---
title: "SE-0172: One-sided Ranges"
framework: swift-evolution
role: article
path: swift-evolution/0172-one-sided-ranges
---

# SE-0172: One-sided Ranges

* Proposal: [SE-0172](0172-one-sided-ranges.md)
* Authors: [Ben Cohen](https://github.com/airspeedswift), [Dave Abrahams](https://github.com/dabrahams), [Becca Royal-Gordon](https://github.com/beccadax)
* Review Manager: [Doug Gregor](https://github.com/DougGregor)
* Status: **Implemented (Swift 4.0)** 
* Decision Notes: [Rationale](https://forums.swift.org/t/accepted-se-0172-one-sided-ranges/5768)

## Introduction

This proposal introduces the concept of a "one-sided" range, created via  prefix/postfix versions of the existing range operators.

It also introduces a new protocol, `RangeExpression`, to simplify the creation of methods that take different kinds of ranges.

## Motivation

It is common, given an index into a collection, to want a slice up to or from that index versus the start/end.

For example (assuming `String` is once more a `Collection`):

```swift let s = "Hello, World!" let i = s.index(of: ",")! let greeting = s[s.startIndex..<i] ```

When performing lots of slicing like this, the verbosity of repeating `s.startIndex` is tiresome to write and harmful to readability.

Swift 3’s solution to this is a family of methods:

```swift let greeting = s.prefix(upTo: i) let withComma = s.prefix(through: i) let location = s.suffix(from: i) ```

The two very different-looking ways to perform a similar task is jarring. And as methods, the result cannot be used as an l-value.

A variant of the one-sided slicing syntax found in Python (i.e. `s[i:]`) is proposed to resolve this.

## Proposed solution

Introduce a one-sided range syntax, where the "missing" side is inferred to be the start/end:

```swift // half-open right-handed range let greeting = s[..<i] // closed right-handed range let withComma = s[...i] // left-handed range (no need for half-open variant) let location = s[i...] ```

Additionally, when the index is a countable type, `i...` should form a `Sequence` that counts up from `i` indefinitely. This is useful in forming variants of `Sequence.enumerated()` when you either want them non-zero-based i.e. `zip(1..., greeting)`, or want to flip the order i.e. `zip(greeting, 0...)`.

This syntax would supersede the existing `prefix` and `suffix` operations that take indices, which will be deprecated in a later release. Note that the versions that take distances are not covered by this proposal, and would remain.

This will require the introduction of new range types (e.g. `PartialRangeThrough`). There are already multiple range types (e.g. `ClosedRange`, `CountableHalfOpenRange`), which require overloads to allow them to be used wherever a `Range` can be.

To unify these different range types, a new protocol, `RangeExpression` will be created and all ranges conformed to it. Existing overloads taking concrete types other than `Range` can then be replaced with a single generic method that takes a `RangeExpression`, converts it to a `Range`, and then forward the method on.

A generic version of `~=` will also be implemented for all range expressions:

```swift switch i { case 9001...: print("It’s over NINE THOUSAAAAAAAND") default: print("There's no way that can be right!") } ```

The existing concrete overloads that take ranges other than `Range` will be deprecated in favor of generic ones that take a `RangeExpression`.

## Detailed design

Add the following to the standard library:

(a fuller work-in-progress implementation can be found here: https://github.com/apple/swift/pull/8710)

