Checking the value of an Optional Bool

With optional booleans it's needed to make the check explicit:

if boolean == true {
    ...
}

Otherwise you can unwrap the optional:

if boolean! {
    ...
}

But that generates a runtime exception if boolean is nil - to prevent that:

if boolean != nil && boolean! {
    ...
}

Before beta 5 it was possible, but it has been changed as reported in the release notes:

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.

Addendum: as suggested by @MartinR, a more compact variation to the 3rd option is using the coalescing operator:

if boolean ?? false {
    // this code runs only if boolean == true
}

which means: if boolean is not nil, the expression evaluates to the boolean value (i.e. using the unwrapped boolean value), otherwise the expression evaluates to false

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Optional binding

Swift 3 & 4

var booleanValue : Bool? = false
if let booleanValue = booleanValue, booleanValue {
    // Executes when booleanValue is not nil and true
    // A new constant "booleanValue: Bool" is defined and set
    print("bound booleanValue: '\(booleanValue)'")
}

Swift 2.2

var booleanValue : Bool? = false
if let booleanValue = booleanValue where booleanValue {
    // Executes when booleanValue is not nil and true
    // A new constant "booleanValue: Bool" is defined and set
    print("bound booleanValue: '\(booleanValue)'")
}

The code let booleanValue = booleanValue returns false if booleanValue is nil and the if block does not execute. If booleanValue is not nil, this code defines a new variable named booleanValue of type Bool (instead of an optional, Bool?).

The Swift 3 & 4 code booleanValue (and Swift 2.2 code where booleanValue) evaluates the new booleanValue: Bool variable. If it is true, the if block executes with the newly defined booleanValue: Bool variable in scope (allowing the option to reference the bound value again within the if block).

Note: It's a Swift convention to name the bound constant/variable the same as the optional constant/variable such as let booleanValue = booleanValue. This technique is called variable shadowing. You could break from convention and use something like let unwrappedBooleanValue = booleanValue, unwrappedBooleanValue. I point this out to help understand what's happening. I recommend using variable shadowing.

Other Approaches

Nil coalescing

Nil coalescing is clear for this specific case

var booleanValue : Bool? = false
if booleanValue ?? false {
    // executes when booleanValue is true
    print("optional booleanValue: '\(booleanValue)'")
}

Checking for false is not as clear

var booleanValue : Bool? = false
if !(booleanValue ?? false) {
    // executes when booleanValue is false
    print("optional booleanValue: '\(booleanValue)'")
}

Note: if !booleanValue ?? false does not compile.

Force unwrapping optional (avoid)

Force unwrapping increases the chance that someone will make a change in the future that compiles but crashes at runtime. Therefore, I would avoid something like this:

var booleanValue : Bool? = false
if booleanValue != nil && booleanValue! {
    // executes when booleanValue is true
    print("optional booleanValue: '\(booleanValue)'")
}

A General Approach

Though this stack overflow question asks specifically how to check if a Bool? is true within an if statement, it's helpful to identify a general approach whether checking for true, false or combining the unwrapped value with other expressions.

As the expression gets more complicated, I find the optional binding approach more flexible and easier to understand than other approaches. Note that optional binding works with any optional type (Int?, String?, etc.).

---

var enabled: Bool? = true

if enabled == true {
    print("when is defined and true at the same moment")
}

if enabled == false {
    print("when is defined and false at the same moment")
}

if let enabled = enabled, enabled == true {
    print("when is defined and true at the same moment")
}

if let enabled = enabled, enabled == false {
    print("when is defined and false at the same moment")
}

if let enabled = enabled, enabled {
    print("when is defined and true at the same moment")
}

if let enabled = enabled, !enabled {
    print("when is defined and false at the same moment")
}

if enabled ?? false {
    print("when is defined and true at the same moment")
}

if enabled == .some(true) {
    print("when is defined and true at the same moment")
}

if enabled == (true) {
    print("when is defined and true at the same moment")
}

if case .some(true) = enabled {
    print("when is defined and true at the same moment")
}

if enabled == .some(false) {
    print("when is defined and false at the same moment")
}

if enabled == (false) {
    print("when is defined and false at the same moment")
}

if enabled == .none {
    print("when is not defined")
}

if enabled == nil {
    print("when is not defined")
}

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I found another solution, overloading the Boolean operators. For example:

public func < <T: Comparable> (left: T?, right: T) -> Bool {
    if let left = left {
        return left < right
    }
    return false
}

This may not be totally in the "spirit" of the language changes, but it allows for safe unwrapping of optionals, and it is usable for conditionals anywhere, including while loops.

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The answer I found most easy to read is to define a function. Not very complicated but does the work.

func isTrue(_ bool: Bool?) -> Bool {
    guard let b = bool else {
        return false
    }
    return b
}

usage:

let b: Bool? = true
if isTrue(b) {
    // b exists and is true
} else {
    // b does either not exist or is false
}

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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"

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