How to define Macros in forest (e.g. labeling edges)

Question

I want to label single edges and branching ones with some style:

For example, instead of

edge label={node[right,midway,font=\scriptsize]{9}}

I want to have \labelA[right]{\scriptsize}{9} and instead of

node options={label={[label distance=7pt,font=\scriptsize]-90:0}}

\labelB[7pt]{\scriptsize}{0}

I tried example in the package about decision tree but I did not get it completely and I do not want to commit to decision trees.

MWE:

\documentclass[10pt,a4paper]{article}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{forest}
\begin{document}

\begin{forest}
%  decision edge label/.style n args=3{
%    edge label/.expanded={node[midway,auto=#1,anchor=#2,\forestoption{elo}]{\strut$\unexpanded{#3}$}}
%  },
for tree={  align=center, 
            parent anchor=south, 
            child anchor=north, 
            l sep=10mm,
            s sep=10mm}         
[A\\B\rule{0mm}{4mm}
 ,baseline
 [C\\D
  ,edge label={node[right,midway,font=\scriptsize]{9}}
  ,node options={label={[label distance=7pt,font=\scriptsize]-90:0}}
  [E
   [F
    ,edge label={node[left,midway,font=\scriptsize]{3}}
   ]
  ]
  [M
   [P
    ,edge label={node[left,midway,font=\scriptsize]{1}} 
   ]
  ]
 ]
]  
\end{forest}

\end{document}

Answer 1

Note first that node options is rarely needed.

label={[label distance=7pt,font=\scriptsize]-90:0} 

is equivalent. You would only need node options here if you wanted multiple labels.

A macro is not, I think, the best way to do this. In particular, a macro with a standard optional argument of the kind you have in mind is definitely not going to work because the square brackets will confuse the parser, so you'd at least have to protect them.

But there's a more basic issue. Where, exactly, would \labelA or \labelB go? Consider

[node text \labelA...
]

Here the label specification will be parsed as part of the contents of the node. Forest won't mind this, but it won't give the result you want so that's not much use.

Alternatively

[node text, \labelA...
]

Here, \labelA... will be read as a key-value option, which it isn't. Forest will not be at all happy. Nor will TikZ, which Forest will try passing the buck to if it manages to parse things at all.

The only remaining spot is

[node text
]\labelA...

which will result in the macro being entirely ignored, I think.

So even a simple macro, without an optional argument, is not going to work here.

Instead, you should use a custom style as the example you cited suggests. However, you don't need anything as complex as the style the manual gives because you are not trying to automate the customisation or the placement and/or content of the labels.

You probably want something like the following.

We define 2 styles, labelA and labelB. Each takes 3 arguments.

• \labelA={<direction>}{<font>}{<content>} creates an edge label given by the <direction> (e.g. left, right) in the relevant <font> (e.g. \scriptsize, \sffamily) containing the specified <content> (e.g. 9, exotic paper towels).

• labelB={<distance>}{<font>}{<content>} creates a label underneath the main node at a distance of <distance> (e.g. 7pt, 3mm) in the relevant <font> (e.g. \large, \bfseries) containing the specified <content> (e.g. 0, tranquil Malteasers).

For example,

    labelA={right}{\scriptsize}{9},
    labelB={7pt}{\scriptsize}{0},

recreates the labels in the original example for the C\\D node.

Coode:

\documentclass[tikz, border=5pt, multi]{standalone}
\usepackage{forest}
\begin{document}
\begin{forest}
  labelA/.style n args=3{%
    edge label/.expanded={%
      node [midway, auto=#1, font=\unexpanded{#2}] {#3}%
    },
  },
  labelB/.style n args=3{%
    label={%
      [label distance=#1, font=\unexpanded{#2}]-90:{#3}%
    },
  },
  for tree={
    align=center,
    parent anchor=children,
    child anchor=parent,
    l sep'=10mm,
    s sep'=10mm,
  }
  [A\\B\rule{0mm}{4mm},
   baseline
   [C\\D,
    labelA={right}{\scriptsize}{9},
    labelB={7pt}{\scriptsize}{0},
    [E
     [F,
       labelA={left}{\scriptsize}{3}
     ]
    ]
    [M
     [P,
       labelA={left}{\scriptsize}{1}
     ]
    ]
   ]
  ]
\end{forest}
\end{document}