# Starlark — Configuration Language for Build Systems

> A deterministic dialect of Python designed for configuration and build system definitions, used by Bazel, Buck2, and other build tools.

## Install

Save as a script file and run:

# Starlark — Configuration Language for Build Systems

## Quick Use
```python
# BUILD file example (Bazel)
load("@rules_python//python:defs.bzl", "py_binary")

py_binary(
    name = "hello",
    srcs = ["hello.py"],
    deps = [":lib"],
)
```

## 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

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Source: https://tokrepo.com/en/workflows/asset-3ff0e9fb
Author: Script Depot