A declarative language for business rules

Lemma is a pure, declarative and open source programming language for business rules, that is easy to read for non-technical stakeholders. Rules are written in Specs that humans can read and systems can evaluate precisely and deterministically. Lemma validates Rules before they can be evaluated. Errors are impossible: Rules return a value or Veto, and optionally an explanation when you ask for one (--explain / explain: true).

spec pricing 2026-01-01
"""
Applies a tiered discount and VAT to the base price, 
effective from 2026-01-01.
"""

data base_price: 100
data is_member:  false
data quantity:   number

rule discount: 0%
  unless quantity >= 10  then 10%
  unless quantity >= 50  then 15%
  unless is_member       then 20%

rule discount_amount: base_price * discount
rule discounted_price: base_price - discount_amount

rule vat: discounted_price * 21%
rule total: discounted_price + vat

Get started

If you have Cargo installed, you can run:

cargo install lemma

If you are using npm:

npm install -g lemma

Precompiled binaries for Linux, macOS, and Windows are included in the npm package automatically. Docker images are also available. See Installation for all options.

Writing your first Spec

Write a .lemma file and run it. The Learn guide walks you through everything from your first Spec to composing Rules across time and registries. You'll be productive in minute.

Documentation

• Learn — guided path from first spec to composing specs
• Reference — operators, types, syntax, CLI, registry, test coverage, benchmarks
• Tools & SDKs — Rust, Elixir, JavaScript/TypeScript, and more
• Community — contributing and publishing specs