Getting started

Lemma is a pure, declarative language for business rules that stakeholders can read and systems can evaluate precisely. Rules live in Specs: named collections of Data (inputs) and Rules (computed outputs). Lemma validates every Spec before evaluation; after that, each Rule returns a value or a Veto (no value). Explanations are opt-in (lemma run -x / explain: true).

This chapter walks through installation, your first Spec, and running it from the CLI.

Install

Install the CLI from crates.io/crates/lemma:

cargo install lemma

Or via npm:

npm install -g lemma

For library bindings, WASM, editor support, and Docker, see Installation. To embed Lemma in your application, see Tools & SDKs.

Your first Spec

Create shipping.lemma:

spec shipping

data money: measure
  -> unit eur 1.00
  -> unit usd 0.91
  -> decimals 2
  -> minimum 0 eur

data weight: measure
  -> unit kilogram 1
  -> unit gram 0.001

data is_express:     true
data package_weight: 2.5 kilogram

rule express_fee: 0 eur
  unless is_express then 4.99 eur

rule base_shipping: 5.99 eur
  unless package_weight > 1 kilogram  then 8.99 eur
  unless package_weight > 5 kilogram  then 15.99 eur

rule total_cost: base_shipping + express_fee

Three building blocks appear here:

• spec names the Rule set.
• data defines inputs with types and constraints (-> unit, -> decimals, -> minimum).
• rule computes values; unless clauses add conditional logic (the last matching condition wins).

There is no inline comment syntax. The only in-source documentation is a commentary block: triple-quoted """...""" placed immediately after the Spec line. The syntax relies on keywords instead of colons or brackets. Indentation is for readability only; newlines are optional. Apply the standard format with:

lemma format

Run it

lemma run shipping

Output:

┌───────────────┬───────────┐
│ base_shipping ┆ 8.99 eur  │
├───────────────┼───────────┤
│ express_fee   ┆ 4.99 eur  │
├───────────────┼───────────┤
│ total_cost    ┆ 13.98 eur │
└───────────────┴───────────┘

Override Data from the command line:

lemma run shipping is_express=false package_weight="6.0 kilogram"

Output:

┌───────────────┬───────────┐
│ base_shipping ┆ 15.99 eur │
├───────────────┼───────────┤
│ express_fee   ┆ 0.00 eur  │
├───────────────┼───────────┤
│ total_cost    ┆ 15.99 eur │
└───────────────┴───────────┘

Other useful flags:

lemma run shipping --json

Machine-readable output.

lemma run shipping -x

Show how each Rule was evaluated (human reasoning table; with --json, per-rule explanation objects per explanation.v1.json).

lemma show shipping

Show the static interface (data types, constraints, and rules reachable after normalize). Overlay-aware needs for a concrete run come from each rule's missing_data; static types and suggestions are on lemma show only.

See CLI reference for all commands and flags.

Next up

Specs, Data, and Rules: how Specs are structured, open inputs, constraints, and Rule references.