Supporting HTML-like labels

[kchanqvq]

The relevant section of the grammar specification can be found at the newest https://www.graphviz.org/doc/info/lang.html:

An ID is one of the following:

Any string of alphabetic ([a-zA-Z\200-\377]) characters, underscores ('_') or digits([0-9]), not beginning with a digit;
a numeral [-]?(.[0-9]⁺ | [0-9]⁺(.[0-9]*)? );
any double-quoted string ("...") possibly containing escaped quotes (")¹;
an HTML string (<...>).

I think we just need to update format-id, but I'm a bit unsure about what's the best interface. E.g. how to tell if the user intend a string to be printed as double-quoted string or HTML string?

[jingtaozf]

@BlueFlo0d Thanks for your note.
I think we can check the type of id, it can be an atom or cons, and if it is cons, the car can be the ID type (for example, :single-quote, :double-quote, :html, etc).

A pull request to support a full feature ID is very welcome.
https://www.graphviz.org/doc/info/lang.html

[kchanqvq]

Re using type of id: yes it makes sense! And such syntax is very Lispy, I like it.

Apologies for the delayed response, I recall last time I was working on this, I tried to use spinneret (an HTML generator in Lisp) so user can write the HTML using S-expr. It was a disaster — Graphviz turns out to require a “fake” HTML that differs from standard HTML in subtle ways (e.g. use of quoting)… maybe for now we could just let user pass in a raw HTML string and output it verbatim. To support full S-expr we'd need to implement this grammar as well, which could be a future goal.