What is a Guard Statement?

Guard Statements are used to check if preconditions are satisfied before continuing. Guard statements…

check for conditions that must be held for execution to continue
can simultaneously check for conditions while safely unwrapping optionals
must exit scope if conditions are not held
use commas to separate multiple conditions

guard <condition-list> else <code-block>

func canClimberClimb(belayerReady: Bool, climberReady: Bool, safetyChecks: Bool) {
    guard belayerReady else { return }
    guard climberReady else { return }
    guard safetyChecks else { return }
    print("Ready to Climb!")
}

canClimberClimb(belayerReady: true, climberReady: true, safetyChecks: true) // prints "Ready to Climb!"
canClimberClimb(belayerReady: true, climberReady: false, safetyChecks: true) // exits early. nothing is printed

Guard vs If

Guard is used to check preconditions and allow for early exits, whereas if statements are used to change condition paths based on conditions

Guard with Optionals

Guard statements are great for working with optionals.

func canClimberClimb(belayerReady: Bool, climberReady: Bool, safetyChecks: Bool, spotterName: String?) {
    guard let spotterName = spotterName else { return } // guard with optional
    guard belayerReady, climberReady, safetyChecks  else { return }
    print("Ready to Climb!")
}

canClimberClimb(belayerReady: true, climberReady: true, safetyChecks: true, spotterName: "Joe" ) // prints "Ready to Climb!"
canClimberClimb(belayerReady: true, climberReady: false, safetyChecks: true, spotterName: nil) // exits early. nothing is printed

Guard Let/Var vs Guard If

When optional values are used with guard let (or guard var) they are bound as non-optional values and available in the rest of the scope where the guard statement appears, wheras with if let, optionals are bound as non-optional constants. They are only available in the body of the if let statement.

Guard Let/Var

func canClimberClimb(belayerReady: Bool, climberReady: Bool, safetyChecks: Bool, spotterName: String?) {
    guard belayerReady, climberReady, safetyChecks  else { return }
    guard var spotterName = spotterName else { return } // guard with optional
    
    spotterName = spotterName.uppercased() // spotterName is available outside the guard block
    
    print("\(spotterName) says you're ready to climb!")
}

canClimberClimb(belayerReady: true, climberReady: true, safetyChecks: true, spotterName: "Joe" ) // prints "JOE says you're ready to climb!"

Guard If

func canClimberClimb(belayerReady: Bool, climberReady: Bool, safetyChecks: Bool, spotterName: String?) {
    guard belayerReady, climberReady, safetyChecks  else { return }

    if let spotterNameUpperCased = spotterName?.uppercased() {
        print("\(spotterNameUpperCased) says you're ready to climb!") // spotterNameUpperCased is available only inside the if let else block
    }
}

canClimberClimb(belayerReady: true, climberReady: true, safetyChecks: true, spotterName: "Joe" ) // prints "JOE says you're ready to climb!"