Starlark

Starlark is a small, deterministic, curly‑brace–free configuration language that was created as a sandboxed subset of Python. It was designed to be easy to embed in build systems and other tools while ensuring reproducible, side‑effect‑free evaluation. Starlark originated in 2014 as part of Google's Bazel build system, where it replaced the older Skylark language (also known as Starlark). The language is intentionally minimal: it supports immutable basic types such as integers, floats, strings, lists, tuples, and dictionaries, but it does not allow arbitrary code execution, dynamic imports, or variable modification.

The syntax closely resembles CPython, which makes the learning curve short for developers familiar with Python. Function definitions, control flow statements like if/else and for loops, and comprehensions are available in the same style as in Python, but Starlark's built‑in functions and data structures are purposely limited to maintain sandboxing. The language is statically typed at runtime, performing type checks when a value is assigned to a variable that was previously seen with a different type. Notably, assignments are allowed only to new, unbound names; rebinding is prohibited, ensuring that scripts do not leak state between different build or rule evaluations.

Starlark is primarily used as the macro and extension language in Bazel, where BUILD files are written in Starlark to declare targets, dependencies, and custom rules. Other build systems and tools have adopted Starlark for similar purposes, including Buck, Pants, and the Google Cloud Build API. Because Starlark is sandboxed and its semantics are deterministic, it enables reproducible builds and simplifies dependency analysis, which is why it has become a common choice for build configuration in large-scale software projects.