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Date: Wed, 15 Sep 2010 02:16:34 -0400 (EDT)
From: "Erik Frey (JIRA)" <jira@apache.org>
To: dev@avro.apache.org
Subject: [jira] Commented: (AVRO-248) make unions a named type
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    [ https://issues.apache.org/jira/browse/AVRO-248?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12909605#action_12909605 ] 

Erik Frey commented on AVRO-248:
--------------------------------

Just to chime in with a concrete use case:  it would be very useful if Avro allowed me pass around arbitrary-depth arrays of arrays.  In Python I use this to represent n-ary trees.  Right now I'm forced to do something like:

{ "type": "record", name: "NestedArray", "fields": {"name": "value", "type": ["string", "NestedArray"] } }

But the following would be simpler, easier to conceptualize, and easier to integrate into my current stack:

{ "type": "array", name: "NestedArray", "items": ["string", "NestedArray"] }

Would allow x[0][1] instead of x['value'][0]['value'][1]

> make unions a named type
> ------------------------
>
>                 Key: AVRO-248
>                 URL: https://issues.apache.org/jira/browse/AVRO-248
>             Project: Avro
>          Issue Type: New Feature
>          Components: spec
>            Reporter: Doug Cutting
>
> Unions are currently anonymous.  However it might be convenient if they were named.  In particular:
>  - when code is generated for a union, a class could be generated that includes an enum indicating which branch of the union is taken, e.g., a union of string and int named Foo might cause a Java class like {code}
> public class Foo {
>   public static enum Type {STRING, INT};
>   private Type type;
>   private Object datum;
>   public Type getType();
>   public String getString() { if (type==STRING) return (String)datum; else throw ... }
>   public void setString(String s) { type = STRING;  datum = s; }
>   ....
> }
> {code} Then Java applications can easily use a switch statement to process union values rather than using instanceof.
>  - when using reflection, an abstract class with a set of concrete implementations can be represented as a union (AVRO-241).  However, if one wishes to create an array one must know the name of the base class, which is not represented in the Avro schema.  One approach would be to add an annotation to the reflected array schema (AVRO-242) noting the base class.  But if the union itself were named, that could name the base class.  This would also make reflected protocol interfaces more consise, since the base class name could be used in parameters return types and fields.
>  - Generalizing the above: Avro lacks class inheritance, unions are a way to model inheritance, and this model is more useful if the union is named.
> This would be an incompatible change to schemas.  If we go this way, we should probably rename 1.3 to 2.0.  Note that AVRO-160 proposes an incompatible change to data file formats, which may also force a major release.

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