What are expl3 quarks? When, where, why and how should I use them?

Question

I feel particularly bombarded by sub-atomic particles right now (and it's mostly Will's fault :)). First, in Will's answer to this question today, he referred to expl3's \prg_case_str:nnn command. Digging into the package, I noticed the command involves an interestingly named macro called \quark_if_recursion_tail_stop_do:nn. All I can gather from the expl3 documentation about things with the word (particle;)) quark in their name is that:

l3quark A ‘quark’ is a command that is defined to expand to itself! Therefore they must never be expanded as this will generate infinite recursion; they do however have many uses, e.g. as special markers and delimiters within code.

Hmmm.

Coincidentally, Will and Kevin's new hardwrap package was released on CTAN today (many congrats guys!). Digging into the documented code, I notice:

\hw@scanstop

This is a ‘quark’ from expl3 designed to delimit scanning; it will never be executed, else an infinite loop results.

\protected\def\hw@scanstop{\hw@scanstop}

Reading the remainder of hardwrap's code, yes, \hw@scanstop is used in exactly the way described in the definition of l3quark, above.

Still, I'm perplexed. What's so special about self-referencing tokens as opposed to any other token assigned a special-purpose name, i.e., any other vanilla signifier, whether a command sequence or not? Are they self-referencing only for error-catching purposes?

If LaTeX quarks do have interesting properties beyond just being specially defined and used signifiers, where, when, how and why should I best employ them?

Are they really expl3 specific, or is it more a convention that has been adopted into the LaTeX3 programming patterns set?

Finally, out of interest (in the spirit of Yiannis's recent question about Lisp car and cdr "relics" in TeX's lexicon), what's the story about how the word quark found its way into LaTeX3 vocabulary? (Please tell me that LaTeX3 quarks aren't envisioned to possess color, flavor, etc. properties!)

Answer 1

So many questions. I'm only going to address "What's so special about self-referencing tokens?", and I'm giving a plain TeX answer. As I learned from TH (at the end of a rather lengthy comment-discussion here), they can be useful for \ifx tests. If I know that #1 is one token, and I do the test

\ifx#1\my@test@token

to find out if #1 is \my@test@token, then it is good to have the definition

\def\my@test@token{\my@test@token}

For \ifx, two control sequences are the same if they have the same definition (including every detail such as parameters and other stuff), but they won't be expanded for the comparison. (That shouldn't happen as you explained above.) For an example see TH's very nice answer that I linked to above.

Be careful: The \ifx test also works if \my@test@token is undefined, but then it turns out true whenever #1 is an undefined control sequence.