Maven

Embed the Lemma engine in Java, Kotlin, or Scala via com.lemmabase:lemma-engine on Maven Central. The JAR ships prebuilt lemma_jni natives for the same six targets as the Hex package (no musl).

Decimals use java.math.BigDecimal. float / double data values are rejected so binary floating point cannot silently corrupt exact magnitudes. See Precision.

Install

Maven:

<dependency>
  <groupId>com.lemmabase</groupId>
  <artifactId>lemma-engine</artifactId>
  <version>0.9.0</version>
</dependency>

Gradle (Kotlin DSL) — consume the published artifact; this repository builds the package with Maven only:

implementation("com.lemmabase:lemma-engine:0.9.0")

Requires JDK 17+.

Java example

import com.lemmabase.lemma.Engine;
import com.lemmabase.lemma.Response;
import com.lemmabase.lemma.RunRequest;
import com.lemmabase.lemma.RuleResult;
import java.math.BigDecimal;
import java.util.Map;

try (Engine engine = Engine.create()) {
  engine.load("""
      spec order
      data quantity: number
      data unit_price: number
      data tax_rate: number
      rule subtotal: quantity * unit_price
      rule tax: subtotal * tax_rate
      rule total: subtotal + tax
      """);

  Response response = engine.run(
      RunRequest.of("order")
          .data(Map.of(
              "quantity", 3,
              "unit_price", new BigDecimal("19.99"),
              "tax_rate", new BigDecimal("0.21"))));

  RuleResult total = response.results().get("total");
  BigDecimal amount = total.number();
}

Kotlin example

Same artifact:

import com.lemmabase.lemma.Engine
import com.lemmabase.lemma.RunRequest
import java.math.BigDecimal

Engine.create().use { engine ->
    engine.load("""
        spec order
        data quantity: number
        data unit_price: number
        data tax_rate: number
        rule subtotal: quantity * unit_price
        rule tax: subtotal * tax_rate
        rule total: subtotal + tax
        """.trimIndent())

    val response = engine.run(
        RunRequest.of("order").data(
            mapOf(
                "quantity" to 3,
                "unit_price" to BigDecimal("19.99"),
                "tax_rate" to BigDecimal("0.21"),
            )
        )
    )
    val total: BigDecimal = response.results()["total"]!!.number()!!
}

API surface

Method	Role
`Engine.create()` / `Engine.create(ResourceLimits)`	Construct engine
`load(String)` / `load(Map<String,String>)`	Validate and load sources (volatile or labeled)
`run(RunRequest)`	Evaluate; named fields only
`list()` / `show(...)` / `source(...)` / `remove(...)`	Inspect and manage loaded specs
`limits()`	Current resource limits
`Lemma.format(String)`	Format source without an engine
`close()`	Drop native engine (`AutoCloseable` + `Cleaner`)

RunRequest.of(spec) defaults: repository null, effective null, rules null (all rules), explain false. An empty rules list is a request error. With explain(true), RuleResult.explanation() is a JSON tree matching explanation.v1.json.

Errors and veto

Invalid Lemma or bad requests → unchecked LemmaException with WASM-shaped EngineError entries.
Domain veto → still a successful Response; check RuleResult.vetoed() / vetoReason().
Use-after-close or invariant failures → LemmaBugError.

Thread safety

One Engine instance; serialize access across threads (native side holds a mutex). Prefer one engine per worker, or external locking around a shared instance.

Development in this repository

cargo build -p lemma_jni
cd engine/packages/maven && ./mvnw test

cargo precommit builds lemma_jni and runs ./mvnw -B test.