Starlark — Configuration Language for Build Systems

Introduction

Starlark (formerly known as Skylark) is a configuration language designed by Google for use in build systems. It is a strict subset of Python with intentional restrictions — no classes, no exceptions, no mutation after freezing — that guarantee deterministic, hermetic evaluation suitable for parallel and cached builds.

What Starlark Does

• Defines build targets, rules, and dependencies in a Python-like syntax
• Guarantees deterministic evaluation for reproducible builds
• Provides sandboxed execution with no file I/O or network access
• Enables macro and rule authoring for custom build logic
• Powers configuration in Bazel, Buck2, Pants, and other build systems

Architecture Overview

Starlark is an interpreted language with a two-phase execution model: a loading phase that evaluates BUILD files and produces a dependency graph, and an analysis phase that resolves configurations. The language is frozen after loading — all values become immutable — preventing non-determinism. Implementations exist in Go (go.starlark.net), Java (within Bazel), and Rust.

Self-Hosting & Configuration

• Used automatically when writing BUILD or .bzl files for Bazel
• Go implementation available: go get go.starlark.net/starlark
• Rust implementation: starlark-rust crate for embedding
• REPL available via starlark command for interactive testing
• Linting via Buildifier for Bazel-specific Starlark files

Key Features

• Python-like syntax with near-zero learning curve for Python developers
• Guaranteed termination — no while loops or recursion allowed
• Hermetic execution with no side effects or I/O
• First-class functions and comprehensions for expressive configs
• Parallel-safe evaluation due to immutability after load phase

Comparison with Similar Tools

• CUE — Stronger typing and validation; Starlark is more flexible and imperative
• Jsonnet — Similar functional config language; Starlark has Python syntax
• Nickel — Contract-based with gradual typing; newer and less ecosystem support
• HCL (Terraform) — Domain-specific for infrastructure; Starlark is general-purpose for builds

FAQ

Q: Is Starlark the same as Python? A: No. It is a subset with no classes, exceptions, global state, or I/O. Most simple Python expressions work, but programs that depend on mutation or side effects do not.

Q: Can I use Starlark outside of Bazel? A: Yes. The Go and Rust implementations can be embedded in any application as a configuration or scripting language.

Q: Why does Starlark forbid while loops? A: To guarantee termination. All iteration uses for loops over finite sequences, ensuring every evaluation completes in bounded time.

Q: How do I debug Starlark code? A: Use the print() function for logging during evaluation, or use the interactive REPL. Bazel also provides --output=starlark for query debugging.

Sources

• https://github.com/bazelbuild/starlark
• https://bazel.build/rules/language