Rust / crates.io

Decl is a declarative language for describing, generating, and validating structured data — a JSON superset with a strong static type system, constraints with first-class diagnostics, references, physical quantities, generics, and modules. Pure, deterministic, terminating.

This crate is the native Rust implementation of the whole language: the tree-sitter grammar is compiled in, decl checks, evaluates, validates, and formats modules (packages included), and decl-lsp is the language server — no Node.js or wasm involved.

cargo install decl-lang

Command line

decl check schema.decl                         # static checks, module-aware (exit 1 on errors)
decl evaluate site.decl                        # evaluate every output -> {"name": value, ...}
decl evaluate site.decl --output site          # one output -> its canonical JSON on stdout
decl evaluate site.decl --json                 # {"ok", "value", "diagnostics"} report
decl evaluate cfg.decl --input deployed=doc.json --output deployed=out.json   # bind a document, write its completed value
decl evaluate cfg.decl --input deployed=doc.yaml --format yaml --pretty      # a document in YAML in, the outputs as YAML out
decl evaluate site.decl --output gateway                # a root in the form its @render declares: YAML, a template's text, one file per element
decl evaluate site.decl --output units=out --template units=unit.j2   # …or with the template and the destination given here
decl validate cfg.decl --input deployed=doc.json   # bind a document to an input root (diagnostics only)
decl validate tests/validation                 # judge a fixture corpus
decl fmt src/*.decl                            # canonical formatting in place (--check: exit 1 if not canonical)
decl repl site.decl                            # an interactive session: expressions, bindings, edits, undo
decl-lsp                                       # language server over stdio

Diagnostics go to stderr as file: severity [code] id at path: message, or into the JSON report with --json. The exit code is 1 when any error was reported. An output declares the form it is emitted in with @render({ format, indent, template, file, each }) — canonical or indented JSON, YAML, or the text of a template in a small Jinja-like dialect with Decl expressions inside, one file per element — and --format, --indent, --template, --output override it (docs/tooling/05_render.md in the repository).

Library

use decl_lang::{check, evaluate, format_source, render, to_yaml, validate, DeclError, Document, EvaluateOptions, RenderOptions, Rendered, TemplateSource};

fn example() -> Result<(), DeclError> {
    // the exported outputs, by name in declaration order, as canonical JSON text
    let docs = evaluate("site.decl", &EvaluateOptions::default())?;
    let site = &evaluate("site.decl", &EvaluateOptions { outputs: vec!["site".into()], ..Default::default() })?["site"];
    // a document bound to an input root, and that root returned completed (a .yaml file is read as YAML)
    let done = evaluate("cfg.decl", &EvaluateOptions {
        inputs: vec![("deployed".into(), Document::File("doc.yaml".into()))], // or Document::Json(text)
        outputs: vec!["deployed".into()],
    })?;
    // each root in the form its @render declares: a text, or a fan-out root's files by path
    let texts = render("site.decl", &RenderOptions::default())?;
    if let Rendered::Text(yaml) = &texts["site"] { println!("{yaml}"); }
    let conf = render("site.decl", &RenderOptions {
        outputs: vec!["site".into()],
        templates: vec![("site".into(), TemplateSource::File("nginx.conf.j2".into()))], // or TemplateSource::Text(text)
        ..Default::default()
    })?;
    let problems = check(&["schema.decl"]); // empty when clean
    let report = validate("cfg.decl", &[("deployed".into(), Document::Json("{\"host\":\"h\"}".into()))])?;
    let text = format_source("const x=1+2\n")?; // "const x = 1 + 2\n"
    let y = to_yaml("{\"a\":[1,2]}", 2)?; // "a:\n  - 1\n  - 2" — the layouts, as pure functions over JSON text
    assert_eq!(text, "const x = 1 + 2\n");
    Ok(())
}

The functions are the decl command line in its own vocabulary: inputs binds documents by input name (a JSON or YAML file, or JSON text), outputs names the roots to return — outputs, or inputs bound here or demanded through their fallback — and defaults to the entry module’s exported outputs; render returns each root’s text in its declared form (Rendered::Text, or Rendered::Files by path for a fan-out root) with RenderOptions as the overrides, to_json and to_yaml lay a document’s text out; a failure is a DeclError whose diagnostics carry the report. The npm package (evaluate, …) and the PyPI package (decl.evaluate, …) offer the same functions with the same semantics; the modules the functions are built from are public as well.

Library layout

The crate is the same modules as the reference implementation, one language rule in the same-named file everywhere (AGENTS.md in the repository), and each is public:

Module	Holds
api	the high-level API above: evaluate, render, check, validate, evaluate_source, format_source, to_json, to_yaml, DeclError, Diagnostic, Document, EvaluateOptions, RenderOptions, Rendered, TemplateSource, Report
ast	the syntax tree (specification chapter 11): declarations, types, members, expressions, source ranges
parse	the tree-sitter binding: parse_source, source text to ast
semantics	values, the environment (Env), resolved types (RT), diagnostics (Diag), canonical paths, the JSON reader and writers
yaml	documents in YAML: the YAML 1.2 core-schema reader into the JSON model, the block-style writer, the JSON layouts
render	the renderer: @render’s form, the template dialect, the fan-out — one root to its text or files
subsume	the subsumption judgment ⊑ (§3.17) and structural emptiness (§3.19)
infer	expression inference and the static assignability of §4
checker	the static checks of a module (check_module)
engine	binding, lazy evaluation, validation, serialization (Engine)
pipeline	one module end to end (run_pipeline, evaluate_source)
module	modules and the universe (load_modules, run_universe)
package	manifests, the resolver, the lock file (§8.6–8.7)
fmt	the canonical formatter (format)
conformance	the fixture corpus judge (judge_corpus)
session	the evaluation session behind the REPL and the server (Session)
repl	decl repl
lsp	decl-lsp
cli	decl

Scope

The crate covers the whole language: parsing, the static checks of chapters 3–4 (type resolution with generics and dimension algebra, inference, assignability, the absence discipline, match exhaustiveness), binding with lazy slots and cycle detection, $referrers, assertions with diagnostic templates, canonical serialization, modules and packages (decl.toml, decl.lock), the canonical formatter, and the language server. Its output is byte-identical to the reference implementation — the repository’s parity harness (tests/parity/differential.py, run by make verify) diffs the Rust and Python runtimes against the reference over every example and fixture that produces output.

Building from source

The grammar’s C sources are compiled by build.rs. Inside the repository they come from ../tree-sitter-decl/src; the published crate carries a copy under grammar/ (generated by npm run build in decl-ts/).

cargo build --release                       # from the repository root (Cargo workspace)
./target/release/decl evaluate docs/examples/02_config.decl --output prod

License

MIT — see LICENSE.