Swift Programming: The Big Nerd Ranch Guide

Lexing an Input String

The first phase of your expression-evaluating compiler is lexing. Lexing is the process of turning some input into a sequence of tokens. A token is something with meaning, like a number or a plus sign (the two tokens your compiler will recognize). Lexing is sometimes referred to as “tokenizing” because you are turning some meaningless-to-the-compiler input (like a string) into a sequence of meaningful tokens.

Create a new playground named ErrorHandling. Define an enumeration that has cases for the two kinds of token.

Listing 20.1 Declaring the Token type

import Cocoa

var str = "Hello, playground"

enum Token {
    case number(Int)
    case plus
}

Next, start building your lexer. To lex an input string, you will ...