feat(core): handle struct-based UDT literals

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

@@ -151,6 +151,16 @@ public O visit(Expression.StructLiteral expr, C context) throws E {
     return visitFallback(expr, context);
   }
 
+  @Override
+  public O visit(Expression.UserDefinedAnyLiteral expr, C context) throws E {
+    return visitFallback(expr, context);
+  }
+
+  @Override
+  public O visit(Expression.UserDefinedStructLiteral expr, C context) throws E {
+    return visitFallback(expr, context);
+  }
+
   @Override
   public O visit(Expression.NestedStruct expr, C context) throws E {
     return visitFallback(expr, context);

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

@@ -693,21 +693,94 @@ public <R, C extends VisitationContext, E extends Throwable> R accept(
     }
   }
 
+  /**
+   * Base interface for user-defined literals.
+   *
+   * <p>User-defined literals can be encoded in one of two ways as per the Substrait spec:
+   *
+   * <ul>
+   *   <li>As {@code google.protobuf.Any} - see {@link UserDefinedAnyLiteral}
+   *   <li>As {@code Literal.Struct} - see {@link UserDefinedStructLiteral}
+   * </ul>
+   */
+  interface UserDefinedLiteral extends Literal {
+    String urn();
+
+    String name();
+
+    List<io.substrait.type.Type.Parameter> typeParameters();
+  }
+
+  /**
+   * User-defined literal with value encoded as {@link com.google.protobuf.Any}.
+   *
+   * <p>This encoding allows for arbitrary binary data to be stored in the literal value.
+   */
   @Value.Immutable
-  abstract class UserDefinedLiteral implements Literal {
-    public abstract ByteString value();
+  abstract class UserDefinedAnyLiteral implements UserDefinedLiteral {
+    @Override
+    public abstract String urn();
+
+    @Override
+    public abstract String name();
 
+    @Override
+    public abstract List<io.substrait.type.Type.Parameter> typeParameters();
+
+    public abstract com.google.protobuf.Any value();
+
+    @Override
+    public Type.UserDefined getType() {
+      return Type.UserDefined.builder()
+          .nullable(nullable())
+          .urn(urn())
+          .name(name())
+          .typeParameters(typeParameters())
+          .build();
+    }
+
+    public static ImmutableExpression.UserDefinedAnyLiteral.Builder builder() {
+      return ImmutableExpression.UserDefinedAnyLiteral.builder();
+    }
+
+    @Override
+    public <R, C extends VisitationContext, E extends Throwable> R accept(
+        ExpressionVisitor<R, C, E> visitor, C context) throws E {
+      return visitor.visit(this, context);
+    }
+  }
+
+  /**
+   * User-defined literal with value encoded as {@link
+   * io.substrait.proto.Expression.Literal.Struct}.
+   *
+   * <p>This encoding uses a structured list of fields to represent the literal value.
+   */
+  @Value.Immutable
+  abstract class UserDefinedStructLiteral implements UserDefinedLiteral {
+    @Override
     public abstract String urn();
 
+    @Override
     public abstract String name();
 
     @Override
-    public Type getType() {
-      return Type.withNullability(nullable()).userDefined(urn(), name());
+    public abstract List<io.substrait.type.Type.Parameter> typeParameters();
+
+    public abstract List<Literal> fields();
+
+    @Override
+    public Type.UserDefined getType() {
+      return Type.UserDefined.builder()
+          .nullable(nullable())
+          .urn(urn())
+          .name(name())
+          .typeParameters(typeParameters())
+          .build();
     }
 
-    public static ImmutableExpression.UserDefinedLiteral.Builder builder() {
-      return ImmutableExpression.UserDefinedLiteral.builder();
+    public static ImmutableExpression.UserDefinedStructLiteral.Builder builder() {
+      return ImmutableExpression.UserDefinedStructLiteral.builder();
     }
 
     @Override

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

@@ -306,13 +306,51 @@ public static Expression.NestedStruct nestedStruct(boolean nullable, Expression.
     return Expression.NestedStruct.builder().nullable(nullable).addFields(fields).build();
   }
 
