[Tex/LaTex] How to see which package define a particular command/macro

macros

We often encounter multiple definitions of certain commands/macros. Then, we often get the error, e.g.:
! LaTeX Error: Command \P already defined.
Or name \end… illegal, see p.192 of the manual.

See the LaTeX manual or LaTeX Companion for explanation.
Type H for immediate help.
…

l.225 \newcommand{\P}{{\mathcal P}}

When we get such an error, is there anyway to know which package or file predefined that particular command/macro? I'm talking about general cases, not the particular case of '\P' listed above.
It is quite cumbersome if we have to go through all possible files via 'grep' manually. I hope that there's a way to see which file already defined the command that produces the conflict/error.

Thanks a lot!

BVP

Best Answer

You can add

\show\P

to your document as in

 \show\P
\documentclass{article}
 \show\P
\usepackage{graphics}
 \show\P
\usepackage{foo}
 \show\P
\usepackage{bar}
 \show\P
\begin{document}
 \show\P

This will make the TeX stop and show the definition of \P at each step, if it is already defined in the format the first one will show the definition, otherwise it will show undefined until the package that defines it is loaded. (The definition might be delayed with \AtBeginDocument which is why I put the final \show at that point.

Related Solutions

[Tex/LaTex] How to redefine or patch the ‘\newcommand’ command

Tackling the problem as posed is tricky due to the way \newcommand works. Heiko's approach is probably more elegant, but one possible method is to use xparse to deal with the syntax of \newcommand, and letltxmacro to deal with the way \newcommand is set up

\documentclass{article}
\usepackage{letltxmacro,xparse}
\makeatletter
\LetLtxMacro{\saved@newcommand}{\newcommand}
\DeclareDocumentCommand{\newcommand}{s+mo+o+m}{%
  \begingroup
  \edef\x{%
    \endgroup
    \ifdefined#2%
      \unexpanded{%
          \renewcommand{#2}%
            {%
              \PackageError{lazyfail}
                {Expanding ambiguous command \protect #2}%
                \@ehc
            }%
        }
    \else
      \noexpand\saved@newcommand
        \IfBooleanT{#1}{*}%
        {\noexpand#2}%
        \IfNoValueF{#3}{[#3]}%
        \IfNoValueF{#4}{[\unexpanded{#4}]}%
        {\unexpanded{#5}}
    \fi
  }%
  \x
}
\makeatother
\begin{document}
\newcommand{\cmd}{FIRST}   %
\cmd                       % outputs FIRST
\newcommand{\cmd}{SECOND}  % no problem yet
\cmd                       % error: expanding ambiguous command
\renewcommand{\cmd}{THIRD} %
\cmd                       % outputs THIRD
\end{document}

The approach here is to grab all of the arguments to the redefined \newcommand in one go, then work out whether they are to be 'recycled' or not.

Of course, you could set all of this up without xparse, but it would be a pain in the next: lots of \@ifstar and \@ifnextchar ( or slightly better \@testopt) stuff and several auxiliaries.

[Tex/LaTex] Double subscript error with \newcommand

The extent to which a "command defined by \newcommand is 'automatically' a single "object" on its own" varies by context.

Certainly such a command is a token, which is an atomic object in TeX's parser. When TeX is reading input, it receives any token as a whole, regardless of its expansion (if any).
It is also an expandable token, which means that it is not an atomic object in TeX's interpreter. In various situations, the input is subject to expansion, in which all expandable tokens are (successively left-to-right) replaced by their meanings.
It is not, as David explains, a "brace group", which is often treated as a single object, particularly when taken as an "argument" to a macro or function-like primitive operator (such as the subscript symbol).

One of the situations in which tokens are expanded is when a subscript symbol is looking for its operand; it expects either a brace group or a single character (or control sequence equivalent to a character). Thus, most \newcommand macros can't be used openly as subscripts, if they expand to more than one token that aren't braced.

Best Answer

Related Solutions

[Tex/LaTex] How to redefine or patch the ‘\newcommand’ command

[Tex/LaTex] Double subscript error with \newcommand

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