README.md

Toy Lexer

These are notes and a working example of a lexer written in Go. This was developed by going through the Lexing portion of Writing An Interpreter In Go.

This does not contain a working programming language. This repository only contains an example toy lexical analyzer used as the first step in creating a programming language. A lexer takes source code as input and outputs the tokens that represent it. In our toy lexer, you will find the following subset tokens:

Token	Description
ILLEGAL	Denotes a value or set of characters that the lexer does not recognize.
EOF	Denotes the end of file. Every program will end with an EOF token.
IDENT	Denotes a user-defined identifier.
INT	Denotes an integer.

Notes

• A lexer takes source code as input and output the tokens that represent it. It is also known as lexical analysis, lexing, or tokenization.
  • Lexing is the first step in transforming text written by humans to a computer-readable format that is easy to work with.

    Source code:
    let x = 5 + 5;
    
    Lexical output:
    [
        LET,
        IDENTIFIER("x"),
        EQUAL_SIGN,
        INTEGER(5),
        PLUS_SIGN,
        INTEGER(5),
        SEMICOLON
    ]
    

• Tokens are small, categorizable data structures that are fed to a parser, which handles the second transformation step, turning our tokens into an Abstract Syntax Tree (AST).
  • A token is a string with an assigned and identifiable meaning. It is structured as a pair consisting of a token name and an optional token value.
  • The token name is a category of lexical unit. Common token names are
    • identifier: names the programmar chooses
    • keyword: names already in the programming language
    • separator: puncuation characters and paired-delimiters
    • operator: symbols that operate on arguments and produce results
    • literal: numeric, logical, textual, reference literals
    • comment: line, block