Errors and Exceptions in Swift in Swift

struct WalletError: Error {
    let message: String
}

func spend(_ amount: Int) throws {
    if amount > money {
        throw WalletError(message: "not enough money")
    }

    money -= amount
}

do {
    try wallet.spend(80)
} catch let error as WalletError {
    print("error: \(error.message)")
} catch {
    print("error: \(error)")
}

Full lesson:

Some requests can't be done. A wallet with 50 can't spend 80.

The method could return a special value, like -1. But the caller may forget to check it. Then the program goes on with a wrong number.

An error that the method throws is better. The program can't miss it.

Throwing an error

• Any type that adopts the Error protocol can be thrown. WalletError is a struct with a message.
• A method that can throw says throws in its header, before the return type.
• throw WalletError(message: "not enough money") stops the method at once.
• The lines after it don't run. So the wallet keeps its money.

Catching it

• Call the method inside a do block, with try in front of the call.
• When the call throws, the rest of the do block is skipped.
• catch let error as WalletError runs for a WalletError.
• Swift wants every error caught. A last plain catch takes any other kind.
• Then the program goes on below the last catch.

Swift checks this when it compiles. A call that can throw needs try, and its errors must be caught.

Your task

A Wallet starts with 50. Its spend method takes money out and returns what is left.

• In spend, throw a WalletError when amount is more than money. Its message is "not enough money".
• In the loop, call spend with try inside a do block. Print what was spent and what is left.
• Catch the WalletError. Print error: and its message. Then add a plain catch for any other error.

The loop tries to spend 20, then 40, then 30.

Expected output:

spent 20, 30 left
error: not enough money
spent 30, 0 left