The guard Statement

func describe(age: Int) {
    guard age >= 0 else {
        print("Invalid age")
        return
    }
    print("Age is \(age)")
}
describe(age: 25)
describe(age: -5)

func processScore(_ score: Int) {
    guard score >= 0, score <= 100 else {
        print("Score out of range")
        return
    }
    print("Valid score: \(score)")
}
processScore(150)
processScore(85)

1. Forgetting that a guard block's else clause MUST exit the current scope (via return, break, continue, or throw) -- Swift enforces this at compile time.
2. Using guard for the main happy-path logic instead of for validating and exiting early; guard is meant for early exits, if for branching.
3. Not realizing that variables bound in guard let remain available for the REST of the enclosing scope, unlike an if let binding which is scoped to its own block.

• guard checks a condition and requires an else block that exits scope if the condition fails.
• It's typically used for early-exit validation at the top of a function.
• Values unwrapped via guard let remain usable for the rest of the function, improving readability over nested if let.
• The Swift compiler enforces that the else branch of a guard must leave the current scope.