How to Throw Errors in Swift

Use throw in Swift when a function should explicitly signal failure conditions instead of returning invalid values or crashing. This is the cleanest way to model validation, file loading issues, parsing failures, and business-rule checks.

What you’ll build or solve

You’ll learn how to throw errors in Swift with custom error enums, throws functions, and clear failure conditions. You’ll also know when Result is a better fit.

When this approach works best

This approach is the right choice when failure is expected and callers should decide how to recover.

Common real-world scenarios include:

• Input validation
• Login checks
• File loading
• JSON parsing
• Payment validation

This is a bad idea when the failure should be represented as a normal optional absence instead of a real error.

Prerequisites

You only need:

• Basic Swift functions
• Familiarity with enums
• Understanding of do-catch

Step-by-step instructions

Step 1: Define a custom error type

Swift errors usually use enums.

enum SignupError: Error {
    case invalidEmail
    case weakPassword
}

This creates strongly typed failure cases.

Step 2: Mark the function with throws

A throwing function must declare it.

func validate(email: String) throws {
    if !email.contains("@") {
        throw SignupError.invalidEmail
    }
}

This makes failure part of the function contract.

Step 3: Throw based on business rules

Throw whenever the rule fails.

func createPassword(_ password: String) throws {
    if password.count < 8 {
        throw SignupError.weakPassword
    }
}

This keeps invalid state from leaking forward.

Step 4: Throw multiple error types cleanly

Enums scale well for multiple cases.

enum PaymentError: Error {
    case insufficientFunds
    case expiredCard
}

This keeps failure paths explicit and easy to match later.

What to look for:

• Define errors with enums
• Mark throwing functions with throws
• Throw specific cases
• Great for validation and business rules
• Keep error names descriptive

Examples you can copy

Validation

throw SignupError.invalidEmail

Payment

throw PaymentError.expiredCard

File load

throw FileError.missingFile

Common mistakes and how to fix them

Mistake 1: Forgetting throws

What the reader might do:

func login() {
    throw AuthError.failed
}

Why it breaks: Swift requires throwing functions to declare it.

Corrected approach:

Add throws.

Mistake 2: Throwing vague generic errors

What the reader might do:

Use one generic .failed.

Why it breaks: recovery logic becomes weaker.

Corrected approach:

Use specific cases.

Mistake 3: Using throw for normal optional absence

What the reader might do:

Throw when a search result is simply missing.

Why it breaks: this may be a normal optional case.

Corrected approach:

Use optionals for expected absence.

Troubleshooting

If Swift complains, add throws to the function.

If recovery is unclear, make error cases more specific.

If the missing value is normal, return an optional instead.

If many async workflows need explicit success/failure, consider Result.

Quick recap

• Define errors as enums
• Add throws to failing functions
• Throw specific cases
• Great for validation and business rules
• Use optionals for expected absence