Ruud’s Configuration Language, RCL for short, is a domain-specific language optimized for specifying human-written data with just enough abstraction features to avoid repetition. It is a superset of json that extends it into a simple functional programming language that resembles Python and Nix. Use cases include:

  • Querying json documents, like jq, but with a more familiar language.
  • Generating repetitive configuration files, such as GitHub Actions workflows or Terraform configuration.
  • Enabling large repositories to split configuration into small reusable pieces that can be referenced from a single consistent entry point, in the same way that Nix enables this for Nixpkgs.
  • Sharing configuration between tools that do not natively share data. For example, import the same user account definitions into Terraform, Tailscale, Kubernetes, and Ansible.

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

Warning: While RCL is usable, it is still in an early exploratory stage with frequent breaking changes. This is a hobby project without stability promise.


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


// Configuration that defines cloud storage buckets for storing backups of two
// databases, "alpha" and "bravo", at three different frequencies. This schema
// is for a hypothetical infrastructure-as-code tool. See the tutorial for more
// details.
  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 },