\quark_if_recursion_tail_stop:n has edge cases

[wspr]

Bruno has submitted the following re-implementation of \quark_if_recursion_tail_stop:n for edge cases noted below.

% The `:n` quark tests are currently broken: both lines below crash.
%
%   \quark_if_recursion_tail_stop:n {{{{a}}}}
%   \quark_if_recursion_tail_stop:n {{ab\iffalse}\fi}
%
% Here is the proposed fix (changing of course `Mquark` to `quark`). FYI,
%     `\if_catcode:w A \tl_to_str:n {...} A` 
% tests whether {...} is empty. It is a variant of 
%     `\exp_after:wN \if_meaning:w \exp_after:wN A \tl_to_str:n {...} A`
% which is the version used for `\tl_if_empty:n(TF)`.

\cs_new:Npn \Mquark_if_recursion_tail_stop:n #1 {
  \if_catcode:w A \tl_to_str:n \exp_after:wN {
    \Mquark_if_recursion_tail_aux:w #1 \q_recursion_stop
                     \q_recursion_tail \q_recursion_stop \q_stop
  } A
  \exp_after:wN \use_none_delimit_by_q_recursion_stop:w
  \fi:
}
\cs_new:Npn \Mquark_if_recursion_tail_stop_do:nn #1#2 {
  \if_catcode:w A \tl_to_str:n \exp_after:wN {
    \Mquark_if_recursion_tail_aux:w #1 \q_recursion_stop 
                     \q_recursion_tail \q_recursion_stop \q_stop
  } A
  \exp_after:wN \use_i_delimit_by_q_recursion_stop:nw
  \else:
  \exp_after:wN\use_none:n
  \fi:
  {#2}
}
\cs_new:Npn \Mquark_if_recursion_tail_aux:w #1 \q_recursion_tail #2
     \q_recursion_stop #3 \q_stop { #1 #2 }

[blefloch]

This issue should probably be closed now that the code is in the svn. For completeness, the quark tests proposed here do not differentiate between \q_recursion_tail, {}\q_recursion_tail, \q_recursion_tail{}, and {}\q_recursion_tail{}. In practical applications, this should not matter.

[wspr]

Thanks; I agree.