[swift-evolution] [pitch] Eliminate Collection.IndexDistance associated type

[Howard Lovatt]

+1 it seems like a generic type just for the sake of it. Many languages do without a specific type for distance and use Int and it is a better solution because it is simpler. 

-- Howard.

> On 9 Nov 2017, at 8:37 am, Ben Cohen via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
> Hi Swift Evolution,
> 
> A pitch for review, aimed at simplifying generic Collection algorithms.
> 
> Online copy here:
> 
> https://github.com/airspeedswift/swift-evolution/blob/5d1ffda2e83f5b95a88d5ce3948c5fd0d59622f4/proposals/NNNN-eliminate-indexdistance.md
> 
> # Eliminate `IndexDistance` from `Collection`
> 
> * Proposal: [SE-NNNN](NNNN-eliminate-indexdistance.md)
> * Authors: [Ben Cohen](https://github.com/airspeedswift)
> * Review Manager: TBD
> * Status: **Awaiting review**
> * Implementation: [apple/swift#12641](https://github.com/apple/swift/pull/12641)
> 
> ## Introduction
> 
> Eliminate the associated type `IndexDistance` from `Collection`, and modify all uses to the concrete type `Int` instead.
> 
> ## Motivation
> 
> `Collection` allows for the distance between two indices to be any `SignedInteger` type via the `IndexDistance` associated type. While in practice the distance between indices is almost always
> an `Int`, generic algorithms on `Collection` need to either constrain `IndexDistance == Int`, or write their algorithm to be generic over any `SignedInteger`.
> 
> Swift 4.0 introduced the ability to constrain associated types with `where` clauses
> ([SE-142](https://github.com/apple/swift-evolution/blob/master/proposals/0142-associated-types-constraints.md)) and will soon allow protocol constraints
> to be recursive ([SE-157](https://github.com/apple/swift-evolution/blob/master/proposals/0157-recursive-protocol-constraints.md)). With these features,
> writing generic algorithms against `Collection` is finally a realistic tool for intermediate Swift programmers. You no longer need to know to
> constrain `SubSequence.Element == Element` or `SubSequence: Collection`, missing constraints that previously led to inexplicable error messages.
> 
> At this point, the presence of `IndexDistance` is of of the biggest hurdles that new users trying to write generic algorithms face. If you want to
> write code that will compile against any distance type, you need to constantly juggle with explicit type annotations (i.e. you need to write `let i:
> IndexDistance = 0` instead of just `let i = 0`), and perform `numericCast` to convert from one distance type to another.
> 
> But these `numericCasts` are hard to use correctly. Given two collections with different index distances, it's very hard to reason about whether your
> `numericCast` is casting from the smaller to larger type correctly. This turns any problem of writing a generic collection algorithm into both a collection _and_
> problem. And chances are you are going to need to interoperate with a method that takes or provides a concrete `Int` anyway (like `Array.reserveCapacity` inside
> `Collection.map`). Much of the generic code in the standard library would trap if ever presented with a collection with a distance greater than `Int.max`.
> Additionally, this generalization makes specialization less likely and increases compile-time work.
> 
> For these reasons, it's common to see algorithms constrained to `IndexDistance == Int`. In fact, the inconvenience of having to deal with generic index
> distances probably encourages more algorithms to be constrained to `Index == Int`, such as [this
> code](https://github.com/airspeedswift/swift-package-manager/blob/472c647dcad3adf4344a06ef7ba91d2d4abddc94/Sources/Basic/OutputByteStream.swift#L119) in
> the Swift Package Manager. Converting this function to work with any index type would be straightforward. Converting it to work with any index distance
> as well would be much trickier.
> 
> The general advice from [The Swift Programming
> Language](https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/TheBasics.html#//apple_ref/doc/uid/TP40014097-CH5-ID309) when writing Swift code is to encourage users to stick to using `Int` unless they have a special reason not to:
> 
> > Unless you need to work with a specific size of integer, always use `Int` for integer values in your code. [...] `Int` is preferred, even when the values to be stored are known to be nonnegative. A consistent use of Int for integer values aids
> code interoperability, avoids the need to convert between different number types, and matches integer type inference[.]
> 
> There are two main use cases for keeping `IndexDistance` as an associated type rather than concretizing it to be `Int`: tiny collections that might
> benefit from tiny distances, and huge collections that need to address greater than `Int.max` elements. For example, it may seem wasteful to force a
> type that presents the bits in a `UInt` as a collection to need to use a whole `Int` for its distance type. Or you may want to create a gigantic
> collection, such as one backed by a memory mapped file, with a size great than `Int.max`. The most likely scenario for this is on 32-bit processors where a collection would be constrained to 2 billion elements.
> 
> These use cases are very niche, and do not seem to justify the considerable impedance to generic programming that `IndexDistance` causes. Therefore,
> this proposal recommends removing the associated type and replacing all references to it with `Int`.
> 
> ## Proposed solution
> 
> Scrap the `IndexDistance` associated type. Switch all references to it in the standard library to the concrete `Int` type:
> 
> ```swift
> protocol Collection {
> 	var count: Int { get }
> 	func index(_ i: Index, offsetBy n: Int) -> Index
> 	func index(_ i: Index, offsetBy n: Int, limitedBy limit: Index) -> Index?
> 	func distance(from start: Index, to end: Index) -> Int
> }
> // and in numerous extensions in the standard library
> ```
> 
> The one instance where a concrete type uses an `IndexDistance` other than `Int` in the standard library is `AnyCollection`, which uses `Int64`. This would be changed to `Int`.
> 
> ## Source compatibility
> 
> This can be split into 2 parts:
> 
> Algorithms that currently constrain `IndexDistance` to `Int` in their `where` clause, and algorithms that use `IndexDistance` within the body of a
> method, can be catered for by a deprecated typealias for `IndexDistance` inside an extension on `Collection`. This is the common case.
> 
> Collections that truly take advantage of the ability to define non-`Int` distances would be source-broken, with no practical way of making this
> compatible in a 4.0 mode. It's worth noting that there are no such types in the Swift source compatibility suite.
> 
> ## Effect on ABI stability
> 
> This removes an associated type and changes function signatures, so must be done before declaring ABI stability
> 
> ## Alternatives considered
> 
> None other than status quo.
> 
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