2

Consider this example:

\documentclass{article}
\begin{document}
\def\something{something}
\something

    {
        \def\something{another thing}

      \something
    }

    \something
\end{document}

From the output it can be seen that there are two variables something each living in their own scope:

enter image description here

Now, I put a variable to live inside a newcommand:

\documentclass{article}
\def\something{something}
\newcommand{\dosomething}{
  invoking ``dosomething''
  \def\something{another thing}

  \something
}
\begin{document}
\something

\dosomething

\something
\end{document}

From output it can be seen that invoking the newcommand altered the previous definition of the variable:

enter image description here

This can lead to obscure errors particularly with libraries involving hundreds of variables. Is there a way to protect my variables from being altered by newcommands? What is the best approach?

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5
  • 3
    Well, in the second example you have no group. \newcommand does not create a group by itself. You could do \newcommand{\dosomething}{{...}}.
    – campa
    Dec 2, 2019 at 10:42
  • 2
    a macro is just expanded inline essentially as a textual replacement, it implies no grouping itself. You can of course define the replacement text of the command to include a group, But mixing \def and \newcommand in the same code is likely to lead to obscure errors clearer to stick to primitive constructs or latex ones, not a mixture of the two.
    – David Carlisle
    Dec 2, 2019 at 11:14
  • @campa better to use the more explicit \begingroup and \endgroup, imho.
    – Skillmon
    Dec 2, 2019 at 11:14
  • Your macros can have names from which you can see what they (are intended to) do. E.g., instead of \dosomething you could name the thing \WithinTheCurrentScopeTurnSomethingIntoAnotherThing. At first glimpse long macro names which have a semantical aspect as they explain what the thing does seem cumbersome. But they can be very nice when the need of debugging arises.
    – Ulrich Diez
    Dec 2, 2019 at 20:27
  • By default (re)definitions in terms of \newcommand/\renewcommand apply to the current scope. In order to prevent this make sure that another local scope is opened up before the (re)definition-assignments in question are carried out. E.g., by means of \begingroup..\endgroup or {..} or \bgroup..\egroup. In case you wish (re)definitions in terms of \(re)newcommand to apply to all superordinate scopes also, look at the package letltxmacro, especially the command \GlobalLetLtxMacro.
    – Ulrich Diez
    Dec 2, 2019 at 20:38

1 Answer 1

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4

TeX offers a macro expansion mechanism; the macro is replaced by the replacement text. Therefore, with your current definition

\newcommand{\dosomething}{
  invoking ``dosomething''
  \def\something{another thing}

  \something
}

the document

\begin{document}
\something

\dosomething

\something
\end{document}

is effectively equivalent to

\begin{document}
\something

invoking ``dosomething''
\def\something{another thing}

\something

\something
\end{document}

(plus a couple of trailing spaces you have been somewhat careless about :-))

If you want changes to \something to be restricted to the execution of \dosomething you must include a level of grouping, either with explicit braces {...} or (more readable) with the primitives \begingroup/\endgroup

\newcommand{\dosomething}{% <-- don't forget this
  \begingroup
  invoking ``dosomething''%
  \def\something{another thing}%

  \something
  \endgroup
}
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