is it possible to use unexpandable cs in \int_eval:n

Question

See the code below, the first two examples are fine while the last gave errors.

The possible reason is that \seq_if_in:NnTF is not expandable.

But I'd like to check the argument and print some error message if the argument is not contained in some sequence, and in the same time, make the command able to be used in \int_eval:n or nested.

Is there some way?

\documentclass{article}
\usepackage{xparse}
\begin{document}
\ExplSyntaxOn
\seq_set_from_clist:Nn \l_tmpa_seq { 1, 2, 3 }
\cs_new:Nn __add_one_or_two:n
  {
    \seq_if_in:NnTF \l_tmpa_seq { #1 }
      { \int_eval:n { #1 + 1} }
      { \int_eval:n { #1 + 2} }
  }
% example 1, fine
__add_one_or_two:n { 3 }
% example 2, fine
__add_one_or_two:n { 4 }
% example 3
\int_eval:n { 2 + __add_one_or_two:n { 5 } }
\ExplSyntaxOff
\end{document}

Short answer: no. Longer answer: you might use \seq_map_function:NN to examine one by one the sequence elements and break the loop in case of a hit; add a sentinel as the last item in the sequence to signal you're at the end. — egreg, Oct 02 '20 at 09:53
Remark: in this particular case, see also expansion - Define an expandable function for comparing a token list to a string in LaTeX3 - TeX - LaTeX Stack Exchange – although just do the unexpandable way if you can. — user202729, Feb 03 '22 at 09:15
Somewhat-related question: macros - What is "expansion"? - TeX - LaTeX Stack Exchange — user202729, Feb 03 '22 at 09:30

score 4 · Accepted Answer · answered Oct 02 '20 at 10:02

\seq_if_in:Nn(TF) is not expandable because the tokens in the sequence might have different catcodes:

\exp_args:NNx \seq_set_from_clist:Nn \l_tmpa_seq { \token_to_str:N : }
\seq_show:N \l_tmpa_seq
\seq_if_in:NnTF \l_tmpa_seq { : }
  { \show\T } { \show\F }

and you can't (reasonably) test that expandably.

Property lists, on the other hand, have the key always made into a string, so \prop_if_in:Nn(TF) can be expandable. If you can change the data type, you can abuse a property list as a catcode-less sequence that allows expandable querying:

\documentclass{article}
\usepackage{xparse}
\begin{document}
\ExplSyntaxOn
\prop_set_from_keyval:Nn \l_tmpa_prop
  { %  v empty values
    1 = ,
    2 = ,
    3 =
  }
\cs_new:Nn __add_one_or_two:n
  {
    \prop_if_in:NnTF \l_tmpa_prop { #1 }
      { \int_eval:n { #1 + 1} }
      { \int_eval:n { #1 + 2} }
  }
% example 1, fine
__add_one_or_two:n { 3 }
% example 2, fine
__add_one_or_two:n { 4 }
% example 3
\int_eval:n { 2 + __add_one_or_two:n { 5 } }
\ExplSyntaxOff
\end{document}

is it possible to use unexpandable cs in \int_eval:n

1 Answers1