Starlark in Rust

An implementation in Rust of the Starlark language

Disclaimer: This is not an officially supported Google product. This project is supported on a best-effort basis and welcome contributions.

Starlark, formerly codenamed Skylark, is a non-Turing complete language based on Python that was made for the Bazel build system to define compilation plugin.

Starlark has at least 3 implementations: a Java one for Bazel, a Go one and this one.

This interpreter was made using the specification from the go version and the Python 3 documentation when things were unclear.

This interpreter does not support most of the go extensions (e.g. bitwise operator or floating point). It optionally includes a set type (by explicitly including starlark::linked_hash_set::global() environment), as an extension which is not specified in the official Starlark specification, but note that this is just an insertion-order-preserving set, and does not have optimisations for nesting as can be found in the starlark Java implementation's depset implementation. It uses signed 64-bit integers.

Usage

Crate

You can depend on the starlark crate, it is documented using docs.rs. Examples are listed under starlark/examples. You can run the examples using cargo run --example, such as

echo "str([x * 2 for x in range(10)])" | cargo run --example starlark-simple-cli

Command line REPL

A command line interpreter is also provided by this project under starlark-repl, it can interpret files passed at the command line and also start a REPL (Read-Eval-Print Loop). The usage of this program is:

Starlark in Rust interpreter

USAGE:
    starlark-rust [FLAGS] [OPTIONS] [file]...

FLAGS:
    -a, --ast           Parse and print AST instead of evaluating.
    -b, --build-file    Parse the build file format instead of full Starlark. See https://docs.rs/starlark/0.3.0-
                        pre/starlark/eval/index.html#build_file
    -h, --help          Prints help information
    -r, --repl          Run a REPL after files have been parsed.
    -V, --version       Prints version information

OPTIONS:
    -c <command>        Starlark command to run after files have been parsed.

ARGS:
    <file>...    Files to interpret

Development

Build

This project build with Cargo. Simply run cargo test to test it, cargo build --release to build a release version and cargo run to run the command-line interpreter.

Possible improvements and optimizations

• Errors:
  • When an identifier is not found, we can suggest close identifier / keyword.
  • Fix suggestions maybe?
  • Better error spans.
  • Recoverable errors (don't stop at the first error, continue parsing).
• Evaluation:
  • Static rewrite of the AST before evaluation (e.g. for constant values)
• Awesome feature:
  • Implement a debugging protocol server side (compatible with the Java one, see (bazelbuild/vscode-bazel#6)).