-  public static Expression.UserDefinedLiteral userDefinedLiteral(
-      boolean nullable, String urn, String name, Any value) {
-    return Expression.UserDefinedLiteral.builder()
+  /**
+   * Create a UserDefinedAnyLiteral with google.protobuf.Any representation.
+   *
+   * @param nullable whether the literal is nullable
+   * @param urn the URN of the user-defined type
+   * @param name the name of the user-defined type
+   * @param typeParameters the type parameters for the user-defined type (can be an empty list)
+   * @param value the value, encoded as google.protobuf.Any
+   */
+  public static Expression.UserDefinedAnyLiteral userDefinedLiteralAny(
+      boolean nullable,
+      String urn,
+      String name,
+      java.util.List<io.substrait.type.Type.Parameter> typeParameters,
+      Any value) {
+    return Expression.UserDefinedAnyLiteral.builder()
+        .nullable(nullable)
+        .urn(urn)
+        .name(name)
+        .addAllTypeParameters(typeParameters)
+        .value(value)
+        .build();
+  }
+
+  /**
+   * Create a UserDefinedStructLiteral with Struct representation.
+   *
+   * @param nullable whether the literal is nullable
+   * @param urn the URN of the user-defined type
+   * @param name the name of the user-defined type
+   * @param typeParameters the type parameters for the user-defined type (can be an empty list)
+   * @param fields the fields, as a list of Literal values
+   */
+  public static Expression.UserDefinedStructLiteral userDefinedLiteralStruct(
+      boolean nullable,
+      String urn,
+      String name,
+      java.util.List<io.substrait.type.Type.Parameter> typeParameters,
+      java.util.List<Expression.Literal> fields) {
+    return Expression.UserDefinedStructLiteral.builder()
         .nullable(nullable)
         .urn(urn)
         .name(name)
-        .value(value.toByteString())
+        .addAllTypeParameters(typeParameters)
+        .addAllFields(fields)
         .build();
   }
 

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

@@ -312,14 +312,24 @@ public interface ExpressionVisitor<R, C extends VisitationContext, E extends Thr
   R visit(Expression.NestedStruct expr, C context) throws E;
 
   /**
-   * Visit a user-defined literal.
+   * Visit a user-defined any literal.
    *
    * @param expr the user-defined literal
    * @param context visitation context
    * @return visit result
    * @throws E on visit failure
    */
-  R visit(Expression.UserDefinedLiteral expr, C context) throws E;
+  R visit(Expression.UserDefinedAnyLiteral expr, C context) throws E;
+
+  /**
+   * Visit a user-defined struct literal.
+   *
+   * @param expr the user-defined literal
+   * @param context visitation context
+   * @return visit result
+   * @throws E on visit failure
+   */
+  R visit(Expression.UserDefinedStructLiteral expr, C context) throws E;
 
   /**
    * Visit a switch expression.

core/src/main/java/io/substrait/expression/proto/ExpressionProtoConverter.java

@@ -1,7 +1,5 @@
 package io.substrait.expression.proto;
 
-import com.google.protobuf.Any;
-import com.google.protobuf.InvalidProtocolBufferException;
 import io.substrait.expression.ExpressionVisitor;
 import io.substrait.expression.FieldReference;
 import io.substrait.expression.FunctionArg;
@@ -377,21 +375,51 @@ public Expression visit(
 
   @Override
   public Expression visit(
-      io.substrait.expression.Expression.UserDefinedLiteral expr, EmptyVisitationContext context) {
+      io.substrait.expression.Expression.UserDefinedAnyLiteral expr,
+      EmptyVisitationContext context) {
     int typeReference =
         extensionCollector.getTypeReference(SimpleExtension.TypeAnchor.of(expr.urn(), expr.name()));
     return lit(
         bldr -> {
-          try {
-            bldr.setNullable(expr.nullable())
-                .setUserDefined(
-                    Expression.Literal.UserDefined.newBuilder()
-                        .setTypeReference(typeReference)
-                        .setValue(Any.parseFrom(expr.value())))
-                .build();
-          } catch (InvalidProtocolBufferException e) {
-            throw new IllegalStateException(e);
-          }
+          Expression.Literal.UserDefined.Builder userDefinedBuilder =
+              Expression.Literal.UserDefined.newBuilder()
+                  .setTypeReference(typeReference)
+                  .addAllTypeParameters(
+                      expr.typeParameters().stream()
+                          .map(typeProtoConverter::toProto)
+                          .collect(java.util.stream.Collectors.toList()))
+                  .setValue(expr.value());
+
+          bldr.setNullable(expr.nullable()).setUserDefined(userDefinedBuilder).build();
+        });
+  }
+
+  @Override
+  public Expression visit(
+      io.substrait.expression.Expression.UserDefinedStructLiteral expr,
+      EmptyVisitationContext context) {
+    int typeReference =
+        extensionCollector.getTypeReference(SimpleExtension.TypeAnchor.of(expr.urn(), expr.name()));
+    return lit(
+        bldr -> {
+          Expression.Literal.Struct structLiteral =
+              Expression.Literal.Struct.newBuilder()
+                  .addAllFields(
+                      expr.fields().stream()
+                          .map(this::toLiteral)
+                          .collect(java.util.stream.Collectors.toList()))
+                  .build();
+
+          Expression.Literal.UserDefined.Builder userDefinedBuilder =
+              Expression.Literal.UserDefined.newBuilder()
+                  .setTypeReference(typeReference)
+                  .addAllTypeParameters(
+                      expr.typeParameters().stream()
+                          .map(typeProtoConverter::toProto)
+                          .collect(java.util.stream.Collectors.toList()))
+                  .setStruct(structLiteral);
+
+          bldr.setNullable(expr.nullable()).setUserDefined(userDefinedBuilder).build();
         });
   }
 

