Instructions on how to debug your Logos lexer.

Visualizing Logos Graph

Logos works by creating a graph that gets derived from the tokens that you defined. This graph describes how the lexer moves through different states when processing input.

Hence, it may be beneficial during debugging to be able to visualize this graph, to understand how Logos will match the various tokens.

If we take this example:

use logos::Logos;

#[derive(Debug, Logos, PartialEq)]
enum Token {
    // Tokens can be literal strings, of any length.


    // Or regular expressions.
fn main() {
    let input = "Create ridiculously fast Lexers.";

    let mut lexer = Token::lexer(input);
    while let Some(token) = {
        println!("{:?}", token);

Logos actually constructs a graph that contains the logic for matching tokens:

graph = {
    1: ::Fast,
    2: ::Period,
    3: ::Text,
    4: {
        [A-Z] ⇒ 4,
        [a-z] ⇒ 4,
        _ ⇒ 3,
    7: [
        ast ⇒ 8,
        _ ⇒ 4*,
    8: {
        [A-Z] ⇒ 4,
        [a-z] ⇒ 4,
        _ ⇒ 1,
    9: {
        . ⇒ 2,
        [A-Z] ⇒ 4,
        [a-e] ⇒ 4,
        f ⇒ 7,
        [g-z] ⇒ 4,

This graph can help us understand how our patterns are matched, and maybe understand why we have a bug at some point.

Let's get started by trying to understand how Logos is matching the . character, which we've tokenized as Token::Period.

We can begin our search by looking at number 9 for the character .. We can see that if Logos matches a . it will jump => to number 2. We can then follow that by looking at 2 which resolves to our ::Period token.

Logos will then continue to look for any matches past our . character. This is required in case there is potential continuation after the . character. Although, in the input we provided, there are no any additional characters, since it is the end of our input.

We also can try to identify how the token fast works by looking at 9, first, and seeing that f will cause Logos to jump to 7. This will then resolve the last letters of our word fast by matching ast which jumps to 8. Since our provided input to the lexer does not include alphabetic characters after the word "fast", but rather a whitespace, the token ::Fast will be recognized. Then, the graph will look for further potential continuation (here, [g-z] => 4)


To enable debugging output you can define a debug feature in your Cargo.toml file, like this:

// Cargo.toml
logos = { version = "1.2.3", features = ["debug"] }

Next, you can build your project with cargo build and the output will contain a debug representation of your graph(s).