L4 with IDE
L4 is a domain-specific programming language for law. It formalizes legal rules and contracts as executable specifications, bringing software engineering rigor to legal drafting and analysis.
This repository contains the L4 compiler, IDE tooling (VS Code extension, LSP, REPL, web editor), and a decision service that exposes L4 rules as REST APIs and MCP tools.
What You Get From a Single L4 File
- REST APIs — Expose functions as HTTP endpoints with
@exportannotations - AI tool integration — MCP server for Claude, Cursor, and VS Code Copilot; WebMCP for in-browser agents
- Interactive visualizations — Ladder diagrams and evaluation traces rendered via GraphViz
- Audit-grade explainability — Every evaluation produces a trace you can follow from the top-level question down to the deciding condition
- Test suites — Golden-file tests and assertions
- Generated schemas — OpenAPI 3.0 specs and JSON schemas for integration
Getting Started
- Install the VS Code extension: L4 Rules-as-code on the Marketplace
- Try the web editor: https://jl4.legalese.com/
- Learn L4: Foundation Course — no prior programming experience required
- Full documentation: doc/README.md
Repository Layout
Haskell (Cabal):
| Package | Purpose |
|---|---|
| jl4-core | Core language (parser, typechecker, evaluator) |
| jl4 | CLI tool and JSON schema generator |
| jl4-lsp | Language Server Protocol for IDE support |
| jl4-repl | Interactive REPL |
| jl4-service | REST API for decision evaluation |
| jl4-websessions | Session persistence service |
| jl4-query-plan | Query planning utilities |
| jl4-actus-analyzer | Static analyzer classifying L4 contracts by ACTUS / FIBO |
| jl4-wasm | WebAssembly build of L4 for browser/Node.js |
| jl4-mlir | MLIR/WASM compiler backend: L4 → .wasm decision binary |
TypeScript (npm workspaces + Turborepo):
| Package | Purpose |
|---|---|
| ts-apps/vscode | VS Code extension |
| ts-apps/jl4-web | Web-based editor (Svelte) |
| ts-shared | Shared libraries (RPC client, visualizers) |
Documentation: See doc/ for the language reference, tutorials, courses, and concept guides.
Building From Source
cabal build all # Haskell
npm ci && npm run build # TypeScript
See AGENTS.md for repository conventions and workflow, and dev-start.sh for running services locally.
Requirements: GHC 9.10.2, Cabal 3.10+, Node.js ≥ 20, and GraphViz (dot). Nix users can run nix-shell nix/shell.nix for a ready environment.
The REPL
An interactive Read-Eval-Print Loop for exploring L4 code:
cabal run jl4-repl -- path/to/file.l4
The REPL provides live evaluation, module reloading (:load, :reload), query planning (:decides, :queryplan), and trace output (:trace, :traceascii, :tracefile). See jl4-repl/README.md for the full command list.
VS Code Extension
Provides syntax highlighting, type checking, inline evaluation, @export previews, deployment management, and an automatically-registered local MCP server that bridges Claude Code, Cursor, and VS Code Copilot to your L4 code. Connects to Legalese Cloud or a self-hosted jl4-service instance. See ts-apps/vscode/README.md.
Trace Visualization
Every tool in this repo can render an L4 evaluation as a GraphViz diagram:
- CLI:
l4 trace myfile.l4 > trace.dot, thendot -Tsvg trace.dot > trace.svg(orl4 trace myfile.l4 --format svg -o out/to generate SVG files directly) - REPL:
:trace <expression>, or:tracefile traces/sessionto capture numbered.dotfiles - Decision Service:
POST /deployments/{id}/functions/{fn}/evaluation?trace=full&graphviz=true
Install GraphViz with brew install graphviz or apt-get install graphviz to render PNG/SVG outputs.
Application Libraries
L4 ships with foundational libraries for building legal and commercial applications:
- Jurisdiction — ISO 3166 country codes, US states, Canadian provinces, EU
- Currency — ISO 4217 currency codes with integer minor-unit storage
- Legal Persons — individuals, corporations, partnerships, LLCs, trusts
- Holdings — ownership structures and beneficial ownership
Real-World Impact
L4 has been piloted with organizations in both public and private sectors:
- Government regulatory compliance — encoded secondary legislation to auto-generate citizen-facing web wizards; formal verification discovered a double-bind where contradictory clauses required and prohibited the same action.
- Insurance policy analysis — formalized contracts from major global providers, uncovering payout-formula ambiguities linked to significant claims leakage.
- Legislative drafting — working with government drafting offices on rules-as-code initiatives.
- Commercial agreements — transformed complex fee schedules and payment terms into L4, served via REST for enterprise integration.
Community
- Discord — chat with the community
- GitHub Issues — report bugs, request features
- Legalese — professional implementation services
L4 is published under the Apache-2.0 License.