the following snippet:
>
> struct Person {
>   var age: Int
>   var name: String
> }
> func getAges(people: [Person]) -> [Int] {
>   return people.flatMap { $0.age }
> }
>
> What happens inside getNames is: thanks to the implicit promotion to
> Optional, the result of the closure gets wrapped into a .some, then
> immediately unwrapped by the implementation of flatMap, and appended to
> the result array. All this unnecessary wrapping and unwrapping can be
> easily avoided by just using map instead.
>
> func getAges(people: [Person]) -> [Int] {
>   return people.map { $0.age }
> }
>
> It gets even worse when we consider future code modifications, like the
> one where Swift 4 introduced a Stringconformance to the Collection protocol.
> The following code used to compile (due to the flatMap overload in
> question).
>
> func getNames(people: [Person]) -> [String] {
>   return people.flatMap { $0.name }
> }
>
> But it no longer does, because now there is a better overload that does
> not involve implicit promotion. In this particular case, the compiler error
> would be obvious, as it would point at the same line where flatMap is
> used. Imagine however if it was just a let names = people.flatMap { $
> 0.name } statement, and the names variable were used elsewhere. The
> compiler error would be misleading.
>
> <https://gist.github.com/moiseev/2f36376c8ef4c2b1273cff0bfd9c3b95#proposed-solution>Proposed
> solution
>
> We propose to deprecate the controversial overload of flatMap and
> re-introduce the same functionality under a new name. The name being
> filteredMap(_:) as we believe it best describes the intent of this
> function.
>
> For reference, here are the alternative names from other languages:
>
>    - Haskell, Idris  mapMaybe :: (a -> Maybe b) -> [a] -> [b]
>    - Ocaml (Core and Batteries)  filter_map : 'a t -> f:('a -> 'b option)
>    -> 'b t
>    - F#  List.choose : ('T -> 'U option) -> 'T list -> 'U list
>    - Rust  fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F> where F:
>    FnMut(Self::Item) -> Option<B>
>    - Scala  def collect[B](pf: PartialFunction[A, B]): List[B]
>
>
> <https://gist.github.com/moiseev/2f36376c8ef4c2b1273cff0bfd9c3b95#source-compatibility>Source
> compatibility
>
> Since the old function will still be available (although deprecated) all
> the existing code will compile, producing a deprecation warning and a
> fix-it.
>
> <https://gist.github.com/moiseev/2f36376c8ef4c2b1273cff0bfd9c3b95#effect-on-abi-stability>Effect
> on ABI stability
>
> This is an additive API change, and does not affect ABI stability.
>
> <https://gist.github.com/moiseev/2f36376c8ef4c2b1273cff0bfd9c3b95#effect-on-api-resilience>Effect
> on API resilience
>
> Ideally, the deprecated flatMap overload would not exist at the time when
> ABI stability is declared, but in the worst case, it will be available in a
> deprecated form from a library post-ABI stability.
>
> <https://gist.github.com/moiseev/2f36376c8ef4c2b1273cff0bfd9c3b95#alternatives-considered>Alternatives
> considered
>
> It was attempted in the past to warn about this kind of misuse and do the
> right thing instead by means of a deprecated overload with a
> non-optional-returning closure. The attempt failed due to another implicit
> promotion (this time to Any).
>
> The following alternative names for this function were considered:
>
>    - mapNonNil(_:)  Does not communicate what happens to nil’s
>    - mapSome(_:)  Reads more like «map some elements of the sequence, but
>    not the others» rather than «process only the ones that produce an
>    Optional.some»
>    - filterMap(_:)  Does not really follow the naming guidelines and
>    doesn’t seem to be common enough to be considered a term of art.
>
>
>
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>
>
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