Re: [Qemu-devel] Non-flat command line option argument syntax

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Markus Armbruster (armbru@redhat.com) wrote:
> "Dr. David Alan Gilbert" <dgilbert@redhat.com> writes:
> 
> > * Markus Armbruster (armbru@redhat.com) wrote:
> >> "Dr. David Alan Gilbert" <dgilbert@redhat.com> writes:
> >> 
> >> > * Markus Armbruster (armbru@redhat.com) wrote:
> [...]
> >> >> === Structured values ===
> >> >> 
> >> >> The dotted key convention messes with KEY syntax to permit structured
> >> >> values.  Works, but the more conventional way to support structured
> >> >> values is a syntax for structured values.  
> >> >> 
> >> >> An obvious one is to use { KEY=VALUE, ...} for objects, and [ VALUE,
> >> >> ... ] for arrays.  Looks like this:
> >> >> 
> >> >>     -drive 'driver=quorum,
> >> >>             child=[{ driver=file, filename=disk1.img },
> >> >>                    { driver=host_device, filename=/dev/sdb },
> >> >>                    { driver=nbd, host=localhost } ]'
> >> >> 
> >> >> Again, lines broken and indented for legibility; you need to join them
> >> >> for actual use.
> >> >> 
> >> >> There's a syntactic catch, though: a value of the form [ ... ] can
> >> >> either be an array or a string.  Which one it is depends on the type of
> >> >> the key.  To parse this syntax, you need to know the types, unlike JSON
> >> >> or traditional QemuOpts.  Unless we outlaw strings starting with '{' or
> >> >> '[', which feels impractical.
> >> >
> >> > I don't understand why [ could imply a string.
> >> 
> >> Consider
> >> 
> >>     -drive 'driver=quorum,
> >>             child=[{ driver=file, filename={"foolish":"name"} },
> >>                    { driver=host_device, filename=/dev/sdb },
> >>                    { driver=nbd, host=[::1] } ]'
> >> 
> >> Three KEY=VALUE have their VALUE start with '[' or '{':
> >> 
> >> * child=[{ driver=file, ...
> >> 
> >>   This is an array, not a string, because child is an array.
> >> 
> >> * host=[::1]
> >> 
> >>   This is a string, not an array containing the string "::1", because
> >>   host is a string.
> >
> > Why is that accepted as valid input? Can't that just spit a
> > 'I wanted a string not an array' ?
> 
> Because "[::1]" is a perfectly valid string.
> 
> It's also a valid array.  Unless the parser knows what type to expect
> here, it cannot decide how to parse it.  That's why I wrote "to parse
> this syntax, you need to know the types".

I think that's fine; the parser would just parse something starting with
a [ as an array, always.  You just define that and then everyone knows
that you'd better " it if you've got a [ in your string.

Dave
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK