LaTeX3: tl_map with spaces

Question

Is there a version of tl_map that does not ignore spaces that are part of a token list?

\documentclass{article}
\usepackage{expl3}
\begin{document}
\ExplSyntaxOn
\tl_new:N \l_token_tl
\tl_set:Nn \l_token_tl { a~b~c }
%\tl_show:N \l_token_tl
\tl_map_inline:Nn \l_token_tl { (#1) }
\ExplSyntaxOff
\end{document}

This example does only three iterations, ignoring all the spaces.

Answer 1

You have two strategies available.

Strategy 1: replace spaces with some function that you can redefine depending on the current state

\tl_new:N \l_mhchem_input_tl
\tl_set:Nn \l_mhchem_input_tl { a ~ b ~ c } % this would come from a macro argument
\tl_replace_all:Nnn \l_mhchem_input_tl { ~ } { \__mhchem_space_do: } 
\tl_map_inline:Nn \l_mhchem_input_tl { whatever with #1 }

Strategy 2: split the token list into a sequence that you can subsequently use.

\seq_new:N \l_mhchem_split_input_seq
\seq_set_split:Nnn \l_mhchem_split_input_seq { ~ } { a ~ b ~ c } % this would come from a macro argument
\seq_use:Nn \l_mhchem_split_input_seq { do something in place of spaces }

With \seq_set_map:NNn you can set another sequence adding something around the values stored in \l_mhchem_split_input_seq and use this new sequence.

Answer 2

I prepared the mentioned \seq approach for you. I append a space to every item of the sequence and remove the resulting trailing space using \tex_unksip:D.

\documentclass{article}
\usepackage{expl3}
\begin{document}
\ExplSyntaxOn
\cs_generate_variant:Nn \seq_set_split:Nnn { NnV }

\tl_new:N \l_token_tl
\tl_set:Nn \l_token_tl { a~b~c }

\seq_new:N \l_token_seq
\seq_set_split:NnV \l_token_seq { ~ } \l_token_tl
\seq_map_inline:Nn \l_token_seq { (#1)~ }
\tex_unskip:D % remove trailing space

\ExplSyntaxOff
\end{document}