Re: [Qemu-devel] [RFC PATCH v2 0/7] qapi: Specify default value for optional argument in schema json

[Markus Armbruster]

Fam Zheng <address@hidden> writes:

> On Tue, 05/20 13:13, Eric Blake wrote:
>> On 05/20/2014 03:07 AM, Fam Zheng wrote:
>> > Please first take a look at patch 7 to see what is supported by this 
>> > series.
>> > 
>> > Patch 1 ~ 3 allows some useful basic types in schema.
>> > 
>> > Patch 4 ~ 6 implements the new syntax.
>> > 
>> > Note: The introduced '@arg' sigil, just like the preexisting '*arg', is
>> > reducing the cleanness of the syntax. We should get rid of both of them in 
>> > long
>> > term. Here, this series compromises on this and introduces '@arg' because:
>> > 
>> >   - We have to distinguish the argument property dictionary from nested 
>> > struct:
>> > 
>> >     I.e.:
>> > 
>> >         'data': {
>> >             'arg1': { 'member1': 'int', 'member2': 'str' }
>> >             '@arg2': { 'type': 'int', 'default': 100 }
>> >          }
>> > 
>> >     Until we completely drop and forbid the 'arg1' nested struct use case.
>> > 
>> >   - Forbidding 'arg1' it's doable, but doing it now means we pull in many
>> >     distractive patches to this series.
>> 
>> Question - since we WANT to get rid of nested struct, why not reverse
>> the sense?  Mark all existing nested structs (weren't there just three
>> that we found?) with the '@' sigil, and let the new syntax be
>> sigil-free.  Then when we clean up the nesting, we are also getting rid
>> of the bad syntax, plus the sigil gives us something to search for in
>> knowing how much to clean up.  But if you stick the sigil on the new
>> code, instead of the obsolete code, then as more and more places in the
>> schema use defaults, it gets harder and harder to remove the use of the
>> sigil even if the nested structs are eventually removed.
>> 
>
> It makes not much difference I can see. The hard part is actaully dropping
> nested, converting from sigil <-> non-sigil is easy. Of course, nothing is
> seriously hard, there are only three nested structs plus some more qapi-schema
> test code.

Adding three ugly sigils and making everybody include one when they add
a nested struct feels much better to me than ugly sigils all over the
place.

> A question before that is, if we are determined to drop '@' sigil (whether 
> from
> nested or property dict), are we as determined to drop '*' sigil as well?

We decided to wait and see how many optionals pick up defaults.  '*' is
only for optionals without defaults.