Re: [Cbor] A CBOR tag for alternatives/unions, request for comments

[Carsten Bormann <[e-mail]>]

On 2022-02-23, at 16:55, Michael Peyton Jones <[e-mail]> wrote:
> 
> I do think we would prefer to keep the overlapping encoding. Many other kinds of CBOR encodings have redundancy, starting right from the basics with the encoding of numbers.

Yes.  In 2013, for a while I was hoping to fix this for the entirety of CBOR (like we did in CoAP), but was advised that was too radical.  There also was the occasional use case for non-preferred encoding.

But I don’t think we are out there actively creating more of these cases.

> We believe that not only is this approach consistent, but it's actually helpful: it allows users to decide whether they care more about flexibility or regularity.

Well, in the non-overlapping approach, they get both! :-)

> If you want, you *can* just encode everything with the long form and not worry about the compact form. This simplicity is sometimes helpful.

I think that spending two ifs in the encoder is OK here.
(The decoder will need to handle all cases anyway, so there is no advantage.)

> Of course, we can (and should) require that the canonical version be the most compact.

Which is the case when we do non-overlapping encodings.

Grüße, Carsten

(Leaving the rest of the message intact as that is still in some moderator queue.)

> 
> Best wishes,
> Michael
> 
> On Wed, 23 Feb 2022 at 15:06, Emile Cormier <[e-mail]> wrote:
> On Wed, Feb 23, 2022 at 10:48 AM Carsten Bormann <[e-mail]> wrote:
> I’m not currently proposing to standardize this, but the cbor diagnostic tools support a transform to tunnel CBOR data items through JSON [0]:
> 
> $ echo "122(h'1234')" | diag2cbor.rb | cbor2json.rb -j
> {
>   "@@!t122": {
>     "@@!b": "EjQ"
>   }
> }
> 
> This works by hijacking JSON object keys starting with @@!; I’m not sure the code handles all cases so we have achieved full transparency yet, but that is a SMOP [1].
> 
> Grüße, Carsten
> 
> [0]: https://github.com/cabo/cbor-diag/blob/master/lib/cbor-transform-j.rb
> [1]: SMOP = Small matter of programming
> 
> I was thinking something as simple as:
> 
> [<alt_index>, <value>] 
> 
> where <alt_index> is a JSON number and <value> is the datum transcoded to JSON. However, transcoding this back to CBOR would be lossy without any other context.
> 
> 
> -- 
> 
> Michael Peyton Jones
> Software Engineering Lead | London, UK
> 
> Website: www.iohk.io
> Skype: michael.s.pj
> Twitter: @mpeytonjones
> PGP Key ID: 29F64616
> 
>  
> 
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