Java / Kotlin

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>0.9.8</version>
</dependency>

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

implementation("com.lemmabase:lemma-engine:0.9.8")

Requires JDK 21+.

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

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 an ExplanationNode.Rule whose children are a sealed ExplanationNode tree. show(...) returns Show; each Show.data entry is ShowData. Non-veto RuleResult flattens RuleResultValue (display() / typed accessors).

Errors and veto

• Invalid Lemma or bad requests → unchecked LemmaException with EngineError entries (same wire shape as api.v1.json).
• Domain veto → still a successful Response; check RuleResult.vetoed() / vetoReason().
• Use-after-close or invariant failures → LemmaBugError.

Thread safety

Engine is thread-safe: every method acquires an internal ReentrantLock. Multiple threads may share one instance without external synchronization. Long-running evaluations block other callers on that lock, so prefer one engine per worker for throughput.

Native library loading

The JAR embeds lemma_jni natives. On first use, the SDK extracts the native for the current platform to a version-keyed cache and loads it. Override the automatic resolution:

The cache path is ~/.cache/lemma-jni/{version}-{triple}/. Override the cache root with the system property lemma.native.cache.dir.

ExplanationNode dispatch

ExplanationNode is a sealed interface. Each variant record implements ExplanationNode.type() returning its discriminator (rule, compose, data, data_unused, conversion, veto). Use pattern matching (Java 21+) or switch on type():

switch (node.type()) {
    case "rule" -> handleRule((ExplanationNode.Rule) node);
    case "compose" -> handleCompose((ExplanationNode.Compose) node);
    case "data" -> handleData((ExplanationNode.Data) node);
    case "data_unused" -> handleDataUnused((ExplanationNode.DataUnused) node);
    case "conversion" -> handleConversion((ExplanationNode.Conversion) node);
    case "veto" -> handleVeto((ExplanationNode.Veto) node);
    default -> throw new IllegalStateException("unknown explanation type: " + node.type());
}

LemmaType follows the same pattern with kind().