feat: exhaustive copy on write visitors

core/src/main/java/io/substrait/expression/FieldReference.java

@@ -24,6 +24,10 @@ public Type getType() {
     return type();
   }
 
+  public static ImmutableFieldReference.Builder builder() {
+    return ImmutableFieldReference.builder();
+  }
+
   public <R, E extends Throwable> R accept(ExpressionVisitor<R, E> visitor) throws E {
     return visitor.visit(this);
   }

core/src/main/java/io/substrait/relation/CopyOnWriteUtils.java

@@ -0,0 +1,59 @@
+package io.substrait.relation;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.Optional;
+import java.util.function.Supplier;
+
+/** Provides common utilities for copy-on-write visitations */
+public class CopyOnWriteUtils {
+
+  public static boolean allEmpty(Optional<?>... optionals) {
+    return Arrays.stream(optionals).noneMatch(Optional::isPresent);
+  }
+
+  /** The `or` method on Optional instances is a Java 9+ feature */
+  public static <T> Optional<T> or(Optional<T> left, Supplier<? extends Optional<T>> right) {
+    if (left.isPresent()) {
+      return left;
+    } else {
+      return right.get();
+    }
+  }
+
+  @FunctionalInterface
+  public interface TransformFunction<T, E extends Exception> {
+    Optional<T> apply(T t) throws E;
+  }
+
+  /**
+   * Applies the given transformation function to each item in the list. If any of the list items
+   * are transformed, returns a new list in which each item is either
+   *
+   * <ul>
+   *   <li>a transformed new item replacing an old item
+   *   <li>the original item in the position it was in
+   * </ul>
+   *
+   * @param items the list of items to transform
+   * @param transform the transformation function to apply to each item
+   * @return An empty optional if none of the items have changed. An optional containing a new list
+   *     otherwise.
+   */
+  public static <ITEM, E extends Exception> Optional<List<ITEM>> transformList(
+      List<ITEM> items, TransformFunction<ITEM, E> transform) throws E {
+    List<ITEM> newItems = new ArrayList<>();
+    boolean listUpdated = false;
+    for (ITEM item : items) {
+      Optional<ITEM> newItem = transform.apply(item);
+      if (newItem.isPresent()) {
+        newItems.add(newItem.get());
+        listUpdated = true;
+      } else {
+        newItems.add(item);
+      }
+    }
+    return listUpdated ? Optional.of(newItems) : Optional.empty();
+  }
+}