Check before validate in case of "when" statement

[jashmikant]

container c{
leaf name {
type string;
}
leaf description {
when '../name="lag-1"';
type int32;
}
}
Here I want description option to only show if name is "lag-1" .
Currently this is only checked at validation in CLI.
But for above use case, it's better to check before validation.

[olofhagsand]

This topic has to do with layering, YANG is a higher layer modeling, CLIgen uses more primitive pattern matching.
The "when" statement is a YANG constraint and is validated during YANG validate (in clixon).
Core CLIgen works independent of YANG, a YANG constraint cannot be checked for in CLIgen code.
Also, XPath is not implemented in CLIgen.
However, there are indirect mechanisms to add YANG semantics to CLIgen:

1. Add corresponding checks in CLIgen and generate code from YANG at start. One example is type and ranges, etc. However, a generic XPath check seems unpractical, one could possibly make some simpler constraints.
2. Add a callback at points in code where YANG rules can be checked. One example is custom-expand functions. This seems the way to go.

That is, add a new callback that checks whether a symbol ("description" in this case) is a valid keyword and computed in run-time.