The RCL Configuration Language

Getting Started · Documentation · Changelog · Online Playground

RCL is a domain-specific language for generating configuration files and querying json documents. It is a superset of json that extends it into a simple, gradually typed, functional programming language that resembles Python and Nix.

RCL can be used through the rcl command-line tool that can export documents to json, yaml, toml, and more. It can also be used through a native Python module, with an interface similar to the json module.

About

RCL solves the following problems:

• Copy-pasted yaml blocks that differ by a single value.
• Broken configs due to whitespace and escaping footguns in templating engines.
• Drift between tools due to lack of a single source of truth.
• Struggling to remember jq syntax.

It does that as follows:

• A real language. Use variables, loops, imports, and functions to eliminate duplication.
• Familiar syntax. Have you used Python, TypeScript, or Rust before? Then you will find RCL obvious to read and natural to write.
• Generate rather than template. Manipulate data structures, not strings. Generate correct json, yaml, and toml.
• Built to integrate. Generate configs for tools that do not natively talk to each other, all from a single source of truth. Integrate with your existing build tools, use the Python module, or the built-in rcl build to update generated files.
• Gradual types. Add type annotations where they help to eliminate bugs and make code self-documenting, omit them in straightforward code.
• Built-in json queries. A language for manipulating structured data makes a pretty good query tool. Run map and filter pipelines straight from your command-line.

Example

Given this input:

{
  // Generate backup buckets for each database and retention period.
  backup_buckets = [
    let retention_days = { hourly = 4, daily = 30, monthly = 365 };
    for database in ["alpha", "bravo"]:
    for period, days in retention_days:
    {
      name = f"{database}-{period}",
      region = "eu-west",
      lifecycle_policy = { delete_after_seconds = days * 24 * 3600 },
    }
  ],
}

RCL generates:

{
   "backup_buckets": [
      {
         "name": "alpha-hourly",
         "region": "eu-west",
         "lifecycle_policy": { "delete_after_seconds": 345600 }
      },
      {
         "name": "alpha-daily",
         "region": "eu-west",
         "lifecycle_policy": { "delete_after_seconds": 2592000 }
      },
      // And 4 similar entries, omitted here for brevity.
   ]
}

For an interactive demo in your browser, see https://rcl-lang.org.

Getting started

After the interactive examples on the website, the manual is the best resource for further information. The most useful chapters to get started:

• Installation
• Tutorial
• Syntax guide

You may also find the examples in the examples directory instructive. Some helpful commands after a clone:

# Build
cargo build --release

# Print usage
target/release/rcl
target/release/rcl eval --help

# Evaluate an RCL expression to json
target/release/rcl eval --format=json examples/tags.rcl

# Query an RCL or json document
target/release/rcl query examples/tags.rcl input.tags.ams01

# Autoformat and highlight an RCL expression (non-destructive, prints to stdout)
target/release/rcl fmt examples/tags.rcl

Status

RCL is a hobby project without stability promise. It is usable and useful, well-tested, and well-documented, but also still experimental, and it may have breaking changes. Syntax highlighting is available for major editors like Vim, Emacs, Helix, and Zed.

Support RCL

One thing that holds RCL back from being useful to more people is the lack of widespread support for syntax highlighting on platforms such as GitHub. If RCL is useful to you, you can help by using RCL publicly in a GitHub repository to demonstrate traction. Use it seriously of course, please don’t game the metric. Other things you can help with are getting RCL packaged for your favorite package manager, and developing syntax highlighting for your favorite editor if it is not yet supported.

Development

Run all tests and checks below in one command:

nix flake check

Run golden tests:

cargo build
golden/run.py

Check the grammar for ambiguities:

bison -Werror=all grammar/bison/grammar.y

Run unit tests and lints:

cargo test
cargo clippy

Typecheck Python sources

mypy --strict --exclude pyrcl .
mypy --strict pyrcl

Check formatting:

cargo fmt
black .

View coverage of the golden tests:

nix build .#coverage --out-link result
xdg-open result/index.html

For how to run the fuzzers, see docs/testing.md.

Building WASM

See the readme in the wasm directory.

License

RCL is licensed under the Apache 2.0 license. It may be used in free software as well as closed-source applications, both for commercial and non-commercial use under the conditions given in the license. If you want to use RCL in your GPLv2-licensed software, you can add an exception to your copyright notice. Please do not open an issue if you disagree with the choice of license.