introduce if-module

[mbj4668]

it would be useful to have a statement "if-module" that can be used like if-feature:

leaf my-interface {
  if-module ietf-interfaces;
  type if:interface-ref;
}

[rgwilton]

This seems useful to me. Presumably it would be allowed wherever if-feature is allowed.

[kwatsen]

Kent: could use if-feature statement, but this is simpler Lada: could use 'when' statements, but would require live configuration

Andy: would this support 'not'? Martin: ...and boolean expressions?

Robert: perhaps have implicit features? Andy: but then YANG library would have to report them

[llhotka]

The new YANG library (7895bis) makes this more complicated because a module may be present only in a subset of datastores. I still think that it would be better and more flexible to permit somehow XPath expressions querying YANG library in config=true data.

[mbj4668]

I don't think it is more complicated than if-feature, which also can vary across module-sets.

I prefer explicit statements b/c otherwise we couple the module definition to the exact layout of the YANG library.

[kwatsen]

Andy: what if there are deviations. James: would like a use case. Importance dependent on the use-case. Juergen: how different than using if-feature statements (workaround). Juergen: one benefit is that it doesn't rely on the server to have to enable a feature, it happens automatically.

[jgcumming]

Should clarify in this issue whether this is

1. Feature exists
2. Feature 'implemented'
3. Feature 'import-only'