rustlex 0.1.2

RustLex: lexical analysers generator for Rust

RustLex is a lexical analysers generator, i.e. a program that generate lexical analysers for use in compiler from a description of the language using regular expressions. It is similar to the well-known Lex but is written in Rust and outputs Rust code as the analyser. It differs from Lex by using Rust's new syntax extensions system as the interface for defining lexical analysers. The description of the analyser thus can be directly embedded into a Rust source file, and the generator code will be called by Rustc at the macro-expansion phase.

Rustlex availibility and rust compatibility

Rustlex is available through cargo.

[dependencies.rustlex]
git = "https://github.com/LeoTestard/rustlex"

Rustlex using syntax extensions, it has to deal with rustc libsyntax. Libsyntax is more or less the compiler guts, and it has been explicitely excluded from the rust 1.0 roadmap. Bottom line is, rustlex inline syntax generation will not be usable with rust 1.0.

When rust 1.0 will be out, we'll draft a procedure to statically generate lexer code. Compiling with --pretty=expanded already shows the generated code. Depending on what cargo will be able to do in 1.0, the worst case should be requiring a copy and paste of the lexer code to your project.

Defining a lexer

Defining a lexical analyser is done using the rustlex! syntax extension. To made this syntax extension available, you first need to tell Rustc to load the RustLex library by adding the following at the top of your crate:

#![feature(plugin)]
#[plugin] extern crate rustlex;
#[macro_use] extern crate log;

You can then invoke the rustlex! macro anywhere. The macro will expand into a single lexer structure and implementation describing the lexical analyser.

The rustlex! macro takes as argument the name of the structure and the description of the lexical analyser. The description consists of two parts:

• definitions of regular expressions
• definitions of rules

A minimum lexer will look like:

rustlex! SimpleLexer {
    // expression definitions
    let A = 'a';

    // then rules
    A => |&: lexer:&mut SimpleLexer<R>| Some(TokA ( lexer.yystr() ))
}

More complex regular expression definition examples can be found in a more complex example. It is worth noting that:

• characters (standalone or in character class) and strings have to be quoted as in rust or C (simple quote for character, double quote for strings)
• an expression definition can be "called" by its identifier in another expression

Using a lexer

The lexer will read characters from a standard rust Reader and implement a Token iterator.

let inp = BufReader::new("aa".as_bytes());
let mut lexer = SimpleLexer::new(inp);
for tok in lexer {
    ...
}

Advanced lexer features

Token enumeration

By default, rustlex! assumes the existence of a token enumeration named Token in the same module, but this name can be overriden when needed as is the case for the OtherLexer from this example.

Conditions

As in flex, conditions can be defined to have the lexer switch from one mode to another.

Check out this example.

Arbitrary lexer properties and methods

It is possible to add specific fields to the lexer structure using the property keyword as shown there.

Lexer methods (to be called from action code) can also be defined by a normal impl section.