6f746f3f8c
I was thinking today about how the | operator is way briefer than writing a comprehension in a collection, but writing the comprehension is nice to format and | is awkward. Also when writing down the type of the | operator ... it's a challenge (also to typecheck it). Perhaps, adding a spread/unpack operator will resolve both issues. It would be slightly more verbose, but not terribly so. And I believe it would be more readable too. |
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| etc | ||
| examples | ||
| fuzz | ||
| golden | ||
| ideas | ||
| src | ||
| tools | ||
| .gitignore | ||
| .gitmodules | ||
| Cargo.lock | ||
| Cargo.toml | ||
| CONTRIBUTING.md | ||
| flake.lock | ||
| flake.nix | ||
| LICENSE | ||
| mkdocs.yml | ||
| README.md | ||
| rust-toolchain.toml | ||
Ruud’s Configuration Language
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.
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.
Getting started
See the manual for more information. The most useful chapters to get started:
You may also find the examples in the examples directory instructive.
Rationale
Why another config language?
-
HCL is too ad-hoc to be suitable for any serious abstraction (
setunionis variadic so it only works with a statically known number of sets;flattenrecursively flattens so it can’t be typed properly and breaks generic code, for comprehensions can’t do nested loops,for_eachsyntax is bolted on, etc.) -
Nix-the-language is great but forces the entire Nix store on you when all I want is to evaluate expressions.
-
Python is great but requires some boilerplate for doing the IO if you want to use it as a configuration language. Also the syntactic order of list comprehensions prevents autocomplete in editors.
-
Dhall has the right ideas but the syntax and heavy use of Unicode symbols make it look ugly.
-
CUE and Nickel were not invented here.
Classification
-
Purely functional: RCL consists of expressions and has no statements. It has immutable values and no mutable objects. Functions are values.
-
Gradually typed: Optional type annotations can be used to prevent bugs and to make code more self-documenting. All type annotations are enforced.
-
Vaporware, not fully implemented: RCL is a superset of json.
Usage
Build:
cargo build --release
Print usage:
target/release/rcl
target/release/rcl eval --help
Evaluate an RCL expression to json:
target/release/rcl eval examples/tags.rcl
Query an RCL or json document:
target/release/rcl query examples/tags.rcl input.tags.ams01
Autoformat an RCL expression (non-destructive, prints to stdout):
target/release/rcl fmt examples/tags.rcl
Highlight an RCL expression in your terminal:
target/release/rcl highlight examples/tags.rcl
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 src/grammar.y
Run unit tests and lints:
cargo test
cargo clippy
Typecheck Python sources
mypy --strict .
Check formatting:
cargo fmt
black .
View coverage of the golden tests:
nix build .#coverage --out-link result
xdg-open result/index.html
Run the fuzzer:
cargo +nightly-2023-06-03 fuzz run main -- -dict=fuzz/dictionary.txt
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.