Swift Programming: The Big Nerd Ranch Guide

Handling Errors by Sticking Your Head in the Sand

You have seen that every call to a function that might throw an error must be marked with try and that any call with try must either be inside a do / catch block or inside a function that itself is marked with throws. These rules work together to make sure you are handling any potential errors. Try modifying your evaluate(_:) function to break one of these rules.

Listing 20.19  Modifying evaluate(_:) illegally

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

    do {
        let tokens = try lexer.lex() print("Lexer output: \(tokens)") let parser = Parser(tokens: tokens) let result = try parser.parse() print("Parser ...