Equivalence principle

[cabo]

Describe the equivalence principle, and claim it for reference tags.

Add a default "join" function for array/string combinations.

[chrysn]

If there is an implicit join, does it even still need its own tag?

Ad equivalence: If we go with this, we might not manage to close that box again, and tags could let themselves in to a lot of places.

I think it's good to describe the principle, but also caution that it's still up to the CBOR protocol to decide which equivalences may be used in which places, going from an HTML style free-for-all (document authors are advised to keep compatibility in mind or rely on graceful fallback) to the-CDDL-is-the-law cases (maybe around COSE).

Speaking of CDDL: How would that equivalence be described on that side? Sure a protcol designer can already say item = integer / #6, but should there be a control for equivalence like item = integer .or-equivalent "any" or item = integer .or-equivalent [2, 3]? (where 2 and 3 might be explicitly admitted tags, ignoring the fact that integer already does include these from the onset).

[cabo]

If there is an implicit join, does it even still need its own tag?

Yes, because the implicit join only works for strings (and arrays of strings).

[cabo]

I added some more text about limitations and usage considerations of the tag equivalence concept.
I think this should be published as a -07 to get discussion going before the IETF 114 CBOR meeting.