Lemma Engine

A language that means business.

Lemma Engine is the Rust crate behind the Lemma language. It lets you parse, validate, and evaluate Lemma docs from your own applications while keeping the same natural, auditable semantics that the CLI exposes.

Status

Lemma is still early-stage and not yet recommended for production use. Expect breaking changes, evolving semantics, and incomplete tooling while the project matures.

Why Lemma?

Readable by business stakeholders – rules look like the policies people already write
Deterministic and auditable – every evaluation returns a full trace explaining the result
Type-aware – dates, percentages, units, and automatic conversions are first-class
Composable – specs extend and reference each other without boilerplate
Multi-platform – use the engine from Rust, power the CLI/HTTP server, or ship via WebAssembly

Quick start

Add the crate:

[dependencies]
lemma-engine = "0.8.4"

Minimal example

use lemma::Engine;
use lemma::parsing::ast::DateTimeValue;
use std::collections::HashMap;

let mut engine = Engine::new();

engine.load(r#"
    spec compensation
    fact base_salary: 60000
    fact bonus_rate: 10%
    rule bonus: base_salary * bonus_rate
    rule total: base_salary + bonus
"#, Some("example.lemma"))?;

let now = DateTimeValue::now();
let response = engine.run("compensation", Some(&now), HashMap::new())?;

for result in response.results {
    if let Some(value) = result.result {
        println!("{}: {}", result.rule_name, value);
    }
}

Providing values at runtime

use lemma::Engine;
use lemma::parsing::ast::DateTimeValue;
use std::collections::HashMap;

let mut engine = Engine::new();

engine.load(r#"
    spec shipping

    fact weight: 5 kilogram
    fact destination: "domestic"

    rule rate: 10
      unless weight > 10 kilogram           then 15
      unless destination is "international" then 25

    rule valid: weight <= 30 kilogram
      unless veto "Package too heavy for shipping"

"#, Some("example.lemma"))?;

let mut values = HashMap::new();
values.insert("weight".to_string(), "12 kilogram".to_string());
values.insert("destination".to_string(), "international".to_string());

let now = DateTimeValue::now();
let response = engine.run("shipping", Some(&now), values)?;

Features

Rich type system – percentages, mass, length, duration, temperature, pressure, power, energy, frequency, and data sizes
Automatic unit conversions – convert between units inside expressions without extra code
Spec composition – extend specs, bind facts, and reuse rules across modules
Audit trail – every evaluation returns the operations that led to each result
WebAssembly build – npm install @benrogmans/lemma-engine to run Lemma in browsers and at the edge

Constraint-style inversion (what inputs would yield a given outcome?) is planned; it is not documented as a supported API yet.

Installation options

As a library

cargo add lemma-engine

CLI tool

cargo install lemma-cli
lemma run pricing quantity=10

HTTP server

cargo install lemma-cli
lemma server --port 8012

WebAssembly

npm install @benrogmans/lemma-engine

import { Lemma } from '@benrogmans/lemma-engine';
const engine = await Lemma();

Build: node build.js (from engine/packages/npm/). See packages/npm/README.md.

Documentation

Language guide: https://benrogmans.github.io/lemma/
API documentation: https://docs.rs/lemma-engine
Examples: https://github.com/benrogmans/lemma/tree/main/documentation/examples
CLI usage: https://github.com/benrogmans/lemma/blob/main/documentation/CLI.md

Use cases

Compensation plans and employment contracts
Pricing, shipping, and discount policies
Tax and finance calculations
Insurance eligibility and premium rules
Compliance and validation logic
SLA and service-level calculations

Contributing

Contributions are very welcome!

License

Apache 2.0

lemma-engine 0.8.4