Re: [netmod] type equivalence

["Rob Wilton (rwilton)" <[e-mail]>]

> -----Original Message-----
> From: Martin Björklund <[e-mail]>
> Sent: 26 February 2021 16:30
> To: [e-mail]
> Cc: Rob Wilton (rwilton) <[e-mail]>; [e-mail]
> Subject: Re: [netmod] type equivalence
> 
> Andy Bierman <[e-mail]> wrote:
> > On Fri, Feb 26, 2021 at 7:06 AM Martin Björklund <[e-mail]>
> wrote:
> >
> > > "Rob Wilton \(rwilton\)" <rwilton=[e-mail]> wrote:
> > > >
> > > >
> > > > > -----Original Message-----
> > > > > From: netmod <[e-mail]> On Behalf Of Juergen
> > > Schoenwaelder
> > > > > Sent: 24 February 2021 20:39
> > > > > To: [e-mail]
> > > > > Subject: Re: [netmod] type equivalence
> > > > >
> > > > > Here is an attempt to come up with better wording. If people agree
> on
> > > > > a new wording, I volunteer to submit an errata.
> > > > >
> > > > > OLD
> > > > >
> > > > >    o  A "type" statement may be replaced with another "type"
> statement
> > > > >       that does not change the syntax or semantics of the type.
> For
> > > > >       example, an inline type definition may be replaced with a
> > > typedef,
> > > > >       but an int8 type cannot be replaced by an int16, since the
> syntax
> > > > >       would change.
> > > > >
> > > > > NEW
> > > > >
> > > > >    o  A "type" statement may be replaced with another "type"
> statement
> > > > >       that does not change the semantics of the type or the
> underlying
> > > > >       built-in type.  For example, an inline type definition may
> be
> > > > >       replaced with a semantically equivalent typedef derived from
> the
> > > > >       same built-in type, but an int8 type cannot be replaced by
> an
> > > > >       int16, since the underlying built-in type would change.
> > >
> >
> >
> > I think the NEW text captures the original intent and is OK for an
> errata.
> 
> +1
> 
> 
> > I believe the use-case discussed at the time of writing was simply
> > replacing an inline
> > type with the identical type but within a typedef-stmt instead of inline
> > within a leaf or leaf-list.
> >
> > Perhaps this rule is too strict.
> > There is a simple way to defeat it:
> >
> > Change all
> >    type foo {  ... }
> > to
> >    type union {
> >       type foo { ... }
> >     }
> >
> > Now you can add new values and semantics without taking away the
> original
> > syntax and semantics.
> >
> >  type union {
> >       type foo { ... }
> >       type bar { ... }   // note new member types added at end of list
> >     }
> >
> > But it is not clear that this would be legal or completely BC. It
> certainly
> > could change the encoding in JSON and CBOR.
> 
> It is not allowed by sec 11 in 7950, since it changes the syntax of
> the type.
[RW] 

But the proposed errata removes the text about not changing the syntax, or are you referring to other text?

Rob


> 
> 
> /martin
> 
> 
> 
> >
> >
> > Andy
> >
> >
> > > [RW]
> > > >
> > > > Would the text be more clear it is just specified what is allowed,
> e.g.,
> > > >
> > > >      o  A "type" statement may be replaced with another "type"
> statement
> > > >         that resolves to the same underlying built-in type.  For
> example,
> > > >         ...
> > > >
> > > >
> > > > What does "semantics of the type" cover?
> > >
> > > Suppose you have:
> > >
> > >    typedef "timestamp" {
> > >      type yang:date-time;
> > >      description
> > >        "The time that an event occurred";
> > >    }
> > >
> > > then you can't change it to:
> > >
> > >    typedef "timestamp" {
> > >      type yang:date-time;
> > >      description
> > >        "The time that an event was received.";
> > >    }
> > >
> > > The syntax is the same, but the semantics are different.
> > >
> > >
> > > /martin
> > >
> > >
> > >
> > >
> > > >
> > > > If I have this type:
> > > >
> > > >   typedef "timestamp" {
> > > >     type "string";
> > > >     description
> > > >       "The time of day that an event occurred, in any format";
> > > >   }
> > > >
> > > > then can I replace it with this definition:
> > > >
> > > >   typedef "timestamp" {
> > > >     type "string";
> > > >     description
> > > >       "The time of day, and optionally date, that an event
> > > >        occurred, in any format";
> > > >   }
> > > >
> > > >
> > > >
> > > > Tangentially, it is worth noting the RFC 8342 also writes about
> syntactic
> > > > constraints covering types:
> > > >
> > > > 5.3.  The Operational State Datastore (<operational>)
> > > >
> > > >    Syntactic constraints MUST NOT be violated, including
> hierarchical
> > > >    organization, identifiers, and type-based constraints.  If a node
> in
> > > >    <operational> does not meet the syntactic constraints, then it
> > > >    MUST NOT be returned, and some other mechanism should be used to
> flag
> > > >    the error.
> > > >
> > > > I'm not sure how clear RFC 8342 section 5.3 is about returning
> values
> > > > that can be represented by the underlying built-in-type, but are
> outside
> > > > the value space defined by a range, length, or pattern statement.
> > > >
> > > > My memory during the discussions was that it is allowed to return a
> value
> > > > outside arange, length, pattern statement, as long as it is
> contained
> > > > in the value space of the built-in-type.  E.g., cannot return 257 in
> a
> > > > uint8, but can return 11 even if the type range is 1..10.
> > > >
> > > > But, I'm not sure that is what the text actually states.
> > > >
> > > > Regards,
> > > > Rob
> > > >
> > > >
> > > > >
> > > > > /js
> > > > >
> > > > > On Mon, Feb 22, 2021 at 03:20:02PM +0100, Carsten Bormann wrote:
> > > > > > On 2021-02-22, at 15:17, Juergen Schoenwaelder
> > > <j.schoenwaelder@jacobs-
> > > > > university.de> wrote:
> > > > > > >
> > > > > > > I guess considering the built-in types as incompatible is the
> most
> > > > > > > robust approach. If we agree that RFC 7950 tried to say this,
> we
> > > could
> > > > > > > file an errata and propose clearer language.
> > > > > >
> > > > > > Right.  And we can keep the COMI key-to-URL mapping as is, as
> this
> > > > > clarification is necessary for its correct functioning.
> > > > > >
> > > > > > Grüße, Carsten
> > > > > >
> > > > >
> > > > > --
> > > > > Juergen Schoenwaelder           Jacobs University Bremen gGmbH
> > > > > Phone: [phone]         Campus Ring 1 | 28759 Bremen |
> Germany
> > > > > Fax:   [phone]         <https://www.jacobs-
> university.de/>
> > > > >
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