Re: [netmod] Canonical value representations in typedefs [was RE: 6021 ipv4-prefix]

Robert Varga <nite@hq.sk> Thu, 02 May 2019 15:42 UTC

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Subject: Re: [netmod] Canonical value representations in typedefs [was RE: 6021 ipv4-prefix]
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On 02/05/2019 15:12, Ladislav Lhotka wrote:
>> But this really means that YANG has two sorts of typedefs:
>>
>> (i) A regular typedef which is just an alias for a base type, perhaps with some additional refinements that can all be handled automatically (e.g. pattern statements, etc).  In this scenario, I think that a compiler can ignore the description, and process the typedef automatically.
>> (ii) An enhanced typedef that has additional semantics embedded into the description statement that requires custom implementation of the typedef.
>>
>> Being able to distinguish these two cases in a programmatic way seems
>> useful to me, but perhaps it is just unnecessary noise.
> YANG assumes "powerful" descriptions in general. An option would be to find a way for specifying the canonical
> format in a machine-readable form.

The way I understand typedefs (and use of restricted types in
leaf/leaf-list) as that the first such use defines a value domain and
any further references must strictly refer to a subset of that domain.

Notably it does not seem to be sane to redefine the canonical format
once it has been set.

I think the entire canonical format problem comes from the fact
ipv4-address is, unlike case (i), only a representation of a value
domain which is semantically 32 bits.

That value domain has two functions that are different from plain old
string:
- equality
- total ordering

Canonical representation deals with the first one, while also providing
interoperability with XPath. There is no current mechanism to address
the second one.

I find having a machine-readable marker (such as an extension) that a
value domain is being defined immediately useful -- it allows the
compiler to warn that a specific typedef needs additional work to get it
working as intended. This one's really easy.

Having some (other) machine-readable marker of how a canonical value
looks like (i.e. similar to pattern) is also useful: it allows the
runtime to check if an incoming value is in canonical format and accept
it irrespective whether specific value domain support (from above) is
present or not. This one is moderately hard.

Finally, some third marker, which would describe how to transform a
non-canonical string into a canonical string would obviously be awesome,
as now the runtime can work automatically. This one is really hard,
especially since the operations probably involve operating not on a
string, but some other representation.

As in implementer, I really care about the first two -- they are easy to
do and provide immense value in making things work 'out of the box' and
automatically catching cases when they do not.

Regards,
Robert