How do I expand exactly once a macro which takes an argument

Question

I thought I was finally understanding expansion, but then I wrote something along the following lines, and it has me a bit stumped.

\documentclass{article}
\usepackage{xcolor}
\usepackage{etoolbox}

\makeatletter

\def\ae@test#1{\textcolor{red}{\fbox{#1}}}
\edef\aetest#1{<a bunch of expandable material>\expandonce\ae@test{#1}<more expandable material>}

\makeatother

\begin{document}

\aetest{Hello}

\end{document}

I suspect the issue has something to do with the need to look ahead for the arguments, but I don't even know how to test that theory.

So, I have two questions:

• What's actually happening that the expansion isn't working as I would like to think it should?

• How do I write my macro \aetest so that it does what I want? Namely,

  \aetest{<arg>}->\textcolor{red}{\fbox{<arg>}}
  

I suppose there might be some trick I could do using tokens (since tokens within an \edef expand just once), but I'm a bit cloudy on that approach since I haven't played much with tokens.

Incidentally, I wrote \ae@test as I did because I'm not interested in the following sort of scenario:

\def\ae@test#1{\textcolor{red}{#1}}

where with \aetest the following would accomplish what I want

\edef\aetest#1{\expandafter\noexpand\ae@test{#1}}

The issue is that I want to expand a macro which may have several fragile elements within it.

I did try something along the following lines

\edef\aetest#1{\unexpanded\expandafter{\ae@test{#1}}}

thinking that \ae@test would thus be able to grab its argument, but still the same issue.

Answer 1

The # gets doubled inside \unexpanded. A workaround with \expandafter:

\documentclass{article}
\usepackage{xcolor}

\makeatletter

\def\ae@test#1{\textcolor{red}{\fbox{#1}}}
\expandafter\def\expandafter\aetest\expandafter#\expandafter1\expandafter{%
  \ae@test{#1}%
}
\makeatother

\begin{document}
  \aetest{Hello}
\end{document}

Then \aetest has the following definition:

macro:#1->\textcolor {red}{\fbox {#1}}

Of course in this case, a simple \let would be easier to get the same meaning:

\let\aetest\ae@test

---

A quite complex way is to replace #1 by a different token and exchange it afterwards:

\usepackage{etoolbox}
\def\ae@test#1{\textcolor{red}{\fbox{#1}}}
\let\myarg\relax
\edef\aetest#1{\expandonce{\ae@test{\myarg/}}}
\patchcmd\aetest{\myarg/}{#1}{}{%
  \errmessage{Patching \noexpand\aetest failed}%
}

The last step needs to be repeated to the number of occurrences of #1 inside the definition text of \ae@test. If the number is unknown, then a loop can be used, which aborts at the first error:

\let\myarg\relax
\edef\aetest#1{\expandonce{\ae@test{\myarg/}}}
\count@=\@ne
\@whilenum\count@>\z@\do{%
  \edef\process@me{%
    \noexpand\patchcmd\noexpand\aetest{\noexpand\myarg/}{\string#1}%
  }\process@me{}{\count@=\z@}%
}

Scratch counter \count@ signalizes the success of the patching. If it is set to one, then the replacement of \myarg to #1 is tried. If the patching failed, because \myarg could not be found anymore, then \count@ is set to zero and the loop is aborted. But it is not detected, if there are other reasons for the patch failure.

The circumvention of \process@me is a trick to prevent doubling of # in the third argument of \patchcmd, since it is used as argument of another command (\@whilenum).

Remark:

• \myarg/ is used for #1, otherwise \patchcmd is not happy, if a space if following #1.

Improvement (by egreg)

Package regexpatch provides \xpatchcmd with a star form, which replaces all matches without the need to program a loop manually:

\documentclass{article}
\usepackage{xcolor}
\usepackage{etoolbox}
\usepackage{regexpatch}

\makeatletter
\def\ae@test#1{%
  \typeout{\detokenize{** #1 **}}%
  \textcolor{red}{\fbox{#1}}%
}
\let\myarg\relax
\edef\aetest#1{\expandonce{\ae@test{\myarg/}}}
\xpatchcmd*{\aetest}{\myarg/}{#1}{}{%
  \errmessage{Patching \noexpand\aetest failed}%
}
\makeatother

\begin{document}
  \aetest{Hello}
\end{document}  

\myarg/ instead of \myarg is also needed for \xpatchcmd, because a space is following #1 in \ae@test.

