[augeas-devel] Default label for a node in a path expression

[Raphaël Pinson]

On Mon, Jan 6, 2014 at 11:04 PM, David Lutterkort <lutter at watzmann.net>wrote:

> Hi Raphael,
>
>
> On Fri, Dec 20, 2013 at 2:24 PM, Raphaël Pinson <
> raphael.pinson at camptocamp.com> wrote:
>
>> Hello,
>>
>>
>> There's regularly people asking how to write idempotent set expressions.
>> With labeled nodes, we have the usual trick:
>>
>>     set foo[.='bar'] 'bar'
>>
>> which creates the 'foo' node with the 'bar' value if it doesn't exist.
>>
>
> Yes, I agree with both - that it's not wel supported right now, and that
> it is needed. The underlying problem is that there's no good way to infer
> from a path expression where the newly created node  should go in case the
> path expression doesn't match.
>
> Rather than put something in the XPath language, I think it would be
> cleaner to do this explicitly in a dedicated command. I can think of a few
> ways to do that. The simplest would be a aug_set_or_create(expr, path,
> value) that goes something like this:
>
> if aug_match(expr) == 1
>   aug_set(expr, value)
> elsif aug_match(expr) == 0
>   aug_set(path, value)
> else
>   complain
> end
>


Come to think of it, one very useful call for idempotent set commands is
defnode, which I've been using extensively lately, such as in this
situation: http://stackoverflow.com/a/21342882/232250

Defnode is very useful in that it sets a variable pointing to the newly
create node (or to the nodeset already matching). The thing that's missing
in defnode is precisely this default label thing, especially when dealing
with seq entries.

So maybe rather than adding an aug_set_or_create() call, we could have a
defnode_default() call, which would allow to pass a default path to
defnode, the advantage being that we would always get a variable pointing
to the resulting path.


-- 
Raphaël Pinson
Infrastructure Developer & Trainer
+33 479 26 57 93
+33 781 90 00 79

Camptocamp France
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BP 352
48, avenue du Lac du Bourget
73372 Le Bourget du Lac, Cedex
www.camptocamp.com
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