Swift Programming: The Big Nerd Ranch Guide, 3rd Edition

Lexing an Input String

The first phase of your expression-evaluating compiler is lexing. In computer science, 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 input that is meaningless to the compiler (like a string) into a sequence of meaningful tokens.

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

Example 23.1. Declaring the Token type

import Cocoa

var str = "Hello, playground"

enum Token {
    case number(Int)
    case plus
}

When building something like a lexer, logging ...