About Koto

Koto is a simple and expressive programming language, usable as an extension language for Rust applications, or as a standalone scripting language.

print 'Hello, World!'
# -> Hello, World!

square = |n| n * n
'8 squared is {square 8}'
# -> 8 squared is 64

(2, 4, 6, 8)
  .each square
# -> [4, 16, 36, 64]


Koto was started in 2020 with the goal to create an ideal language for adding scripting to applications developed in Rust. Of particular interest were interactive systems like animation or game engines, where rapid iteration demands a lightweight programming interface that compiles and runs quickly.

The guiding design principle is that Koto should be simple, conceptually as well as visually. To that end, a focus throughout the language's development has been on reducing syntax noise and minimizing core concepts wherever possible.

Current State

Koto is a new language and should be considered to have a prominent 'use at your own risk' disclaimer.

With that said, Koto is starting to feel more stable, and although we're still some way from a 1.0 release, breaking changes are becoming much less frequent.

Early adopter feedback is invaluable, so if you do try out Koto please get in touch and share your experiences, positive or otherwise! You're welcome to reach out in Discussions, or on Discord, or by opening an issue.

You can read the guide, try it out in the playground or the CLI, and see how well it works in your existing Rust application.


  • Simple and clean syntax: Koto aims to reduce visual noise and cognitive load wherever possible, while still enabling full intuitive control of your program.
  • Easy integration with Rust: Koto is implemented in Rust, and is designed to be easily added to existing applications. Custom value types can be added to the Koto runtime by implementing the KotoObject trait.
  • Fast compilation: The compiler has been written with rapid iteration in mind, with the goal of compiling a script as quickly as possible.
  • Rich iterator support: Koto has a focus on using iterators for data manipulation, with a large collection of iterator generators, adaptors, and consumers available in the core library's iterator module.
  • Built-in testing: Automated testing has first-class support in Koto, making it natural to write tests along with your code.

Missing/Incomplete Features

  • Type Checking: Koto is currently a dynamically typed language without support for type checking, but type hints could be added in the future.
  • async tasks: Koto doesn't have support for asynchronous tasks, support could be added in the future.
  • Integration with other languages: There's currently no C API for Koto, which would allow it to be integrated with other languages.
  • Tooling: Some basic editor support is available for Koto, but modern tooling like an LSP implementation, auto-formatting, or linting are still topics for the future.


Basic editor support is available for Koto: