Re: [Qemu-devel] [PATCH v5 17/28] qapi: Allow true, false and null in schema json

[Eric Blake]

On 04/01/2015 03:33 AM, Markus Armbruster wrote:

>>> Longhand:
>>>
>>>     # mandatory
>>>     'name': { 'type': 'str' }
>>>     # optional, with a default
>>>     'flag': { 'type': 'bool', 'default': true }
>>>     # optional, no default
>>>     'string': { 'type': 'str', 'default': null }
>>>
>>> Presence of 'default' implies optional.
>>>
>>> Equivalent shorthand, if any:
>>>
>>>     'name': 'str'
>>>     '*string': 'str'
>>
>> A nice shorthand for defaults would be:
>>
>>     '*name': 'str' = 'default'
>>
>> Though that would be neither valid JSON nor Python any more. Do we
>> actually rely on this property anywhere or is it only parsed by the QAPI
>> generator anyway and we can extend the language in such ways?
> 
> I guess JSON / Python was chosen as QAPI schema language to save us the
> bother of defining a syntax and building the tools to work with it, like
> an Emacs mode.  JSON's not exactly my favourite choice, but at least
> it's not XML.
> 
> What we have now isn't JSON, but it's still a subset of Python, and the
> Python tools work.  If we go beyond Python, they'll break.

Well, we were a subset of Python, until this patch added true, false,
and null (the Python way would have been True, False, and None).  We are
also similar to JSON5, http://json5.org/

Among other things, JSON5 allows trailing commas, allows unquoted keys
in a dictionary, allows single-quoted strings, and allows C-style comments.

> 
> If we decide to sacrifice these tools for readability, then we can just
> as well replace the syntax entirely.  Preferably by something where I
> don't have to put every identifier in quotes.
> 
> In short, you're welcome to hack up qapi.py some more for schema
> readability, but either keep Emacs Python mode working, or provide a
> replacement :)

Since we're not quite python or JSON, we've already rolled our own
parser; so rewriting QAPI to use a syntax of our own choosing is not
that much of a leap.  But not for this series.

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

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