I like it, but only if the API is symmetric. However this would require the break existing API to make `firstIndex(where/of:)`.
One question: Can we maybe add retroactively an extra parameter with a default value? Something like: `func index(where predicate: (Element) throws -> Bool, traverseDirection: TraverseDirection = .leftToRight)` However this comes at a cost of loosing the trailing closure :/ Am 27. November 2017 um 17:31:19, Nate Cook via swift-evolution ([email protected]) schrieb: Hello, Swift Evolution! This is a revision of a previous proposal for adding methods to sequences/collections for searching from the end. Feedback welcome! Nate ————— Add last(where:) and lastIndex(where:) Methods Proposal: SE-NNNN Author: Nate Cook Status: Awaiting review Review manager: TBD Implementation: Branch on natecook1000/swift Related Bug: [SR-1504] RFE: index(of:) but starting from end Introduction The standard library should include methods for finding the last element in a sequence, and the index of the last element in a collection, that match a given predicate. Swift-evolution thread: [swift-evolution] (Draft) Add last(where:) and lastIndex(where:) methods Motivation The standard library currently has methods that perform a linear search to find an element or the index of an element that matches a predicate: Swift let a = [20, 30, 10, 40, 20, 30, 10, 40, 20] a.first(where: { $0 > 25 }) // 30 a.index(where: { $0 > 25 }) // 1 a.index(of: 10) // 2 Unfortunately, there are no such methods that search from the end. Finding the last of a particular kind of element has multiple applications, particularly with text, such as wrapping a long string into lines of a maximum length or trimming whitespace from the beginning and end of a string. You can work around this limitation by using the methods above on a reversed view of a collection, but the resulting code is frankly appalling. For example, to find the corresponding last index to a.index(where: { $0 > 25 }), something like this unholy incantation is required: Swift (a.reversed().index(where: { $0 > 25 })?.base).map({ a.index(before: $0) }) Proposed solution The Sequence protocol should add a last(where:) method, and the Collectionprotocol should add lastIndex(where:) and lastIndex(of:) methods. These new methods create symmetry with the existing forward-searching APIs that are already part of Sequence and Collection. These additions remove the need for searching in a reversed collection and allow code like this: Swift a.last(where: { $0 > 25 }) // 40 a.lastIndex(where: { $0 > 25 }) // 7 a.lastIndex(of: 10) // 6 Much better! Detailed design last(where:) and lastIndex(where:) will be added to the standard library as Sequence and Collection protocol requirements, respectively. These methods will have default implementations in their respective protocols and in BidirectionalCollection, which can provide a more efficient implementation. lastIndex(of:) will be provided in Collection and BidirectionalCollection extensions constrained to Equatable elements. The new APIs are shown here: Swift protocol Sequence { // Existing declarations... /// Returns the last element of the collection that satisfies the given /// predicate, or `nil` if no element does. The sequence must be finite. func last(where predicate: (Element) throws -> Bool) rethrows -> Element? } protocol Collection { // Existing declarations... /// Returns the index of the last element of the collection that satisfies /// the given predicate, or `nil` if no element does. func lastIndex(where predicate: (Element) throws -> Bool) rethrows -> Index? } extension BidirectionalCollection { func last(where predicate: (Element) throws -> Bool) rethrows -> Element? { ... } func lastIndex(where predicate: (Element) throws -> Bool) rethrows -> Index? { ... } } extension Collection where Element: Equatable { /// Returns the index of the last element equal to the given element, or /// no matching element is found. func lastIndex(of element: Element) -> Index? { ... } } extension BidirectionalCollection where Element: Equatable { func lastIndex(of element: Element) -> Index? { ... } } You can explore the usage (but not really the performance) of these methods in this Swift sandbox. Source compatibility The addition of the last(where:), lastIndex(where:), and lastIndex(of:)methods is strictly additive and should have no impact on existing code. Effect on ABI stability & API resilience This change does not affect ABI stability or API resilience beyond the addition of the new methods. Alternatives considered A previous proposal limited the new methods to the BidirectionalCollection protocol. This isn't a necessary limitation, as the standard library already has methods on sequences and forward collections with the same performance characteristics. Another previous proposal included renaming index(of:) and index(where:) to firstIndex(of:) and firstIndex(where:), respectively. This version of the proposal removes that source-breaking change. _______________________________________________ swift-evolution mailing list [email protected] https://lists.swift.org/mailman/listinfo/swift-evolution
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