Writing a macro for an edge label in tikz-qtree

Question

I'm attempting to make my code more readable by replacing a long bit of text I intend to use several times with a macro.

Before I use the macro, I have the following code:

\documentclass{minimal}
\usepackage{tikz-qtree}
\newcommand{\llabel}[1]{\edge node[auto=left]{#1};}
\begin{document}
\begin{tikzpicture}
  \Tree [.$P_1$ [.$P_2$ ]
                \edge node[auto=left]{1,2}; [.$P_2$ ] ]
\end{tikzpicture}
\end{document}

This produces the correct output:

When I replace the edge label with my macro, the following code produces the following error:

\documentclass{minimal}
\usepackage{tikz-qtree}
\newcommand{\llabel}[1]{\edge node[auto=left]{#1};}
\begin{document}
\begin{tikzpicture}
  \Tree [.$P_1$ [.$P_2$ ]
                \llabel{1,2} [.$P_2$ ] ]
\end{tikzpicture}
\end{document}

Undefined control sequence.
\llabel #1->\edge 
                  node[auto=left]{#1};
l.8 \end
        {tikzpicture}

Is there a way to do this?

Answer 1

If a different package is an option, Forest can support something like this quite easily. Rather than a macro, it would make sense to use a style llabel in this case. It is also possible to drop the dollar signs by using math content for the tree, reducing visual clutter.

In this example, I use the linguistics library to set up some suitable defaults. The style my tree sets the content of nodes to maths mode and enables the llabel=<text> style.

\documentclass[tikz,border=10pt,multi]{standalone}
\usepackage[linguistics]{forest}
\forestset{%
  my tree/.style={%
    for tree={%
      math content,
    },
    llabel/.style={%
      edge label={node [midway, auto=left]{##1}}
    }
  }
}
\begin{document}
\begin{forest}
  my tree
  [P_1
    [P_2]
    [P_2, llabel={1,2}]
  ]
\end{forest}
\end{document}