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

Mon Oct 23 22:38:34 CDT 2017

+1 good idea.

Re. the naming I would suggest `mapFilterNil` since it says what it does
and filter, nil, and map are all understood already in Swift. (I have
sympathy for people wanting `mapFilteringNil`, but Swift chose `filter`.)

The problems I see with `filterMap` are that:

  1. It sounds like it is a merged `filter` and `map` and therefore you
would expect it to have two arguments, one to filter and one to map, i.e.
`filterMap<R>(filter: (T) -> Bool, map: (T) -> R) -> [R]`.
  2. It sounds like it will filter the incoming values (for `nil`, but see
1 above) and then map, i.e. `filterMap<R>(map: (T?) -> R) -> [R]`, note
`T?` *not* `R?`.

  -- Howard.

On 24 October 2017 at 11:56, BJ Homer via swift-evolution <
swift-evolution at swift.org> wrote:

> I agree with Xiaodi; I like ‘filterMap’ more than ‘filteredMap’. But both
> are superior to ‘flatMap’ in this context.
>
> -BJ
>
> On Oct 23, 2017, at 5:22 PM, Max Moiseev <moiseev at apple.com> wrote:
>
> It occurred to me that filteringMap(_:) should be even more descriptive,
> still conform to the guidelines, although similarly unprecedented and
> un-googlable.
>
> Max
>
> On Oct 23, 2017, at 3:52 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>
> +1 in general. As to the name: since 'map' is used as a term of art,
> 'filterMap' seems superior to 'filteredMap', which half follows naming
> guidelines and half is a term of art; neither is immediately comprehensible
> but 'filterMap' can be googled and has precedents in other languages.
> On Mon, Oct 23, 2017 at 17:24 BJ Homer via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>> I strongly agree! In fact, I just started writing up a similar proposal
>> the other day, but hadn’t had time to finish it yet.
>>
>> The current name for this particular filtering variant is not
>> particularly descriptive. It’s certainly not obvious to newcomers that
>> ‘flatMap’ will filter out results. And it’s not true to the existing usage
>> of ‘flatMap' from other languages; you have to really squint at it to see
>> how any “flattening” is happening at all.
>>
>> So yes, a big +1 from me. Thanks!
>>
>> -BJ Homer
>>
>> 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 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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