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I'd like to "add to" an existing command by writing something like

\renewcommand{\somecommand}{\begin{something}\somecommand\end{something}}

but (not surprisingly) I get errors about being "too deeply nested".

Is there a way to take an existing command, add to it (by appending or prepending additional commands or text), and then have that "augmented" command replace the existing command?

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8  
You may also want to look at the etoolbox package. –  mbork Mar 9 '12 at 6:55

4 Answers 4

up vote 28 down vote accepted

You can use \let to save a command under another name, and then use that new name in the \renewcommand.

\documentclass{article}

\let\oldemph\emph
\renewcommand{\emph}[1]{\textbf{\oldemph{#1}}}

\begin{document}
\oldemph{Old emph}
\emph{New emph}
\end{document}

(Edit: the TeX FAQ has much more information about "patching" commands. Edit2: egreg's answer below describes some of the gotchas of \let)

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Sometimes, you need to use the original name of the command, and you cannot use an environment because of some undesirable side effects. I have the case with \LettrineFontHook which is not directly called in my code. Here, the only solution I have found is to use \let to store and restore the command:

\let\tempLettrineFontHook\LettrineFontHook
\renewcommand{\LettrineFontHook}{\fontseries{bx}}
\lettrine{F}{}
\let\LettrineFontHook\tempLettrineFontHook

If I try to define an environment like this:

\newenvironment{boldLettrine}{%
\renewcommand{\LettrineFontHook}{\fontseries{bx}}%
}{}
% let's use this environment in the document now:
\begin{boldLettrine}
\lettrine{F}{}
\end{boldLettrine}

For reasons I ignore, the indentation of the lettrine is broken.

EDIT: From egreg remark, in your example, this method cannot apply since the new command is defined with the help of the original one, leading to possible infinite loops. So I cancel the application to your use case.

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1  
not that I follow like a detective all your moves but viz For reasons I ignore, the indentation of the lettrine is broken. : there must be a \par. Because the closing of the environment cleans up the \parshape set up by lettrine package. Please always post complete MWE rather than just a few lines of code. Apart from that this uses your own \Lettrine from some other question but should be with \lettrine. (but then one needs a second, possibly empty, argument {}) –  jfbu May 22 at 17:53
1  
In my answer I tell exactly why \let\oldemph\emph is to be avoided. –  egreg May 22 at 18:13
    
@egreg, in the case you detail, the copied command is used in the definition of the original one, what can makes infinite loops in the conditions you precise. I think it is not the case in my use case, but I should not have generalized. –  lalebarde May 22 at 19:53
    
@jfbu: you are welcome, I do appreciate your help. I can reword my post with only lettrine. –  lalebarde May 22 at 20:00
1  
@lalebarde Follow \lettrine{F}{} by a \par before closing the environment if you want to get the indentation as expected. Did you notice that the original post dates back to Mar 9 '12 at 5:31? –  jfbu May 22 at 20:11

While something such as

\let\oldemph\emph
\renewcommand{\emph}[1]{\textbf{\oldemph{#1}}}

seems to work, it really doesn't, because \emph is defined with \DeclareRobustCommand. Where doesn't it work? suppose you write that code and then use \emph in a caption:

\caption{This is \emph{emphasized}}

what is written in the .aux file is

\@writefile{lof}{\contentsline {figure}{\numberline {1}{\ignorespaces This is \textbf  {\emph  {emphasized}}}}{1}}

This is because the original expansion of \emph is

\protect\emph§

(where with § I'm denoting a space in the name). So the \emph{emphasized} in the caption gets expanded successively into

\textbf{\oldemph{emphasized}}
\protect\textbf§{\protect\emph§{emphasized}}
\textbf§{\emph§{emphasized}

and written as

\textbf  {\emph  {emphasized}}

(because TeX adds a space after a command name when writing). In this case, when reading the file for building the List of Figures or Tables, TeX will find

\textbf{\textbf{\oldemph{emphasized}}}

which is innocuous. But it's not very difficult to imagine situations where this can lead to infinite loops.

The package letltxmacro prevents this problem:

\usepackage{letltxmacro}
\LetLtxMacro{\oldemph}{\emph}
\renewcommand{\emph}[1]{\textbf{\oldemph{#1}}}

would be a safer way to proceed.

Never use \let on commands defined with \DeclareRobustCommand or commands defined with \newcommand and having an optional argument.

The former can be recognized by the fact that \show\command returns

> \command=macro:
->\protect \command  .

The latter are distinguished by the fact that \show\command returns

> \command=macro:
->\@protected@testopt \command \\command {...}.

(where ... is actually the default for the optional argument).

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Haha, I picked a perfect bad example! (Thanks for explaining the mistake!) –  dbaupp Mar 9 '12 at 10:34

Yes, this is possible using \g@addto@macro or some expanded definition (using \expandafter). Here are some examples:

enter image description here

\documentclass{article}
\setlength{\parindent}{0pt}% Just for this example
\begin{document}
\newcommand{\somecmd}{some \textbf{text}}
\somecmd \par
\makeatletter
\g@addto@macro\somecmd{ and some \textit{more} text}
\makeatother
\somecmd

\renewcommand{\somecmd}{some \textbf{text}}
\somecmd \par
\makeatletter
\expandafter\def\expandafter\somecmd\expandafter{\somecmd{} and some \textit{more} text}
\makeatother
\somecmd
\end{document}

The latter example, just like the former, appends contents to a macro. The placement of \somecmd in the latter could be used to prepend something.

However, nothing prevents you from making a new command that includes the old command as part of its expansion, since memory problems tend to be a thing of the past.

You run into problems when the contents of the newly-defined macro does not expand properly. Using your case as an example: although \somecommand may expand as expected, the center environment does not.

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