Checking the value of an Optional Bool

Question

When I want to check if an Optional Bool is true, doing this doesn\'t work:

var boolean : Bool? = false
if boolean{
}

It results in this error:

Answer 1

As Antonio said

Optionals no longer implicitly evaluate to true when they have a value and false when they do not, to avoid confusion when working with optional Bool values. Instead, make an explicit check against nil with the == or != operators to find out if an optional contains a value.

I spent a few hours trying to understand a line of code I stumbled upon, but this thread put me on the right track.

This quote is from august 2014, and since then Apple introduced Never following proposal SE-0102 and latter made it conform to Equatable, Hashable, Error and Comparable

It is now possible to check if a boolean is nil using Never? :

var boolean: Bool? = false
boolean is Never? // false
boolean = true
boolean is Never? // false
boolean = nil
boolean is Never? // true

You can actually use any other uninhabitable types :

public enum NeverEver { }
var boolean: Bool? = false
boolean is NeverEver? // false
boolean = true
boolean is NeverEver? // false
boolean = nil
boolean is NeverEver? // true

That being said, it's also possible to use a property wrapper now :

@propertyWrapper struct OptionalBool {
    public var wrappedValue: Bool?
    public var projectedValue: Bool { wrappedValue ?? false }
    public init(wrappedValue: Bool?) {
        self.wrappedValue = wrappedValue
    }
}

struct Struct {
    @OptionalBool var predicate: Bool?
    var description: String {
        if $predicate {
            return "predicate is true"
        }
        return "predicate is false"
    }
}

var object = Struct()
object.description // "predicate is false"
object.predicate = false
object.description // "predicate is false"
object.predicate = true
object.description // "predicate is true"

or even:

@propertyWrapper struct OptionalBool {
    var wrappedValue: Bool?
    var projectedValue: OptionalBool { self }
    var isNil: Bool { wrappedValue is Never? }
    var value: Bool { wrappedValue ?? false }
    
    init(wrappedValue: Bool?) {
        self.wrappedValue = wrappedValue
    }
}

struct Struct {
    @OptionalBool var predicate: Bool?
    var description: String {
        if $predicate.value {
            return "predicate is true"
        }
        if !$predicate.isNil {
            return "predicate is false"
        }
        return "predicate is nil"
    }
}

var object = Struct()
object.description // "predicate is nil"
object.predicate = false
object.description // "predicate is false"
object.predicate = true
object.description // "predicate is true"