SE-0045: Add prefix(while:) and drop(while:) to the stdlib

Introduction

Add 2 new Sequence functions prefix(while:) and drop(while:), with overrides as appropriate on Collection, LazySequenceProtocol, and LazyCollectionProtocol.

Swift-evolution thread: Proposal: Add scan, takeWhile, dropWhile, and iterate to the stdlib

Review

Motivation

The Swift standard library provides many useful sequence manipulators like dropFirst(:), filter(:), etc. but it's missing a few common methods that are quite useful.

Proposed solution

Modify the declaration of Sequence with two new members:

protocol Sequence {
  // ...
  /// Returns a subsequence by skipping elements while `predicate` returns
  /// `true` and returning the remainder.
  func drop(while predicate: (Self.Iterator.Element) throws -> Bool) rethrows -> Self.SubSequence
  /// Returns a subsequence containing the initial elements until `predicate`
  /// returns `false` and skipping the remainder.
  func prefix(while predicate: (Self.Iterator.Element) throws -> Bool) rethrows -> Self.SubSequence
}

Also provide default implementations on Sequence that return AnySequence, and default implementations on Collection that return a slice.

LazySequenceProtocol and LazyCollectionProtocol will also be extended with implementations of drop(while:) and prefix(while:) that return lazy sequence/collection types. Like the lazy filter(_:), drop(while:) will perform the filtering when startIndex is accessed.

Detailed design

In addition to the above declarations, provide default implementations based on AnySequence, similarly to how functions like dropFirst(:) and prefix(:) are handled:

extension Sequence where
  SubSequence : Sequence,
  SubSequence.Iterator.Element == Iterator.Element,
  SubSequence.SubSequence == SubSequence {

  public func drop(while predicate: (Self.Iterator.Element) throws -> Bool) rethrows -> AnySequence<Self.Iterator.Element>
  public func prefix(while predicate: (Self.Iterator.Element) throws -> Bool) rethrows -> AnySequence<Self.Iterator.Element>
}

These default implementations produce an AnySequence that wraps an Array (as the functions must be implemented eagerly so as preserve the convention of not holding onto a user-provided closure past the function call without the explicit appearance of .lazy).

Provide default implementations on Collection as well:

extension Collection {
  func drop(while predicate: (Self.Iterator.Element) throws -> Bool) rethrows -> Self.SubSequence
  func prefix(while predicate: (Self.Iterator.Element) throws -> Bool) rethrows -> Self.SubSequence
}

Also provide overrides as needed on AnySequence, AnyCollection, AnyBidirectionalCollection, and AnyRandomAccessCollection.

Extend LazySequenceProtocol with lazy versions of the functions:

extension LazySequenceProtocol {
  func drop(while predicate: (Self.Iterator.Element) -> Bool) -> LazyDropWhileSequence<Self.Elements>
  func prefix(while predicate: (Self.Iterator.Element) -> Bool) -> LazyPrefixWhileSequence<Self.Elements>
}

The types LazyDropWhileSequence and LazyPrefixWhileSequence conform to LazySequenceProtocol.

Extend LazyCollectionProtocol with collection variants for the functions:

extension LazyCollectionProtocol {
  func drop(while predicate: (Self.Iterator.Element) -> Bool) -> LazyDropWhileCollection<Self.Elements>
  func prefix(while predicate: (Self.Iterator.Element) -> Bool) -> LazyPrefixWhileCollection<Self.Elements>
}

The types LazyDropWhileCollection and LazyPrefixWhileCollection conform to LazyCollectionProtocol.

Impact on existing code

None, this feature is purely additive.

Alternatives considered

Naming

The names here are likely to cause some bikeshedding. Here are some alternatives I've heard proposed:

• suffixFrom(firstElementSatisfying:) instead of drop(while:) – Not only is

it rather long, it's also focusing on taking a suffix while the actual expected usage of the function is focused around skipping elements at the start. There's also the potential confusion around whether it's the first element from the beginning or the first element from the end (since the term "suffix" implies working from the end backwards).

• skip(while:) instead of drop(while:) – I'm actually partial to this one,

but we'd need to rename dropFirst(_:) as well. The benefit of this is it removes the potential confusion around whether the method is mutating.

• take(while:) instead of prefix(while:) – This was actually the original

name proposed, and it matches precedent from other languages, but I eventually decided that consistency with prefix(_:) was desired. However, there is an argument to be made that prefix(while:) is using the term "prefix" like a verb instead of a noun, and the verb form means something different.

• prefix(to:) instead of prefix(while:) – The name here doesn't make it

obvious that the argument is a predicate, and this also requires inverting the meaning of the predicate which I don't like. The focus of this function is on retaining the initial elements that have a desired characteristic, which suggests that the predicate should describe the characteristic the desired elements have, not the inverse.

• prefix(having:) instead of prefix(while:) – Reasonable. I chose

prefix(while:) for consistency with drop(while:) but prefix(having:) makes more grammatical sense (since we're using the noun meaning of prefix rather than the verb meaning).

Previous versions

Previous versions of this proposal included global functions scan(:combine:) and unfold(:applying:) (see [revision 3][rev-3]). This proposal was partially accepted, with scan(:combine:) rejected on grounds of low utility and unfold(:applying:) rejected on grounds of poor naming (see [rationale]).

[rationale]: https://forums.swift.org/t/accepted-with-modifications-se-0045-add-scan-prefix-while-drop-while-and-unfold-to-the-stdlib/2466