The Roc Programming Language

Roc is a language for making delightful software.

If you already know Elm, then Roc for Elm Programmers may be of interest.

If you're curious about where the language's name and logo came from, here's an explanation.

Getting started

1. Install Rust
2. Build from source
3. In a terminal, run this from the root folder:

   cargo run repl
   

4. Check out these tests for examples of using the REPL

Applications and Platforms

Applications are often built on a framework. Typically, both application and framework are written in the same language.

• Rails applications are written in Ruby, and so is Rails.
• Angular applications are written in TypeScript, and so is Angular.
• Phoenix applications are written in Elixir, and so is Phoenix.

Some programs support plugins. Often the plugins are written in the same language as the underlying program.

• Webpack plugins are written in JavaScript, and so is Webpack.
• Eclipse plugins are written in Java, and so is Eclipse.
• Leiningen plugins are written in Clojure, and so is Leiningen.

All of these can be considered examples of a platform/application relationship. There is an underlying platform, and many applications are built on top of it. (Plugins are a type of application in this sense.)

Sometimes, platforms and their applications are written in different languages.

• Neovim is written in C for performance, and its plugins can be written in languages such as Python, JS, and Ruby.
• NGINX is written in C for performance, and its plugins can be written in a subset of JavaScript.
• Unity is written in C++ for performance, and Unity applications (such as games) can be written in C#, Boo, or a JavaScript dialect called UnityScript.

Like in the previous examples, application authors building on these platforms get to use high-level languages with automatic memory management. They make no ergonomics sacrifices, and may not even be aware that the underlying platform is written in a lower-level language.

By using systems-level programming languages like C and C++, platform authors sacrifice development speed, but unlock the highest possible performance characteristics. This is a tradeoff many platform authors are happy to accept, for the sake of having applications built on their platforms run very fast.

Roc's Design

Roc is designed to make the "systems-level platform, higher-level application" experience as nice as possible.

• Application authors code exclusively in Roc. It's a language designed for nice ergonomics. The syntax resembles Ruby or CoffeeScript, and it has a fast compiler with full type inference.
• Platform authors code almost exclusively in a systems-level language like C, C++, Rust, or Zig, except for the thin Roc API they expose to application authors. Roc application code compiles to machine code, and production builds of Roc apps benefit from the same LLVM optimizations that C++, Rust, and Zig do. Roc application authors do not need to know this lower-level code exists; all they have to interact with is the platform's API, which is exposed as an ordinary Roc API.

Every Roc application is built on top of exactly one Roc platform. There is no such thing as a Roc application that runs without a platform, and there is no default platform. You must choose one!

The core Roc language and standard library include no I/O operations, which gives platform authors complete control over which effects they want to support. Some of the implications of this include:

• A high-performance build tool (or text editor) written in Rust can be a Roc platform with a strong plugin security model. For example, it could expose only operations allowing plugin authors to modify the contents of certain files, rather than allowing plugins arbitrary read/write access to the entire filesystem.
• A VR or Arduino platform can expose uncommon I/O operations supported by that hardware, while omitting common I/O operations that are unsupported (such as reading keyboard input from a terminal that doesn't exist).
• A high-performance Web server written in Rust can be a Roc platform where all I/O operations are implemented in terms of Streams or Observables rather than a more traditional asynchronous abstraction like Futures or Promises. This would mean all code in that platform's ecosystem would be necessarily built on a common streaming abstraction.

Each Roc platform gets its own separate package repository, with packages built on top of the API that platform exposes. This means each platform has its own ecosystem where everything is built on top of the same shared set of platform-specific primitives.

Project Goals

Roc is in relatively early stages of development. It's currently possible to build both platforms and applications (see the examples folder for some examples that aren't particularly organized at the moment), although documentation is in even earlier stages than the compiler itself.

Besides the above language design, a separate goal is for Roc to ship with an ambitiously boundary-pushing graphical editor. Not like "an IDE," but rather something that makes people say "I have never seen anything remotely like this outside of Bret Victor demos."

One of the reasons this editor is coupled with the language itself is to allow package authors to include custom editor tooling inside packages.

A trivial example: suppose I'm writing a Roc app for an Arduino platform. I install a platform-specific package for displaying text on a grid of LEDs. Because I've installed this package, at the call site where I call the function to specify the color of the text on the LEDs, my Roc editor displays an inline color picker. As I move a slider around to try out different colors, not only does my code change to reflect that value in realtime, but the physical LEDs in my room change color in realtime as well. As the application author, all I did to get that experience was to install the "text on an LED grid" package, nothing else.

The goal is for this to be one of the most trivial, bare minimum examples of what the editor experience would be like. Hopefully, people in the future will look back on this example and say "that's so embarrassingly basic; why didn't you talk about one of the actually great things in the seamless editor plugin ecosystem?"

Finally, some implementation goals:

• The web server for the package manager is written in Roc (with an underlying Rust platform for the web server, for example warp).
• The editor plugins are written in Roc (with an underlying Rust platform for the editor itself, for example using gfx-hal).
• The CLI (for building Roc projects on CI platforms) has its user interface written in Roc (with an underlying Rust platform for fast compilation and basic CLI interactions).

It's an ambitious project! It'll take a long time to get where it's going, but hopefully it'll be worth the wait.

Getting Involved

The number of people involved in Roc's development has been steadily increasing over time - which has been great, because it's meant we've been able to onboard people at a nice pace. (Most people who have contributed to Roc had previously never done anything with Rust and also never worked on a compiler, but we've been able to find beginner-friendly projects to get people up to speed gradually.)

If you're interested in getting involved, check out CONTRIBUTING.md which has more info and a link to our Zulip chat!