---
nav_title: Rust
nav_order: 10
---
# Rust
`lemma-engine` is the engine itself. Crate name `lemma-engine`, imported as `lemma`.
## Install
```bash
cargo add lemma-engine --rename lemma
```
## Usage
```rust
use lemma::{DateTimeValue, Engine, SourceType};
use std::collections::HashMap;
use std::path::PathBuf;
use std::sync::Arc;
let mut engine = Engine::new();
engine.load([(
SourceType::Path(Arc::new(PathBuf::from("example.lemma"))),
r#"
spec compensation
data base_salary: 60000
data bonus_rate: 10%
rule bonus: base_salary * bonus_rate
rule total: base_salary + bonus
"#
.to_string(),
)])?;
let now = DateTimeValue::now();
let response = engine.run(
None,
"compensation",
Some(&now),
HashMap::new(),
None,
false,
)?;
for (rule_name, rule_result) in &response.results {
if !rule_result.vetoed {
println!("{rule_name}: {}", rule_result.display().unwrap_or(""));
}
}
```
## Providing values at runtime
```rust
use lemma::{DateTimeValue, Engine, SourceType};
use std::collections::HashMap;
use std::path::PathBuf;
use std::sync::Arc;
let mut engine = Engine::new();
engine.load([(
SourceType::Path(Arc::new(PathBuf::from("example.lemma"))),
r#"
spec shipping
data weight: 5 kilogram
data destination: "domestic"
rule rate: 10
unless weight > 10 kilogram then 15
unless destination is "international" then 25
rule valid: weight <= 30 kilogram
unless weight > 30 kilogram then veto "Package too heavy for shipping"
"#
.to_string(),
)])?;
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(
None,
"shipping",
Some(&now),
values,
None,
false,
)?;
```
## Show vs run discovery
`Engine::show` returns the static planning interface: data used by the spec's rules, plus local rule result types.
For requirements on a partial run, call `run` and inspect each rule's `missing_data` (`string[]` input keys). Types, prefilled literals, and `-> suggest` hints are on `Engine::show` (`Show.data` values are `ShowData`) only. Bound inputs (caller run bindings or spec prefilled) are omitted from `missing_data`; suggestions do not bind until supplied in `run`'s data. Non-veto rule results flatten `RuleResultValue` onto each result (`display()` / typed fields). Pass `explain: true` as the last `run` argument to attach per-rule explanation trees ([api.v1.json](../schemas/api.v1.json)).
```rust
let response = engine.run(
None,
"shipping",
Some(&now),
HashMap::new(),
Some(&["rate".to_string()]),
false,
)?;
for key in &response.results["rate"].missing_data {
println!("need: {key}");
}
```
## Embedded units stdlib
`Engine::new()` loads `repo lemma` / `spec units` at compile time (import with `uses lemma units`). It always appears in `Engine::list`. Formatted source: `engine.source(Some("lemma"), None, None)?`.
## API docs
Full Rust API documentation is published on [docs.rs](https://docs.rs/lemma-engine).