[Cbor] Referencing external files/URLs from CBOR diagnostic notation

[Carsten Bormann <[e-mail]>]

In the context of discussing the module structure for CDDL, Ned Smith
has pointed out that a way to reference external files/URLs might
be useful for CBOR diagnostic notation as well.

Now, fortunately, this is right within the reach of application extensions:

    [1, 2, ref'http://tzi.de/~cabo/123.diag']

123.diag is a file containing

    [1, 2, 3]

so the result is

    [1, 2, [1, 2, 3]]

(Unfortunately, the same extension point does not work for splicing in CBOR sequences, so if a referenced file parses as a CBOR sequence this is currently treated as an error.)

I have implemented a proof of concept in the cbor-diag-ref gem.

    gem install cbor-diag-ref

To use the PoC with the cbor-diag gem, you need to explicitly add the "ref" application extension, e.g., by giving `-aref` to the diag2x.rb commands:

$ echo "[1, 2, ref'http://tzi.de/~cabo/123.diag']" >my.diag
$ diag2diag.rb -aref my.diag
[1, 2, [1, 2, 3]]

➔ Is this application extension useful?
➔ Do we want to enable use of other external forms beyond diagnostic mode (cbor-pretty == hex; actual binary CBOR)?

(Note that this proof of concept does NO SANITIZING of URLs or file names at all; to make this a permanent addition to EDN, we’d need to think about the right restrictions for external references like this.)

Grüße, Carsten