NOTE: The following is subject to change depending on pending compiler features. Methods may actually be on underscored protocols, and then moved once recursive protocols are implemented. Types may be collapsed using conditional conformance. This should not matter from a usage perspective – users are not expected to use these types directly or override any of the behaviors in their own types. Any final implementation will follow the below in spirit if not in practice.

```swift public protocol RangeExpression {     associatedtype Bound: Comparable

/// Returns `self` expressed as a range of indices within `collection`.     ///     /// -Parameter collection: The collection `self` should be     ///                        relative to.     ///     /// -Returns: A `Range<Bound>` suitable for slicing `collection`.     ///           The return value is *not* guaranteed to be inside     ///           its bounds. Callers should apply the same preconditions     ///           to the return value as they would to a range provided     ///           directly by the user.     func relative<C: _Indexable>(to collection: C) -> Range<Bound> where C.Index == Bound

func contains(_ element: Bound) -> Bool }

extension RangeExpression {   public static func ~= (pattern: Self, value: Bound) -> Bool }

prefix operator ..< public struct PartialRangeUpTo<T: Comparable>: RangeExpression {   public init(_ upperBound: T) { self.upperBound = upperBound }   public let upperBound: T } extension Comparable {   public static prefix func ..<(x: Self) -> PartialRangeUpTo<Self> }

prefix operator ... public struct PartialRangeThrough<T: Comparable>: RangeExpression {   public init(_ upperBound: T)   public let upperBound: T } extension Comparable {   public static prefix func ...(x: Self) -> PartialRangeThrough<Self> }

postfix operator ... public struct PartialRangeFrom<T: Comparable>: RangeExpression {   public init(_ lowerBound: T)   public let lowerBound: T } extension Comparable {   public static postfix func ...(x: Self) -> PartialRangeFrom<Self> }

// The below relies on Conditional Conformance. Pending that feature, // this may require an additional CountablePartialRangeFrom type temporarily. extension PartialRangeFrom: Sequence    where Index: _Strideable, Index.Stride : SignedInteger

extension Collection {   public subscript<R: RangeExpression>(r: R) -> SubSequence    where R.Bound == Index { get } } extension MutableCollection {   public subscript<R: RangeExpression>(r: R) -> SubSequence    where R.Bound == Index { get set } }    extension RangeReplaceableCollection {   public mutating func replaceSubrange<C: Collection, R: RangeExpression>(     _ subrange: ${Range}<Index>, with newElements: C   ) where C.Iterator.Element == Iterator.Element, R.Bound == Index

public mutating func removeSubrange<R: RangeExpression>(     _ subrange: ${Range}<Index>   ) where R.Bound == Index } ```

Additionally, these new ranges will implement appropriate protocols such as `CustomStringConvertible`.

It is important to note that these new methods and range types are _extensions only_. They are not protocol requirements, as they should not need to be customized for specific collections. They exist only as shorthand to expand out to the full slicing operation.

Where `PartialRangeFrom` is a `Sequence`, it is left up to the type of `Index` to control the behavior when the type is incremented past its bounds. In the case of an `Int`, the iterator will trap when iterating past `Int.max`. Other types, such as a `BigInt` that could be incremented indefinitely, would behave differently.

The `prefix` and `suffix` methods that take an index _are_ currently protocol requirements, but should not be. This proposal will fix that as a side-effect.

## Source compatibility

The new operators/types are purely additive so have no source compatibility consequences. Replacing the overloads taking concrete ranges other than `Range`  with a single generic version is source compatible. `prefix` and `suffix` will be deprecated in Swift 4 and later removed.

## Effect on ABI stability

The `prefix`/`suffix` methods being deprecated should be eliminated before declaring ABI stability.

## Effect on API resilience

The new operators/types are purely additive so have no resilience consequences.

## Alternatives considered

`i...` is favored over `i..<` because the latter is ugly. We have to pick one, two would be redundant and likely to cause confusion over which is the "right" one. Either would be reasonable on pedantic correctness grounds – `(i as Int)...` includes `Int.max` consistent with `...`, whereas `a[i...]` is interpreted as `a[i..<a.endIndex]` consistent with `i..<`.

It might be nice to consider extend this domain-specific language inside the subscript in other ways. For example, to be able to incorporate the index distance versions of prefix, or add distance offsets to the indices used within the subscript. This proposal explicitly avoids proposals in this area. Such ideas would be considerably more complex to implement, and would make a good project for investigation by an interested community member, but would not fit within the timeline for Swift 4.