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 Wed, 05/21 07:54, Markus Armbruster wrote:
>> 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.
>
> Well, I could use some background here. Why did we introduce nested structure
> in the first place?

Because we could?

Felt like a good idea at the time?

I quick glance at commit 0f923be and fb3182c suggests they have been
supported since the beginning.  There is no design rationale.

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