Re: [Qemu-devel] [PATCH v5 06/28] qapi: Add some union tests

[Eric Blake]

On 03/31/2015 12:15 PM, Eric Blake wrote:

>>> Cool.  Or more concretely,
>>>
>>> { 'union': 'Simple', 'data': { 'one': 'str', 'two': 'int' } }
>>>
>>> is identical on the wire to:
>>>
>>> { 'enum': 'MyEnum', 'data': ['one', 'two'] }
>>> { 'type': 'Base', 'data': { 'type': 'MyEnum' } }
>>> { 'type': 'Branch1', 'data': { 'data': 'str' } }
>>> { 'type': 'Branch2', 'data': { 'data': 'int' } }
>>> { 'union': 'Flat', 'base': 'Base', 'discriminator': 'type',
>>>   'data': { 'one': 'Branch1', 'two': 'Branch2' } }
>>
>> Perhaps we should expose all unions for schema introspection in a way
>> similar to this (possibly not splitting out Branch1 and Branch2 as
>> independent types, though). We would just have to give a name to
>> implicitly generated enums and base types.
> 
> So maybe we update the schema to allow anonymous types.  That is:
> 
> { 'union': 'Simple',
>   'data': { 'one': { 'name': 'str', 'value': 'int' } } }
> 
> would be nicer than the current requirement of:
> 
> { 'type': 'Branch1', 'data': { 'name': 'str', 'value': 'int' } }
> { 'union': 'Simple',
>   'data': { 'one': 'Branch1' } }

Hmm; maybe we could support a notion of a 'common' dictionary for simple
unions, containing all non-discriminator fields present in all branches,
where (using anonymous types for compactness), and assuming the enum
definition:

{ 'union': 'Simple',
  'common': { 'readonly': 'bool' },
  'data': { 'one': 'str', 'two': 'int' } }

is shorthand for:

{ 'union': 'Flat',
  'base': { 'readonly': 'bool', 'type': 'Enum' },
  'discriminator': 'type',
  'data': { 'one': { 'data': 'str' },
            'two': { 'data': 'int' } } }

that is, where 'base' for flat unions is the union of the 'common' type
and 'discriminator':'discriminator-type' member of simple unions.

At any rate, any changes along these lines will be a later series.  Time
for me to get back to work on publishing v6 :)

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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