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0207-containsOnly.md

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Add an allSatisfy algorithm to Sequence

Introduction

It is common to want to confirm that every element of a sequence equals a value, or matches certain criteria. Many implementations of this can be found in use on GitHub. This proposal adds such a method to Sequence.

Motivation

You can achieve this in Swift 3 with contains by negating both the criteria and the result:

// every element is 9
!nums.contains { $0 != 9 }
// every element is odd
!nums.contains { !isOdd($0) }

but these are a readability nightmare. Additionally, developers may not make the leap to realize contains can be used this way, so may hand-roll their own for loop, which could be buggy, or compose other inefficient alternatives:

// misses opportunity to bail early
nums.reduce(true) { $0.0 && $0.1 == 9 }
// the most straw-man travesty I could think of...
Set(nums).count == 1 && Set(nums).first == 9

Proposed solution

Introduce an algorithm on Sequence which tests every element and returns true if they all match a given predicate:

nums.allSatisfy(isOdd)

on the basis that it aids readability and avoids performance pitfalls from the composed alternatives.

Detailed design

Add the following extensions to Sequence:

extension Sequence {
  /// Returns a Boolean value indicating whether every element of the sequence
  /// satisfies the given predicate.
  func allSatisfy(_ predicate: (Element) throws -> Bool) rethrows -> Bool
}

Source compatibility

This change is purely additive so has no source compatibility consequences.

Effect on ABI stability

This change is purely additive so has no ABI stability consequences.

Effect on API resilience

This change is purely additive so has no API resilience consequences.

Alternatives considered

Not adding it, since it can be trivially (if confusingly) composed.

Much name bikeshedding has ensued. Names considered included containsOnly and all. all has strong precedent in other languages, but was considered unclear at the call site (adding an argument label does not help here given trailing closures omit them). Naming it all suggests a renaming of contains to any would be appropriate – but this is already a fairly heavily-used term elsewhere in Swift, and is less explicit. containsOnly is more explicit, and echoes the existing contains, but is too easily misread at the use-site as “contains one instance equal to,” especially when considering empty collections. contains(only:) was discounted due to trailing closures dropping the argument label, rendering it indistinguishable from contains(where:).