core/src/main/java/io/substrait/expression/proto/ProtoExpressionConverter.java

@@ -506,10 +506,36 @@ public Expression.Literal from(io.substrait.proto.Expression.Literal literal) {
         {
           io.substrait.proto.Expression.Literal.UserDefined userDefinedLiteral =
               literal.getUserDefined();
+
           SimpleExtension.Type type =
               lookup.getType(userDefinedLiteral.getTypeReference(), extensions);
-          return ExpressionCreator.userDefinedLiteral(
-              literal.getNullable(), type.urn(), type.name(), userDefinedLiteral.getValue());
+          String urn = type.urn();
+          String name = type.name();
+          List<io.substrait.type.Type.Parameter> typeParameters =
+              userDefinedLiteral.getTypeParametersList().stream()
+                  .map(protoTypeConverter::from)
+                  .collect(Collectors.toList());
+
+          switch (userDefinedLiteral.getValCase()) {
+            case VALUE:
+              return ExpressionCreator.userDefinedLiteralAny(
+                  literal.getNullable(), urn, name, typeParameters, userDefinedLiteral.getValue());
+            case STRUCT:
+              return ExpressionCreator.userDefinedLiteralStruct(
+                  literal.getNullable(),
+                  urn,
+                  name,
+                  typeParameters,
+                  userDefinedLiteral.getStruct().getFieldsList().stream()
+                      .map(this::from)
+                      .collect(Collectors.toList()));
+            case VAL_NOT_SET:
+              throw new IllegalStateException(
+                  "UserDefined literal has no value (neither 'value' nor 'struct' is set)");
+            default:
+              throw new IllegalStateException(
+                  "Unknown UserDefined literal value case: " + userDefinedLiteral.getValCase());
+          }
         }
       default:
         throw new IllegalStateException("Unexpected value: " + literal.getLiteralTypeCase());

core/src/main/java/io/substrait/extension/DefaultExtensionCatalog.java

@@ -22,6 +22,7 @@ public class DefaultExtensionCatalog {
       "extension:io.substrait:functions_rounding_decimal";
   public static final String FUNCTIONS_SET = "extension:io.substrait:functions_set";
   public static final String FUNCTIONS_STRING = "extension:io.substrait:functions_string";
+  public static final String EXTENSION_TYPES = "extension:io.substrait:extension_types";
 
   public static final SimpleExtension.ExtensionCollection DEFAULT_COLLECTION =
       loadDefaultCollection();
@@ -44,6 +45,8 @@ private static SimpleExtension.ExtensionCollection loadDefaultCollection() {
             .map(c -> String.format("/functions_%s.yaml", c))
             .collect(Collectors.toList());
 
+    defaultFiles.add("/extension_types.yaml");
+
     return SimpleExtension.load(defaultFiles);
   }
 }

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

@@ -214,7 +214,13 @@ public Optional<Expression> visit(Expression.NestedStruct expr, EmptyVisitationC
 
   @Override
   public Optional<Expression> visit(
-      Expression.UserDefinedLiteral expr, EmptyVisitationContext context) throws E {
+      Expression.UserDefinedAnyLiteral expr, EmptyVisitationContext context) throws E {
+    return visitLiteral(expr);
+  }
+
+  @Override
+  public Optional<Expression> visit(
+      Expression.UserDefinedStructLiteral expr, EmptyVisitationContext context) throws E {
     return visitLiteral(expr);
   }