0199-bool-toggle.md

Adding toggle to Bool

• Proposal: SE-0199
• Author: Chris Eidhof
• Review Manager: Ben Cohen
• Status: Implemented (Swift 4.2)
• Decision notes: Rationale
• Implementation: apple/swift#14586
• Review thread: Swift evolution forum

Introduction

I propose adding a mutating func toggle to Bool. It toggles the Bool.

• Swift-evolution thread: Discussion thread topic for that proposal
• Swift forums thread: pitch: adding toggle to Bool

Motivation

For Bool variables, it is common to want to toggle the state of the variable. In larger (nested) structs, the duplication involved can become especially annoying:

myVar.prop1.prop2.enabled = !myVar.prop1.prop2.enabled

It's also easy to make a mistake in the code above if there are multiple Bool vars.

Proposed solution

Add a method toggle on Bool:

extension Bool {
  /// Equivalent to `someBool = !someBool`
  ///
  /// Useful when operating on long chains:
  ///
  ///    myVar.prop1.prop2.enabled.toggle()
  mutating func toggle() {
    self = !self
  }
}

This allows us to write the example above without duplication:

myVar.prop1.prop2.enabled.toggle()

! and toggle() mirror the API design for - and negate(). (Thanks to Xiaodi Wu for pointing this out).

Detailed design

N/A

Source compatibility

This is strictly additive.

Effect on ABI stability

N/A

Effect on API resilience

N/A

Alternatives considered

Other names could be:

• invert
• negate
• flip

From the brief discussion on SE, it seems like toggle is the clear winner.

Some people also suggested adding a non-mutating variant (in other words, a method with the same semantics as the prefix ! operator), but that's out of scope for this proposal, and in line with commonly rejected proposals.