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

[Fam Zheng]

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?

Fam

> 
> > 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.