Answer 2

You can use \unexpanded from eTeX using the following code:

\def\aeAtest#1{\textcolor{red}{\fbox{#1}}}

\def\eonce#1{\unexpanded\expandafter{#1{##1}}}
\edef\tmp{\def\noexpand\OUT##1{expanded before \eonce{\aeAtest} after.}}\tmp

\message{\meaning\OUT}
% \OUT#1 -> expanded before \textcolor {red}{\fbox {#1}} after.

Edit Another usage of the same idea defines the \oedef macro:

\def\eonce#1#2{\unexpanded\expandafter{#1{#2}}}
\def\eodef#1#2{\edef\tmp{\def\noexpand#1####1{#2}}\tmp}

\def\aeAtest#1{\textcolor{red}{\fbox{#1},#1}}
\eodef\aetest{expanded ##1+y before \eonce\aeAtest{#1+x} after.}

\message{\meaning\aetest}
% \aetest#1 -> expanded #1+y before \textcolor {red#1}{\fbox {#1+x},#1+x} after.

Unfortunately, the \eodef macro has a curious syntax: the parameter can't be presented in the parameter mask and this parameter must be typed as ##1 in "normal" positions but as #1 in the parameter of \eonce.

Answer 3

To work around the expansion issues point to by @HeikoOberdiek, I've come up with the following work around

\edef\aetest#1{\noexpand\def\noexpand\ae@tmp{#1}%%
    \unexpanded\expandafter{\ae@test{\ae@tmp}}}

though I'll have to test it out a bit before I'm sure that it really does accomplish what I want.

Answer 4

A solution with expl3:

\documentclass{article}
\usepackage{xcolor}
\usepackage{expl3}

\ExplSyntaxOn
\cs_new_protected:Npn \ae_test:n #1
 {
  \textcolor{red}{\fbox{#1}}~
  \textcolor{green}{\fbox{#1}}~
  \textcolor{blue}{\fbox{#1}}
 }

\group_begin:
\tl_set:No \l_tmpa_tl { \ae_test:n { #1 } }
\use:x
 {
  \group_end:
  \cs_new:Npn \exp_not:N \aetest ##1
   {
    something~expandable~
    \exp_not:V \l_tmpa_tl
   }
 }
\ExplSyntaxOff

\begin{document}

\raggedright

\texttt{\meaning\aetest}

\bigskip

\aetest{Hello}

\end{document}

Explanation: in \l_tmpa_tl, we get the replacement text of \ae_test:n with ##1 in place of #1. So we just need to expand once this token list inside another (temporary) definition. The \use:x trick provides this without the need of defining another temporary macro. I used \group_begin: and \group_end: not to leave \l_tmpa_tl modified, but it's not necessary.

Answer 5

This is too long for a comment I suspect, so I'll just put it here. Something that helped me a lot but I don't see it mentioned anywhere is the following. It might be really obvious, but it might also not.

Take the situation as follows. You have your two macros, one which you want to expand, the other which must hold the expanded definition:

\def\expandthis#1{\textcolor{red}{\fbox{#1}}}
\def\macrowithexpandeddefinition#1{\expandthis{#1}}

In order to get what you want, you have to work your way backwards (it will be apparent soon what I mean with this). This requires some knowledge about the idea of tokens, which I'm going to assume you're familiar with. The trickery has to happen in the line with \def\macrowithexpandeddefinition#1. In order to expand \expandthis once, we just have to reach the escape character \ of that macro without expanding anything before that.

Step 1: jump over the last token before \expandthis, which is a brace {:

\def\macrowithexpandeddefinition#1\expandafter{\expandthis{#1}}
                                   ^^^^^^^^^^

Step 2: We must now reach the first \expandafter. There last token before it is 1:

\def\macrowithexpandeddefinition#\expandafter1\expandafter{\expandthis{#1}}
                                  ^^^^^^^^^^

Step 3 and follows: The last token before that is the # (hash) token, continue:

\def\macrowithexpandeddefinition\expandafter#\expandafter1\expandafter{\expandthis{#1}}
                                 ^^^^^^^^^^

And repeat with the last two tokens \macrowithexpandeddefinition and \def respectively:

\expandafter\def\expandafter\macrowithexpandeddefinition\expandafter#\expandafter1\expandafter{\expandthis{#1}}

You can repeat these steps for any macro you like for fine grained expansion control, but it won't make the code very readable. Also note that what I end up with is exactly the same as Heiko's "workaround".