szehon-ho commented on code in PR #15994:
URL: https://github.com/apache/iceberg/pull/15994#discussion_r3939127064


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api/src/main/java/org/apache/iceberg/udf/UdfTypes.java:
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@@ -0,0 +1,326 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.iceberg.udf;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.Objects;
+import java.util.stream.Collectors;
+import org.apache.iceberg.relocated.com.google.common.base.Preconditions;
+import org.apache.iceberg.relocated.com.google.common.collect.ImmutableList;
+import org.apache.iceberg.types.Types;
+
+/**
+ * Concrete implementations of {@link UdfType}: {@link PrimitiveType} for 
primitive and
+ * semi-structured types and {@link ListType}, {@link MapType}, {@link 
StructType} for nested types.
+ * {@link NestedField} represents a named field inside a {@link StructType}.
+ */
+public class UdfTypes {
+
+  private UdfTypes() {}
+
+  /**
+   * A UDF primitive or semi-structured type, encoded as a type string (e.g., 
{@code int}, {@code
+   * string}, {@code decimal(9, 2)}, {@code variant}).
+   *
+   * <p>The type string must be a recognized Iceberg primitive or 
semi-structured type as understood
+   * by {@link Types#fromTypeName(String)}. The input is canonicalized to 
Iceberg's standard form
+   * (lowercase, normalized whitespace), so {@code PrimitiveType.of("INT")} 
and {@code
+   * PrimitiveType.of("Decimal( 9 , 2 )")} produce {@code int} and {@code 
decimal(9, 2)}
+   * respectively.
+   */
+  public static final class PrimitiveType implements UdfType {
+
+    private final String typeString;
+
+    public static PrimitiveType of(String typeString) {
+      Preconditions.checkArgument(typeString != null, "Invalid primitive type: 
null");
+      // Validate against Iceberg's primitive/semi-structured type vocabulary 
and use the parsed
+      // type's canonical toString() so callers don't have to worry about 
casing or whitespace.
+      String canonical = Types.fromTypeName(typeString).toString();

Review Comment:
   Please ensure this produces the canonical form required by the UDF spec. The 
UDF spec says type strings and definition IDs must contain no spaces, but 
`Types.fromTypeName("geography").toString()` produces `geography(OGC:CRS84, 
spherical)`. It also preserves CRS casing, so equivalent spatial types can 
produce unequal `PrimitiveType` values. This exposes a conflict with Iceberg 
schema JSON’s geography form that should be resolved here or in the spec.



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