[swift-evolution] [Proposal draft] Introducing `indexed()` collections

[Erica Sadun]

Gist here: https://gist.github.com/erica/2b2d92e6db787d001c689d3e37a7c3f2

Introducing indexed() collections

Proposal: TBD
Author: Erica Sadun <https://github.com/erica>, Nate Cook <https://github.com/natecook1000>, Jacob Bandes-Storch <https://github.com/jtbandes>, Kevin Ballard <https://github.com/kballard>
Status: TBD
Review manager: TBD
 <https://gist.github.com/erica/2b2d92e6db787d001c689d3e37a7c3f2#introduction>Introduction

This proposal introduces indexed() to the standard library, a method on collections that returns an (index, element) tuple sequence.

Swift-evolution thread: TBD <https://gist.github.com/erica/tbd>
 <https://gist.github.com/erica/2b2d92e6db787d001c689d3e37a7c3f2#motivation>Motivation

The standard library's enumerated() method returns a sequence of pairs enumerating a sequence. The pair's first member is a monotonically incrementing integer starting at zero, and the second member is the corresponding element of the sequence. When working with arrays, the integer is coincidentally the same type and value as an Array index but the enumerated value is not generated with index-specific semantics. This may lead to confusion when developers attempt to subscript a non-array collection with enumerated integers. It can introduce serious bugs when developers use enumerated()-based integer subscripting with non-zero-based array slices.

Indices have a specific, fixed meaning in Swift, which are used to create valid collection subscripts. This proposal introduces indexed() to produce a more semantically relevant sequence by pairing a collection's indices with its members. While it is trivial to create a solution in Swift, the most common developer approach shown here calculates indexes twice: 

extension Collection {
    /// Returns a sequence of pairs (*idx*, *x*), where *idx* represents a
    /// consecutive collection index, and *x* represents an element of
    /// the sequence.
    func indexed() -> Zip2Sequence<Self.Indices, Self> {
        return zip(indices, self)
    }
}
Incrementing an index in some collections can be unnecessarily costly. In a lazy filtered collection, an index increment is potentially O(N). We feel this is better addressed introducing a new function into the Standard Library to provide a more efficient design that avoids the attractive nuisance of the "obvious" solution.

 <https://gist.github.com/erica/2b2d92e6db787d001c689d3e37a7c3f2#detailed-design>Detailed Design

Our vision of indexed() bypasses duplicated index generation with their potentially high computation costs. We'd create an iterator that calculates each index once and then applies that index to subscript the collection. Implementation would take place through IndexedSequence, similar to EnumeratedSequence.

 <https://gist.github.com/erica/2b2d92e6db787d001c689d3e37a7c3f2#impact-on-existing-code>Impact on Existing Code

This proposal is purely additive and has no impact on existing code.

 <https://gist.github.com/erica/2b2d92e6db787d001c689d3e37a7c3f2#alternatives-considered>Alternatives Considered

Not yet
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