throw, try, and catch

throw is raising your hand to say something went wrong, try is attempting something that might fail, and catch is what handles it if it does.

In this page:

Declaring and Throwing an Error
Handling Multiple Error Cases

Declaring and Throwing an Error

A function marked throws can use throw to raise an error instead of returning normally when something goes wrong.

Example: Declaring and Throwing an Error

markup

enum ValidationError: Error {
    case empty
}
func validate(_ text: String) throws -> String {
    if text.isEmpty {
        throw ValidationError.empty
    }
    return text
}
do {
    let result = try validate("Hello")
    print("Valid: \(result)")
} catch {
    print("Validation failed: \(error)")
}

Handling Multiple Error Cases

Multiple catch clauses can match specific error cases in order, similar to a switch statement, with a final unmatched catch as a fallback.

Example: Handling Multiple Error Cases

markup

enum ValidationError: Error {
    case empty
    case tooShort
}
func validate(_ text: String) throws -> String {
    if text.isEmpty { throw ValidationError.empty }
    if text.count < 3 { throw ValidationError.tooShort }
    return text
}
do {
    let result = try validate("hi")
    print("Valid: \(result)")
} catch ValidationError.empty {
    print("Text was empty")
} catch ValidationError.tooShort {
    print("Text was too short")
} catch {
    print("Unknown error")
}

Common Mistakes

Forgetting to mark a function that can throw with throws in its signature -- Swift won't let you throw from a non-throwing function.
Calling a throwing function without try in front of it; every call to a throwing function must be prefixed with try, try?, or try!.
Writing a catch block that doesn't match any specific error case; a plain catch at the end acts as a catch-all, but a mismatched typed catch just falls through.

Chapter Summary

A function that can fail is marked with throws after its parameter list.
throw someError raises an error, stopping normal execution of the current function.
Calling a throwing function requires try, and the call is usually wrapped in do { } catch { }.
Multiple catch blocks can match specific error cases, similar to a switch.

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throw, try, and catch

Declaring and Throwing an Error

Handling Multiple Error Cases

Chapter Quiz — Complete all 6 topics to unlock

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