Starlark types

[comius]

Support type annotations and type checking in Starlark

The first goal is: correctness and readability of Starlark code

Other goals are:

• improving efficiency of Starlark interpreter and
• Bazel tech-debt reduction

Roadmap:

Milestone 1 (Q4 2024) - MVP - Bootstraping Starlark types

Design doc: SEP-001 Bootstrapping Starlark types

Starlark specification is incrementally extended under the starlark-with-types branch: https://github.com/bazelbuild/starlark/blob/starlark-with-types/spec.md

Goal: Provide a minimally viable type system in Starlark

• Extend Starlark syntax with type annotations #24407
• Primitive types
• Unions, Collections, functions and constructed types
• Runtime/dynamic type checks
• Bazel/Starlark built-in types (struct)
• Launch MVP

Milestone 2 - Static type checking and type inference

Backlog

• Providers/classes
• Types for Rules & Symbolic macros
• Types for selects
• Types for Java-Starlark interface / Java annotations
• Support types in tooling & IDEs

[fmeum]

Prior art available right now: https://github.com/withered-magic/starpls?tab=readme-ov-file#tips-and-tricks

[withered-magic]

Hey, just came across this - the starpls type checker is definitely still a work in progress, but I do have plans to eventually make it invokable outside of the LSP (i.e. as a command-line tool, similar to how pyright offers a CLI) as well as to publish it as a library if we'd like to re-use some of the existing work here!

[comius]

Are there refences of Buck2's type system beyond https://github.com/facebook/starlark-rust/blob/main/docs/types.md?

[jin]

Related: static typechecking FR, and relevant pointers to earlier discussions #7468

(IIUC this bug aims to implement runtime typechecking, a different goal)

[sluongng]

@comius you could just see how they are used in Buck2's "prelude" (collection of starlark rules Meta opensourced)

https://github.com/search?q=repo%3Afacebook%2Fbuck2+path%3A%2F%5Eprelude%5C%2F%2F++language%3AStarlark+%2Fdef.*-%3E%2F&type=code