On 17/03/2017 14:03, Dale R. Worley wrote:
Robert Wilton <[email protected]> writes:
To quote Joey's example, I think that both of the following modules are
valid (presuming that they are both implemented by a device) regardless
of which features are enabled. Do you agree?
module base-module {
prefix bmod;
feature things;
feature widgets;
container things {
if-feature things;
...
container widgets {
if-feature widgets;
...
}
}
}
module augment-module {
prefix amod;
augment "/bmod:things" {
container other-things {
}
}
augment "/bmod:things/bmod:widgets" {
container other-widgets {
}
}
}
Does it remain valid if base-module is changed to:
module base-module {
prefix bmod;
feature things;
feature widgets;
container things {
if-feature things;
...
}
}
}
No, I don't think that it is valid because this augment fails because
the base node "/bmod:things/bmod:widgets" doesn't exist.
As I analyze it, the augment statement is unconditional, but the
presence of its target node can be (1) unconditional, (2) conditional,
or (3) *never* present.
My preferred approach is that an augment is only valid if it is
"present" only if the target node is present (a condition I think can be
verified statically). But if we allow the augment to silently have no
effect if the target node is not present in the current implementation,
do we still require that there is some possible implementation in which
the target node exists?
I guess that I think that a node with an if-feature stmt is logically
present in the schema (and hence can always be augmented), but just that
it (and its descendant nodes) can be supported/unsupported depending on
what features a device advertises that it is supporting.
But I don't think that means that an augmentation is ever is ignored,
but it might add nodes to the schema tree that aren't supported because
a given feature isn't supported.
Phil's example of a feature that relates to whether an SD card is
present is interesting because it gives an example of when it might be
useful to dynamically modify the set of supported features whilst a
device is running. Likewise, I could see how the features could change
due to adding/removing software components, or different revisions of
hardware.
Thanks,
Rob
Dale
.
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