A LaTeX3 macro to typeset chemical compounds?

Question

Out of curiosity I wonder how would a macro coded in expl3 that does the basics of mhchem (as in section basics in mhchem manual) looks like?

I have found examples in TeX and LuaTeX in http://wiki.contextgarden.net/Programming_in_LuaTeX but their input style is too close to math (in my view, not as convenient asmhchem).

So gurus please showcase the power of expl3.

Answer 1

I wouldn't have a clue if this is sufficient to fully implement what \ce is capable of, but here's a quick example of how I'd go about this. Note that there are no state machines or active characters, just judicious use of search'n'replace. The TeX equivalent, I suppose, of a regexp solution. Who knows if it's efficient, though.

\documentclass{article}
\usepackage{expl3,xparse,
  fixltx2e, % for \textsubscript
  amstext   % for \text
}

\newcommand\textsubsuperscript[2]{$_{\text{#1}}^{\text{#2}}$}
\newcommand\textsupersubscript[2]{$_{\text{#2}}^{\text{#1}}$}

\ExplSyntaxOn

% \catcode 12 strings of otherwise special chars:
\tl_set:Nx \c_underscore_char { \cs_to_str:N \_ }
\tl_set:Nx \c_caret_char      { \cs_to_str:N \^ }
\tl_set:Nx \c_bgroup_char     { \cs_to_str:N \{ }
\tl_set:Nx \c_egroup_char     { \cs_to_str:N \} }

\DeclareDocumentCommand \ce {m} {
  \tl_set:Nx \l_ce_tl { \tl_to_str:n {#1} }

  \tl_replace_all_in:Nnn \l_ce_tl {1} {\ce_sub:w{1}}
  \tl_replace_all_in:Nnn \l_ce_tl {2} {\ce_sub:w{2}}
  \tl_replace_all_in:Nnn \l_ce_tl {3} {\ce_sub:w{3}}
  \tl_replace_all_in:Nnn \l_ce_tl {4} {\ce_sub:w{4}}
  \tl_replace_all_in:Nnn \l_ce_tl {5} {\ce_sub:w{5}}
  \tl_replace_all_in:Nnn \l_ce_tl {6} {\ce_sub:w{6}}
  \tl_replace_all_in:Nnn \l_ce_tl {7} {\ce_sub:w{7}}
  \tl_replace_all_in:Nnn \l_ce_tl {8} {\ce_sub:w{8}}
  \tl_replace_all_in:Nnn \l_ce_tl {9} {\ce_sub:w{9}}
  \tl_replace_all_in:Nnn \l_ce_tl {0} {\ce_sub:w{0}}

  \tl_replace_all_in:Non \l_ce_tl \c_caret_char {\ce_super:w}
  \tl_replace_all_in:Non \l_ce_tl \c_underscore_char {\ce_sub:w}

  \tl_replace_all_in:Nnn \l_ce_tl {+} {\ce_super:w{+}}
  \tl_replace_all_in:Nnn \l_ce_tl {-} {\ce_super:w{$-$}}

  % fix syntax like "^2":
  \tl_replace_all_in:Nnn \l_ce_tl {\ce_super:w \ce_sub:w} {\ce_super:w}

  % finally, fix up braces that were detokenized:
  \tl_rescan:nV {\ExplSyntaxOn} \l_ce_tl
}

\cs_set:Npn \ce_super:w #1 {
  \peek_meaning_remove_ignore_spaces:NTF \ce_sub:w
  {\textsupersubscript{\ce_disable_subsuper: #1}}
  {\textsuperscript{\ce_disable_subsuper: #1}}
}

\cs_set:Npn \ce_sub:w #1 {
  \peek_meaning_remove_ignore_spaces:NTF \ce_super:w
  {\textsubsuperscript{\ce_disable_subsuper: #1}}
  {\textsubscript{\ce_disable_subsuper: #1}}
}

\cs_set:Nn \ce_disable_subsuper: {
  \cs_set_eq:NN \ce_super:w \use:n
  \cs_set_eq:NN \ce_sub:w \use:n
}

% Need these variants:
\cs_generate_variant:Nn \tl_replace_all_in:Nnn {Non}
\cs_generate_variant:Nn \tl_rescan:nn {nV}

\ExplSyntaxOff

\begin{document}
\ce{Y^{99}+}\par
\ce{Y^{99+}}\par
\ce{H20}\par
\ce{Sb2O3}\par
\ce{H+}\par
\ce{CrO4^2-}\par
\ce{AgCl2-}\par
\ce{[AgCl2]-}\par
\ce{H2_{(aq)}}\par
\ce{NO3-}\par
\ce{(NH4)2S}\par
\end{document}

The premise of \ce is that the argument is turned into a string before replacing the various characters with macros to expand to the desired output. First the numbers are replaced by themselves inside a conditional subscript, then ^ and _ are replaced macros to perform "conditional superscripts and subscripts" (resp.), and finally + and - are replaced by themselves superscripted.

In case of syntax like ^2, which would be replaced by [superscript]+[subscript 2], we replace the combination of [superscript]+[subscript] with the conditional superscript alone.

Finally, the string is the "retokenised" using the expl3 wrapper around \scantokens to perform all of the typesetting.

The "conditional subscripts" and "conditional superscripts" that I refer to above are simply text sub-/super-scripts that deactivates themselves for nested material. E.g., [subscript [subscript x]] is interpreted as [subscript x]. This allows things like + inside a superscript not superscripting itself again.