Swift Tutorial · Closures, Generics and Errors · Lesson 21 of 30

Swift Error Handling

Throw, propagate and catch errors with throws, try, do-catch, typed throws, try? and Result.

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throws, try and do-catch

A function that can fail is marked throws; every call to it is marked try, so failure points are visible. Errors are usually enums conforming to Error. do/catch handles them, and catch clauses can match specific cases.

throws, try and do-catch

enum CheckoutError: Error {
    case emptyCart
    case outOfStock(item: String)
}

func checkout(_ cart: [String], stock: [String: Int]) throws -> String {
    guard !cart.isEmpty else { throw CheckoutError.emptyCart }
    for item in cart where stock[item, default: 0] == 0 {
        throw CheckoutError.outOfStock(item: item)
    }
    return "ordered \(cart.count) items"
}

let stock = ["mug": 3, "teapot": 0]
for cart in [["mug"], [], ["mug", "teapot"]] {
    do {
        print(try checkout(cart, stock: stock))
    } catch CheckoutError.outOfStock(let item) {
        print("sorry, \(item) is out of stock")
    } catch {
        print("failed: \(error)")
    }
}

Output

ordered 1 items
failed: emptyCart
sorry, teapot is out of stock

Run this on your computer — see “Get Started” for setup.

try?, typed throws and Result

try? turns an error into nil.
Typed throws (Swift 6): throws(CheckoutError) declares exactly which error type a function throws.
Result<Success, Failure> stores a success or failure as a value, for example to pass it around.

Typed throws, try? and Result

enum ParseError: Error { case notANumber(String), negative(Int) }

func parseQuantity(_ text: String) throws(ParseError) -> Int {
    guard let n = Int(text) else { throw .notANumber(text) }
    guard n >= 0 else { throw .negative(n) }
    return n
}

do {
    _ = try parseQuantity("-2")
} catch {
    print("caught \(error)")          // error is ParseError here, not any Error
}

print(try? parseQuantity("7") as Any)
print((try? parseQuantity("x")) ?? 0)

let result = Result { try parseQuantity("12") }
switch result {
case .success(let n): print("parsed", n)
case .failure(let e): print("error", e)
}

Output

caught negative(-2)
Optional(7)
0
parsed 12

Run this on your computer — see “Get Started” for setup.

Test yourself

What does try? do with an error?

Crashes the program
Converts it to nil
Prints it
Rethrows it

Show answer

Answer: Converts it to nil

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