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Swift Throwing Functions

A throwing function is a function that can fail by throwing an error instead of returning a value. The throws mark in its header warns callers that failure is possible.

An error list

The LoginError enum conforms to Error and lists two ways a login check can fail. The test confirms which case a value holds.

enum LoginError: Error {
    case emptyName
    case shortPassword
}
let problem = LoginError.emptyName
print(problem == LoginError.emptyName)

A call that succeeds

The welcome function throws only for an empty name, and Ana is a fine name here. The do block prints the returned greeting.

enum LoginError: Error {
    case emptyName
}
func welcome(name: String) throws -> String {
    if name.isEmpty {
        throw LoginError.emptyName
    }
    return "Welcome, \(name)"
}
do {
    let message = try welcome(name: "Ana")
    print(message)
} catch {
    print("Login failed")
}

A call that throws

The empty name makes welcome throw before any return, so the greeting print never runs. The catch block prints its fixed line instead.

enum LoginError: Error {
    case emptyName
}
func welcome(name: String) throws -> String {
    if name.isEmpty {
        throw LoginError.emptyName
    }
    return "Welcome, \(name)"
}
do {
    let message = try welcome(name: "")
    print(message)
} catch {
    print("Login failed")
}

Example

enum VendingError: Error {
    case outOfStock
}
func buy(stock: Int) throws -> String {
    if stock == 0 {
        throw VendingError.outOfStock
    }
    return "Dispensed"
}
do {
    let first = try buy(stock: 2)
    print(first)
    let second = try buy(stock: 0)
    print(second)
} catch {
    print("Could not buy")
}

Key points

  • An enum that conforms to Error lists what can go wrong.
  • The throws mark in a function header says the function can throw an error.
  • The throw word stops the function at once and sends the error to the caller.
  • A call to a throwing function takes the try mark.
  • That call sits inside a do block, which gives the call a catch block.
  • The catch block runs only when the call throws.
  • A call that does not throw gives its return value as usual.

In this example, the buy function throws the outOfStock error when the stock is 0 and returns a word the rest of the time. The first call inside the do block has stock 2, so it prints Dispensed. The second call has stock 0 and throws, so the catch block prints its one fixed line Could not buy.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Programiz Swift Compiler

Exercise

What does the throw word do inside a function?