How to set replacement text of function from token list variable?

Question

I'm trying to define a local function with parameters in expl3 where the replacement text of that function should be the content of a token list variable.

My current approach is

\tl_set:Nn \l_tmpa_tl { \textbf{##1} }
\exp_args:NNnV \cs_set:Npn \foo {##1} \l_tmpa_tl
\cs_show:N \foo

where the expected output is something like

\foo #1:->\textbf {#1}

but it results in

! Illegal parameter number in definition of \reserved@b.

How do you handle such dynamic function definitions in expl3?

Answer 1

The main problem is the parameter text that must not be braced. You can skip it by using the “without parameter” syntax. See https://tex.stackexchange.com/search?q=cs_set%3ANV for examples of usage

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn

\tl_set:Nn \l_tmpa_tl { \textbf{#1} }

\exp_args:NNV \cs_new:Nn \foo:n \l_tmpa_tl
\cs_set_eq:NN \foo \foo:n

\cs_show:N \foo

The output on the console is

> \foo=\long macro:#1->\textbf {#1}.

You can also do with the parameters, but it's really ugly:

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn

\exp_args_generate:n { NNNV }

\tl_set:Nn \l_tmpa_tl { \textbf{#1} }
\tl_set:Nn \l_tmpb_tl { #1 }

\exp_args:NNNNV
\exp_last_unbraced:NNV
\cs_new:Npn \foo \l_tmpb_tl \l_tmpa_tl

\cs_show:N \foo

Answer 2

There is the p argument type, which is a wrapper around TeX's #{ argument. The advantage is that anything valid in the parameter text of a macro is valid here without changing the code. The downside is that the argument that follows must be braced, so you'd either need to change the expansion type from V to o (which will work because of an implementation detail), or to pass a braced, single-token argument to V (which will work because of another implementation detail).

To stick with the expl3 syntax and have the braces required for p you can use v:

\documentclass{article}
\usepackage{expl3}
\ExplSyntaxOn
\exp_args_generate:n { Npv } % p = #{
%
\tl_set:Nn \l_tmpa_tl { \textbf{#1} }
\exp_args:NNpv \cs_set:Npn \foo #1 { l_tmpa_tl }
% \foo=\long macro:#1->\textbf {#1}.
\exp_args:NNpv \cs_set:Npn \bold text #1 { l_tmpa_tl }
% \bold=\long macro:text#1->\textbf {#1}.
\ExplSyntaxOff
\begin{document}
\foo{hello} \bold text{world}
\end{document}