Personal command incompatible with tikz calc library

Question

I have written a personal command called \compo that allows you to place a figure next to a text: \compo[.4]{ text }{ figure };

When the figure is created with the tikz calc library, there is this error:

! Package tikz Error: + or - expected.

I installed the tikz babel library and also used \shorthandoff and \shortandon commands, but that doesn't change anything. I guess the problem is my macro. His code is as follows:

%----------------------------------------
% new command \compo to place a figure next to a text
%----------------------------------------
\newlength{\colG}\newlength{\colD}
\newcommand{\compo}[3][0.5]{
\setlength{\colG}{#1\linewidth}
\setlength{\colD}{\linewidth}%
\addtolength{\colD}{-\colG}
\addtolength{\colG}{-10pt}
\addtolength{\colD}{-10pt}%
\par \noindent%
\begin{minipage}[t]{\colG}\vspace{0cm}#2\end{minipage}\hfill\vrule\hfill%
\begin{minipage}[t]{\colD}\vspace{0cm}#3\end{minipage}\par}

Here is a MCE:

\documentclass[a4paper, 11pt]{article} 
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[french]{babel}
\usepackage{tikz}               
\usetikzlibrary{calc}        
\usetikzlibrary{babel}     

%----------------------------------------
% new command \compo to place a figure next to a text
%----------------------------------------
\newlength{\colG}\newlength{\colD}
\newcommand{\compo}[3][0.5]{
\setlength{\colG}{#1\linewidth}
\setlength{\colD}{\linewidth}%
\addtolength{\colD}{-\colG}
\addtolength{\colG}{-10pt}
\addtolength{\colD}{-10pt}%
\par \noindent%
\begin{minipage}[t]{\colG}\vspace{0cm}#2\end{minipage}\hfill\vrule\hfill%
\begin{minipage}[t]{\colD}\vspace{0cm}#3\end{minipage}\par}

\begin{document}

\compo[.5]{
  Un triangle 

}{
\shorthandoff{!:}
\begin{tikzpicture}[handle active characters in code]

\draw(-1,-1) rectangle (4,4);
\coordinate (B) at (0,0);
\coordinate  (C) at (3,0);
\coordinate(D) at (2,0);
\draw(B)--(C)--(D)--cycle;
\coordinate (Ap) at ($(C)!.35!-90:(B)$);
\draw (Ap)--(B)--(C)--cycle;
\end{tikzpicture}
\shorthandon{!:}
}

\end{document}

What is the error in the code of this macro \compo? How do I correct this error?

Answer 1

The problem is a catcode issue. Here is a more minimal demonstration:

\documentclass{article}
\usepackage[french]{babel}
\usepackage{tikz}
\usetikzlibrary{calc}
\usetikzlibrary{babel}

\newcommand\compo[1]{#1} % compo does nothing but return it's argument
\begin{document}
%%% Works:
\shorthandoff{!}
\compo{
    \begin{tikzpicture}[]
    \coordinate (Ap) at ($(0,0)!0!(3,0)$);
    \end{tikzpicture}
}
\shorthandon{!}

%%% Breaks:
\compo{
    \shorthandoff{!}    % \shorthandoff doesn't do anything because characters in 
    \begin{tikzpicture} % argument have already had their catcodes assigned
    \coordinate (Ap) at ($(0,0)!0!(3,0)$); % so the active ! causes an error here
    \end{tikzpicture}
    \shorthandon{!}
}
\end{document} 

Note that I have replaced your command with one that does nothing but return its argument and it still causes the problem. Why does it cause a problem? Because the catcodes of characters are assigned when they are first parsed. Normally, the ! in the body of the tikzpicture is parsed after the \shorthandoff{!} command changes ! from catcode active to catcode other. However, when the ! occurs in the argument of a command, the entire body of the command is parsed before any of the body is evaluated. Thus, the ! is already active, and the catcode change caused by the \shorthandoff{!} command does nothing. The best fix is to do the catcode change inside the \compo command and call a helpre command to grab the arguments:

\documentclass{article}
\usepackage[french]{babel}
\usepackage{tikz}
\usetikzlibrary{calc}
\usetikzlibrary{babel}

% The wrapper command doesn't take any arguments, it just makes the catcode change and 
% calls the helper.
\newcommand\compo{\shorthandoff{!}\compohelper}
% The helper does all the work.
\newcommand\compohelper[1]{#1\shorthandon{!}}
\begin{document}
\compo{
    \begin{tikzpicture}
    \coordinate (Ap) at ($(0,0)!0!(3,0)$);
    \end{tikzpicture}
}
\end{document} 

Now you can use your original definition for \compohelper and it works as expected:

\documentclass{article}
\usepackage[french]{babel}
\usepackage{tikz}
\usetikzlibrary{calc}
\usetikzlibrary{babel}

\newlength{\colG}\newlength{\colD}
% Wrapper command just changes catcodes and calls helper
\newcommand\compo{%
    \shorthandoff{!:}% Change the catcodes
    \compohelper % call helper to grab arguments
}
\newcommand{\compohelper}[3][0.5]{% do actual work
    \setlength{\colG}{#1\linewidth}%
    \setlength{\colD}{\linewidth}%
    \addtolength{\colD}{-\colG}%
    \addtolength{\colG}{-10pt}%
    \addtolength{\colD}{-10pt}%
    \par \noindent%
    \begin{minipage}[t]{\colG}#2\end{minipage}\hfill\vrule\hfill%    
    \begin{minipage}[t]{\colD}#3\end{minipage}%
    \par
    \shorthandon{!:}% restore catcodes
}

\begin{document}
\compo[.5]{
  Un triangle

}{
    \begin{tikzpicture}
        \draw(-1,-1) rectangle (4,4);
        \coordinate (B) at (0,0);
        \coordinate  (C) at (3,0);
        \coordinate(D) at (2,0);
        \draw(B)--(C)--(D)--cycle;
        \coordinate (Ap) at ($(C)!.35!-90:(B)$);
        \draw (Ap)--(B)--(C)--cycle;
    \end{tikzpicture}
}
\end{document}