Starlark

Starlark
Paradigmscripting, procedural (imperative)
First appeared2015; 11 years ago (2015)
Typing disciplineDynamic
OSCross-platform
Filename extensions.star
Websitegithub.com/bazelbuild/starlark/
Major implementations
starlark-go, starlark-rust,
Influenced by
Python

Starlark is a lightweight, high-level programming language designed for embedded use in applications. It uses a subset of the Python syntax. By default, the code is deterministic and hermetic[further explanation needed].

History

Starlark was released in 2015 as part of Bazel under the name Skylark. This first implementation was written in Java. In 2018, the language was renamed Starlark.

In 2017, a new implementation of Starlark in Go was announced.

In 2021, Meta announced an implementation of Starlark written in Rust, to be used for the Buck build system.

Popularity

In addition to the Bazel and Buck build systems, Starlark is used by dozens of projects, including Isopod, skycfg, Uber's Starlark Worker, and Tilt.

On GitHub, Starlark is among the top 50 languages based on the developer activity.

Syntax

Starlark syntax is a strict subset of Python syntax. Similar to Python syntax, Starlark relies on indentation to delimit blocks, using the off-side rule.

Statements and control flow

Starlark's statements include:

  • The = statement to assign a value to a variable
  • The augmented assignment statements to modify a variable
  • The if statement to execute conditionally a block of code (with else or elif)
  • The for statement to iterate over an iterable object
  • The def statement to define a function
  • The break statement to exit a loop
  • The continue statement to skip the rest of the current iteration and continues with the next
  • The pass statement, serving as a NOP, syntactically needed to create an empty code block
  • The return statement to return a value from a function.
  • The load statement, which replaces Python import, to import a value from another module. Unlike Python, the order of load statements does not affect the semantics of the code.

Unlike Python, Starlark statements don't include: while, try, raise, class, with, del, assert, yield, import, match and case.

Freezing

To ensure thread safety and support parallel computing, Starlark has a feature called freezing. At the end of the evaluation of a module, all values become immutable. This means that the values that can be accessed from multiple threads can no longer be modified, which removes the risk of race conditions.

References

  1. 1 2 3 4 5 "starlark/spec.md at master · bazelbuild/starlark". GitHub.
  2. ↑ Le Brun, Laurent (January 2024). "An Overview of the Starlark language". Retrieved 1 September 2025.
  3. 1 2 "A glimpse of the design of Skylark". blog.bazel.build.
  4. ↑ "Starlark". blog.bazel.build.
  5. ↑ Donovan, Alan (18 November 2017). A Go implementation of the Skylark Configuration Language. GothamGo 2017 – via YouTube.
  6. ↑ Mitchell, Neil (8 April 2021). "The Rust Starlark library".
  7. ↑ "Meta open-sources 'significantly faster' build system". InfoWorld.
  8. ↑ "5 Things you didn't know about Buck2". 23 October 2023.
  9. ↑ Antoniucci, Javier (2024). Ultimate Monorepo and Bazel for Building Apps at Scale: Level up Your Large-Scale Application Development with Monorepo and Bazel for Enhanced Productivity, Scalability, and Integration (English Edition) (1st ed.). Orange Education PVT Ltd. ISBN 9788197223914.
  10. ↑ "Starlark Programming Language". starlark-lang.org.
  11. ↑ Pandey, Mohit (12 December 2024). "Starlark is Basically Python, But Not Really Python, and That's Fine". AIM. Retrieved 13 December 2024.
  12. ↑ Xu, Charles; Ilyevskiy, Dmitry (2019). Isopod: An expressive DSL for Kubernetes configuration. Proceedings of the ACM Symposium on Cloud Computing. doi:10.1145/3357223.3365759.
  13. ↑ Norton, Peter (2019). "Other Faces of Python" (PDF). Login Usenix Mag. 44 (2).
  14. ↑ "Open-Sourcing Starlark Worker: Define Cadence Workflows with Starlark". uber.com.
  15. ↑ Sayfan, Gigi (2019). Hands-on microservices with Kubernetes: build, deploy, and manage scalable microservices on Kubernetes (1st ed.). Packt Publishing. p. 353. ISBN 9781789809732.
  16. ↑ "Global Metrics: Programming Languages". Innovation Graph. GitHub.
  17. ↑ "Languish - Programming Language Trends".
  18. ↑ "starlark/spec.md at master · bazelbuild/starlark". GitHub.
  19. ↑ "starlark/spec.md at master · bazelbuild/starlark". GitHub.
  20. ↑ Le Brun, Laurent (December 2024). "A practical introduction to the Starlark language". Retrieved 24 March 2025.
  21. ↑ "starlark/spec.md at master · bazelbuild/starlark". GitHub.
  22. ↑ "starlark/spec.md at master · bazelbuild/starlark". GitHub.