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

[Fam Zheng <famz@redhat.com>]

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.

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?

Fam