[GitHub] [avro] martin-g commented on a change in pull request #1411: AVRO-3257: IDL support for nullable types

[GitBox <gi...@apache.org>]

martin-g commented on a change in pull request #1411:
URL: https://github.com/apache/avro/pull/1411#discussion_r766084596



##########
File path: doc/src/content/xdocs/idl.xml
##########
@@ -301,17 +301,36 @@ record Card {
         <section id="unions">
           <title>Unions</title>
           <p>Union types are denoted as <code>union { typeA, typeB, typeC, ... }</code>. For example,
-          this record contains a string field that is optional (unioned with <code>null</code>):
+          this record contains a string field that is optional (unioned with <code>null</code>), and
+          a field containing either a precise or a imprecise number:
           </p>
           <source>
 record RecordWithUnion {
   union { null, string } optionalString;
+  union { decimal(12, 6), float } number;
 }
           </source>
           <p>
             Note that the same restrictions apply to Avro IDL unions as apply to unions defined in the
-            JSON format; namely, a record may not contain multiple elements of the same type.
+            JSON format; namely, a record may not contain multiple elements of the same type. Also,
+            fields/parameters that use the union type and have a default parameter must specify a
+            default value of the same type as the <em>first</em>em> union type.
           </p>
+          <p>Because it occurs so often, there is a special shorthand to denote a union of
+            <code>null</code> with another type. In the following snippet, the first three fields have
+            identical types:
+          </p>
+          <source>
+record RecordWithUnion {
+  union { null, string } optionalString1 = null;
+  string? optionalString2 = null;
+  string? optionalString3; // No default value
+  string? optionalString4 = "something";
+}
+          </source>
+          <p>Note that unlike explicit unions, the position of the <code>null</code> type is fluid; it will be
+            the first or last type depending on the default value (if any). So in the example above, all fields
+            are valid.</p>

Review comment:
       We should not forget to update the new website once it starts being used (https://lists.apache.org/thread/ktjh7kyd92hf0jsmpp16c3k121dc8grg)




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