Add support for evaluating module output and expressions to ConfigEvaluator

pkl-config-java/src/main/java/org/pkl/config/java/Config.java

@@ -110,7 +110,7 @@ static Config fromPklBinary(InputStream inputStream) {
     return fromPklBinary(inputStream, ValueMapper.preconfigured());
   }
 
-  private static Config makeConfig(Object decoded, ValueMapper mapper) {
+  static Config makeConfig(Object decoded, ValueMapper mapper) {
     if (decoded instanceof Composite composite) {
       return new CompositeConfig("", mapper, composite);
     }

pkl-config-java/src/main/java/org/pkl/config/java/ConfigEvaluator.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -41,6 +41,12 @@ static ConfigEvaluator preconfigured() {
   /** Evaluates the given module source into a {@link Config} tree. */
   Config evaluate(ModuleSource moduleSource);
 
+  /** Evaluates the given module's {@code output.value} property into a {@link Config} tree. */
+  Config evaluateOutputValue(ModuleSource moduleSource);
+
+  /** Evaluates the Pkl expression represented as {@code expression} into a {@link Config} tree. */
+  Config evaluateExpression(ModuleSource moduleSource, String expression);
+
   /**
    * Releases all resources held by this evaluator. If an {@code evaluate} method is currently
    * executing, this method blocks until cancellation of that execution has completed.

pkl-config-java/src/main/java/org/pkl/config/java/ConfigEvaluatorImpl.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -34,6 +34,18 @@ public Config evaluate(ModuleSource moduleSource) {
     return new CompositeConfig("", mapper, module);
   }
 
+  @Override
+  public Config evaluateOutputValue(ModuleSource moduleSource) {
+    var value = evaluator.evaluateOutputValue(moduleSource);
+    return Config.makeConfig(value, mapper);
+  }
+
+  @Override
+  public Config evaluateExpression(ModuleSource moduleSource, String expression) {
+    var value = evaluator.evaluateExpression(moduleSource, expression);
+    return Config.makeConfig(value, mapper);
+  }
+
   @Override
   public ValueMapper getValueMapper() {
     return mapper;

pkl-config-java/src/test/java/org/pkl/config/java/ConfigTest.java

@@ -15,16 +15,20 @@
  */
 package org.pkl.config.java;
 
+import static org.assertj.core.api.Assertions.assertThat;
 import static org.pkl.core.ModuleSource.text;
 
+import org.junit.jupiter.api.Test;
+
 public class ConfigTest extends AbstractConfigTest {
   private static final ConfigEvaluator evaluator = ConfigEvaluator.preconfigured();
 
+  private static final String pigeonText =
+      "pigeon { age = 30; friends = List(\"john\", \"mary\"); address { street = \"Fuzzy St.\" } }";
+
   @Override
   protected Config getPigeonConfig() {
-    return evaluator.evaluate(
-        text(
-            "pigeon { age = 30; friends = List(\"john\", \"mary\"); address { street = \"Fuzzy St.\" } }"));
+    return evaluator.evaluate(text(pigeonText));
   }
 
   @Override
@@ -42,4 +46,20 @@ protected Config getPairConfig() {
   protected Config getMapConfig() {
     return evaluator.evaluate(text("x = Map(\"one\", 1, \"two\", 2)"));
   }
+
+  @Test
+  public void evaluateOutputValue() {
+    var valueConfig =
+        evaluator.evaluateOutputValue(
+            text(pigeonText + "\noutput { value = (outer) { pigeon { age = 99 } } }"));
+    var pigeon = valueConfig.get("pigeon").as(Person.class);
+    assertThat(pigeon.age).isEqualTo(99);
+  }
+
+  @Test
+  public void evaluateExpression() {
+    var addressConfig = evaluator.evaluateExpression(text(pigeonText), "pigeon.address");
+    var address = addressConfig.as(Address.class);
+    assertThat(address.street).isEqualTo("Fuzzy St.");
+  }
 }