logos 0.10.0-rc6

Create ridiculously fast Lexers

Create ridiculously fast Lexers.

Logos works by:

  • Resolving all logical branching of token definitions into a state machine.
  • Optimizing complex patterns into Lookup Tables.
  • Avoiding backtracking, unwinding loops, and batching reads to minimize bounds checking.

In practice it means that for most grammars the lexing performance is virtually unaffected by the number of tokens defined in the grammar. Or, in other words, it is really fast.


extern crate logos;

use logos::Logos;

#[derive(Logos, Debug, PartialEq)]
enum Token {
// Logos requires that we define two default variants,
// one for end of input source,

// ...and one for errors. Those can be named anything
// you wish as long as the attributes are there.

// Tokens can be literal strings, of any length.
#[token = "fast"]

#[token = "."]

// Or regular expressions.
#[regex = "[a-zA-Z]+"]

fn main() {
let mut lexer = Token::lexer("Create ridiculously fast Lexers.");

assert_eq!(lexer.token, Token::Text);
assert_eq!(lexer.slice(), "Create");
assert_eq!(lexer.range(), 0..6);


assert_eq!(lexer.token, Token::Text);
assert_eq!(lexer.slice(), "ridiculously");
assert_eq!(lexer.range(), 7..19);


assert_eq!(lexer.token, Token::Fast);
assert_eq!(lexer.slice(), "fast");
assert_eq!(lexer.range(), 20..24);


assert_eq!(lexer.token, Token::Text);
assert_eq!(lexer.slice(), "Lexers");
assert_eq!(lexer.range(), 25..31);


assert_eq!(lexer.token, Token::Period);
assert_eq!(lexer.slice(), ".");
assert_eq!(lexer.range(), 31..32);


assert_eq!(lexer.token, Token::End);

Token disambiguation

Rule of thumb is:

  • Longer beats shorter.
  • Specific beats generic.

If any two definitions could match the same input, like fast and [a-zA-Z]+ in the example above, it's the longer and more specific definition of Token::Fast that will be the result.

This is done by comparing numeric priority attached to each definition. Every consecutive, non-repeating single byte adds 2 to the priority, while every range or regex class adds 1. Loops or optional blocks are ignored, while alternations count the shortest alternative:

  • [a-zA-Z]+ has a priority of 1 (lowest possible), because at minimum it can match a single byte to a class.
  • foobar has a priority of 12.
  • (foo|hello)(bar)? has a priority of 6, foo being it's shortest possible match.