What if I use a \def name starting with "if"

Question

Following this answer to Is it possible to use some sort of internationalization in LaTeX? I'm setting up my own dictionaries.

When compiling the files I get this error:

! Use of \if doesn't match its definition.

and I guess is why I named a term as if_foreign_buyer

\def \if_foreign_buyer{You can only...}

removing starting if solves the problem

Any solution, other than not starting terms with if?

Moreover I would like to start all terms with a underscore _ so I can grab them with a sed command and create dictionaries from terms in the master file.

Also this seems not allowed. Really?

Answer 1

The problem here is that non-letters like _ are not allowed in macro names and not taken as part of the name. Any text after \def\themacro and the { of the macro replacement text is taken as the parameter text which must match the actual input. With your code you are (re-)defining the TeX primitive \if as a macro which awaits and removes _foreign_buyer after it and replaces it with You can only.... Then the first use of \if (inside a package or a LaTeX core macro) which isn't followed by _foreign_buyer throws a TeX error.

You shouldn't use _ as part of macro names and if you do you need to ensure that they have the correct catcode, namely 11 "letter", while being defined and when used. This can be done using \catcode`\_=11\relax but is not recommended and will break code which relies on the default meaning of _ as an subscript command in math-mode.

Having if in front of a macro name is not a problem and fully OK. The only restriction are \ifxxxs defined by \newif where it is assumed that they start with if. The first two letters are then always stripped to produce \xxxtrue and \xxxfalse. This doesn't affect macro definitions though.

Answer 2

There is almost nothing really bad if you use _ in command names, provided you have

\catcode`\_=11

before starting the definition of those commands and never revert the choice.

You can even continue to use _ in math formulas by adding

\mathcode`\_=\string"8000
\begingroup
\catcode`\_=\active
\global\let_\sb
\endgroup

However such a naming scheme is non standard and definitely not recommendable. With some babel languages it may even go wrong.

I don't see how

\command_with_underscore

will increase readability over

\NEcommand

that's as easily searchable.

An example, just to show how this can work.

\documentclass{article}

% Start of document commands
\catcode`\_=11
\mathcode`\_=\string"8000
\begingroup
\catcode`\_=\active
\global\let_\sb
\endgroup

\newcommand{\foreign_buyer}{whatever}

\begin{document}
Here we use \foreign_buyer, and also a formula $a_{1}$.
\end{document}

Starting a command name with \if is definitely bad practice, unless it is a real conditional, introduced with \newif.

---

Notice that without \catcode`\_=11, when you say

\def\if_foreign_buyer{...}

you're actually (re)defining the very important primitive command \if and this will cause sure disasters with any document.

---

Note. The command \sb is defined by \let\sb=_ in the LaTeX kernel, so its meaning doesn't change when one modifies the category code of the underscore character. So it's always available as a substitute for it.

In this application we are telling TeX that _ in math mode should behave like a command, which expands to \sb, that is it will just do the right thing. However, macros that rely on the presence of _ will be fooled by this, so some package or macro might fail (not commonly used, but I've seen some cases where a macro tests for a following _).

Answer 3

Use the hyphen instead and it is easier:

\documentclass{minimal}
\makeatletter
\@namedef{if-foreign-buyer}{You can only...}
\makeatother
\begin{document}

\csname if-foreign-buyer\endcsname

\end{document}  

Answer 4

\documentclass{article}
\makeatletter
\let\mydef\@namedef
\let\myuse\@nameuse
\makeatother

\begin{document}

\mydef{foreign_buyer}{jfbu}
\mydef{foreign_seller}{egreg}


The foreign seller \myuse{foreign_seller} is definitely infinitely more of a
\TeX pert than the foreign buyer \myuse{foreign_buyer}.


\end{document}