Re: [Qemu-devel] [PATCH 1/2] qapi: allow flat unions with empty branches

[Eric Blake]

On 05/17/2018 03:05 AM, Markus Armbruster wrote:
> > > > > > > QAPI language design alternatives:
> > > > > > >
> > > > > > > 1. Having unions cover all discriminator values explicitly is useful.
> > > >
> > > > > > > 2. Having unions repeat all the discriminator values explicitly is not
> > > > > > > useful.  All we need is replacing the code enforcing that by code
> > > > > > > defaulting missing ones to the empty type.

> > I think I'd vote for 2 (never enforce all-branches coverage) as well.
>
> Eric, what do you think?

I'm sold. Let's go ahead and make the change that for any flat union, a 
branch not listed defaults to the empty type (no added fields) rather 
than being an error, then simplify a couple of the existing flat unions 
that benefit from that.

> One more thought: if we ever get around to provide more convenient flat
> union syntax so users don't have to enumerate the variant names twice,
> we'll need a way to say "empty branch".  Let's worry about that problem
> when we have it.

In other words, our current "simple" unions act in a manner that 
declares an implicit enum type - if we ever add an implicit enum to a 
flat union (where you don't have to declare a pre-existing enum type), 
then you need some syntax to declare additional acceptable enum values 
that form an empty branch.  Indeed, not a problem to be solved right now.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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