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Excerpts from Zane Bitter's message of 2014-11-06 15:35:09 -0800:
> On 06/11/14 20:44, Steven Hardy wrote:
> > On Wed, Nov 05, 2014 at 02:46:43PM +0000, Lee, Alexis wrote:
> >>     I'm considering adding a function which takes a list and returns the first
> >>
> >>     non-null, non-empty value in that list.
> >>
> >>     So you could do EG:
> >>
> >>         some_thing:
> >>
> >>             config:
> >>
> >>                 ControlVIP:
> >>
> >>                     first_nonnull:
> >>
> >>                     - {get_param: ControlVIP}
> >>
> >>                     - {get_attr: [ControlVirtualIP, fixed_ips, 0,
> >>     ip_address]}]}
> >>
> >>
> >>     I'm open to other names, EG "some", "or", "fallback_list" etc.
> >>
> >>     Steve Hardy suggested building this into get_attr or Fn::Select. My
> >>     feeling is that those each do one job well right now, I'm happy to
> >>     take a steer though.
> >
> > Ah, from our IRC discussion I was thinking you wanted primarily list
> > filtering of get_attr output, thus thinking an optional argument would make
> > more sense than a new function.
> >
> > I see now that you're actually looking for something of a poor-mans
> > conditional, so you choose either the ControlVIP parameter, or the
> > ControlVirtualIP attribute, for which your proposal is probably cleaner -
> > my concern is just that we avoid a proliferation of different list
> > select/filter functions, when we could just have one.
> 
> 
> Crazy thought: why not just implement conditionals? We had a proto-spec 
> for them started at one point...
> 

The coalesce/first_nonnull is just a shortcut for a common conditional
problem. I'd agree that conditionals are useful as well, but they might
be better served by more time to bake than this more narrow case.