rumblebars-rustlex

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 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.

The way of using RustLex depends on the version of Rust you are using to build your project.

Nightly

This is the easy way. Just indicate a dependency to rustlex in your Cargo.toml and add the following lines at the top of your crate:

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

This will make rustc load the RustLex plugin which contains everything that is needed to generate the code.

Stable

On the stable channel, you have to use [syntex] (https://github.com/erickt/rust-syntex) to first perform code generation and then include!() the produced code into your project.

Your Cargo.toml should look like that:

[package]
name = "your package"
version = "0.0.0"
build = "build.rs"

[build-dependencies]
rustlex_codegen = { version = "*", features = ["with-syntex"] }
syntex          = { version = "*", optional = true }

[dependencies]
rustlex_codegen = { version = "*", features = ["with-syntex"] }

You will need to write a build.rs file. This file is automatically called by cargo before the build (according to the build variable in the Cargo.toml file). In our case, it will use syntex to process your code and call RustLex's code generation:

pub fn main() {
    extern crate syntex;
    extern crate rustlex_codegen;
    use std::env;
    use std::path::Path;

    let mut registry = syntex::Registry::new();
    rustlex_codegen::plugin_registrar(&mut registry);
    let src = Path::new("src/foo.in.rs");
    let dst = Path::new(&env::var_os("OUT_DIR").unwrap()).join("foo.rs");
    registry.expand("", &src, &dst).unwrap();
}

Replace foo.in.rs by the name of the file in which you will use RustLex. Note that all its submodules are processed with it, so you don't need to add them as well even if they also contain calls to the RustLex macro.

This will generate a file called foo.rs (or however you named it) in Cargo's OUT_DIR. To use this file, add something like this somewhere in your project (for example in a foo.rs file placed alongside foo.in.rs):

If you want to build a project using RustLex that can be built using either stable or nightly, you can write a portable Cargo.toml file using features, and use the #[cfg()] attribute in your build.rs and the rest of your code to make it build using both versions. You can check out the [test project] (http://github.com/LeoTestard/RustLex/tree/master/codegen/tests/Cargo.toml/) for rustlex_codegen for an example.

Defining a lexer

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.