Re: Tag Spec

On Sun, Feb 3, 2013 at 1:31 PM, Oren Ben-Kiki <[email protected]>wrote:

> The question isn't one of ease of parsing (we all know YAML isn't easy to
> parse :-).
>

:-) Indeed!

It is a matter of ambiguity. The !<verbatim> syntax was added very late in
> the game, to allow avoiding the tag prefix games.
>

>
The thinking isn't "domain" vs. "local", the thinking is "full arbitrary
> URI" (inside <...>) vs. "suffix added to some arbitrary URI prefix"
> (without the <...>). The difference between "!", "!!" and "!foo!" is just
> "which URI prefix should we use here".
>

Though in practice that's what people see. Perhaps the better terminology
is "globally unique" vs "local" tags. Although determining if a prefix is
applied would depend on whether the tag contains a `:` or not, sticking
with current spec, !<...> tags would still not resolve. To clarify the
difference:

    %TAG ! tag:foo.org/
    ---
    - !foo                   => !<tag:foo.org/foo>
    - !tag:bar.org/bar   => !<tag:bar.org/bar>

Also, I think this would open up `!<foo>` to be a legal local tag, rather
then the degenerate global tag it is now b/c it is not a valid URI. So,
without a `%TAG !` directive:

    ---
    - !foo                 => !<foo>

So the `!` would no longer have any significance if a tags *name*. It would
be used only to designate a tag and to sub prefixes, but a local tag would
not need to be `!foo` any more, just `foo`.

If we said "we don't attach any prefix if what follows the "!" looks like a
> complete URI" we'd be entering a world of pain. URIs can be in all sort of
> forms: "urn:isbn:0-395-36341-1" is a URI and hence (if someone wanted) !<
> urn:isbn:0-395-36341-1> would be a valid (if somewhat insane) tag. So
> "looking like a complete URI" isn't really easy to define. "Looking like a
> tag URI" is well-defined, but YAML really doesn't insist on using "tag
> URIs", even though we call the "node type tags", well, tags.
>

I'm not seeing how this translates into be a world of pain. Keeping it
simple, as in "does it contain a `:`" should suffice b/c `:` is required of
a valid URI. Local tags can live without them. Some adjustment might be
required by end users in rare cases. Like your example, they could use !<
urn:isbn#0-395-36341-1>. I don't think it's too much to ask of end-users
that local tags not use `:`. (If it really is too much to ask then perhaps
a different escape notion could be allowed, e.g `![urn:isbn:0-395-36341-1]`.

(Note, before I suggested that `/` be an indicator of global tag too. In
that case if no `:` is present, `tag:` would be assumed. That just seemed
like a nice convenience, but it is not necessary.)

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