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:

Optional type '@IvalueBool?' cannot be used as a boolean; test for '!= nil' instead

I don't want to check for nil; I want to check if the value returned is true.

Do I always have to do if boolean == true if I'm working with an Optional Bool?

Since Optionals don't conform to BooleanType anymore, shouldn't the compiler know that I want to check the value of the Bool?

Answer 1

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

Answer 2

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.).

Answer 3

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")
}

Answer 4

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.

Answer 5

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
}

Answer 6

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"