Decl

Describe

Types, defaults, derived members, and constraints live in one flat schema. No domain keywords — graphs, configs, and hardware are all just records, references, and rules.

Generate

Comprehensions and derivations turn a description into canonical JSON, byte-identical on every run and every implementation — and, with @render, into YAML, indented JSON, or text from a template, one file per element.

Validate

Bind external documents to the same types. Every failure is a diagnostic with a path, a code, and a root cause — never a cascade.

One file, three capabilities

services.decl

// describe: types carry the value rules
type Service = {
    name: /[a-z][a-z0-9-]*/
    port?: 1024..65535 = 8080
    replicas?: 1..64 = 1
    timeout?: quantity<Time> = 250ms
    endpoint = `${name}:${port}`   // derived, never supplied
    assert grpc_ports: name != "grpc" || port >= 9000
        else warn `grpc convention is 9000+`
}

// generate: values from the description
export output demo: Service[] = [
    { name: "gateway" }
    { name: "auth", port: 9001, replicas: 2 }
]

// validate: bind external documents to the same schema
input deployed: Service[]

decl evaluate services.decl

{
  "demo": [
    {
      "name": "gateway",
      "port": 8080,
      "replicas": 1,
      "timeout": {
        "value": 0.25,
        "unit": "s"
      },
      "endpoint": "gateway:8080"
    },
    {
      "name": "auth",
      "port": 9001,
      "replicas": 2,
      "timeout": {
        "value": 0.25,
        "unit": "s"
      },
      "endpoint": "auth:9001"
    }
  ]
}

The same Service type that generated demo validates deployed documents from the outside world:

decl validate services.decl --input deployed=prod.json

Install

npm

npm install -g decl-lang

The TypeScript reference implementation: decl and the decl-lsp language server (Node.js ≥ 22).

pip

pip install decl-lang

The native Python implementation: decl, decl-lsp, and a Python API — no Node.js.

cargo

cargo install decl-lang

The native Rust implementation: decl and decl-lsp with no Node or wasm.

brew

brew install luuvish/tap/decl-lang

The npm package through a Homebrew tap.

Tools

The same language server, formatter, and evaluator in every implementation; the editor extensions and the REPL are clients of them.

decl check services.decl                     # the static checks, and nothing else
decl fmt --check services.decl               # the canonical form
decl repl services.decl                      # a session over the module: expressions, :type, :trace, undo
decl-lsp --stdio                             # what VS Code, Zed, Neovim, Helix, and Emacs talk to

Command linecheck, evaluate, validate, fmt — universes, bound inputs, the --json report, exit codes.

REPLA session over the evaluation universe: partial evaluation, session outputs, document edits with exact undo.

Language serverDiagnostics, hover, completion, navigation, rename, code actions, semantic tokens — the same answers from all three servers.

Editor extensionsVS Code and Zed extensions; configurations for Neovim, Helix, Emacs, Vim, and Sublime.

Read on

Decl by exampleOne scenario end to end — describe, generate, validate — with the JSON and diagnostics it produces.

SpecificationThe normative specification: types, expressions, constraints, modules, semantics, grammar, errors, and the standard library.

ExamplesA hardware interconnect, an API config schema, and a fixture sweep — written with no domain keywords.

DesignWhy the language is shaped this way: vision, requirements, and the decision charter.