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

[David Waite]

I'm not attached to the name filteredMap, but having a specific signature for the optional version makes sense. 

I think the problem is that while Optional can be contemplated as a sequence of zero or one items, implicit conversion to Optional means anything can then be interpreted as a collection of one item.

Likewise, if the syntax for passing in closures required any argument label to be specified, it might be clearer for optional vs sequence, but they are not required.

-DW

> On Oct 23, 2017, at 4:15 PM, Max Moiseev via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 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/moiseev/2f36376c8ef4c2b1273cff0bfd9c3b95 <https://gist.github.com/moiseev/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 : Sequence
> Optional.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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