Swift Programming: The Big Nerd Ranch Guide

Catching Errors

To handle errors, Swift uses a control construct you have not yet seen: do/catch, with at least one try statement inside of the do. We will explain in a moment. First, modify evaluate(_:) to use this control flow to handle errors coming from lex().

Listing 20.13 Error handling in evaluate(_:)

...
func evaluate(_ input: String) {
    print("Evaluating: \(input)")
    let lexer = Lexer(input: input)

    do {
        let tokens = try lexer.lex()
        print("Lexer output: \(tokens)")
    } catch {
        print("An error occurred: \(error)")
    }
}

What do all these new keywords mean? do introduces a new scope, much like an if statement. Inside of the do scope, you can write code as normal, like calling print(). In addition, you can call functions ...