TikZ setting the edge of a path to horizontal rather than in=0 out=180 or vice versa

Question

\path[->] (A) edge [out=180, in=0] (B);

This path is written quite often in my document. Sometimes I find myself changing it to:

\path[->] (B) edge [out=0, in=180] (A);

I do not want to touch the -> property for consistance sake. But I would love to write something like:

\path[->] (B) edge [inout = horizontal] (A);

How can I achieve the implied effect?

---

EDIT:

Clarification: I want those two paths to be the same path without manually changing the input output angles.

\path[-] (B) edge [inout = horizontal] (A);
\path[-] (A) edge [inout = horizontal] (B);

Here is a miniexample that illustrates the question better:

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
    \node (A) at (0,0) {A};
    \node (B) at (3,2) {B};

    \node (C) at (3,0) {C};
    \node (D) at (6,2) {D};

    \path[-] (B) edge [out=0, in=180] (A); %not desired - longer path
    \path[-] (A) edge [out=0, in=180] (B); %desired path - shorter path

    %\path[-] (C) edge [inout = automagic] (D); %desired path - shorter path
    %\path[-] (D) edge [inout = automagic] (C); %desired path - shorter path 
    \path[-, ultra thick, red] (C) edge [out=0, in=180] (D); %desired result of [inout = automagic]
\end{tikzpicture}
\end{document}

Answer 1

I provide three styles:

• the curve - style that produces a “horizontal” curve (the in and out direction are 0 and 180),
• the curve | style that produces a “vertical” curve (the in and out direction are 90 and 270),
• the underlying curve + style that, if used directly, decides on its own whether a “vertical” or “horizontal” curve shall be used.

Code

\documentclass[tikz]{standalone}
\makeatletter
\tikzset{
  curve +/.default=2,
  curve +/.style={
    to path={
      \pgfextra
        \tikz@scan@one@point\pgfutil@firstofone(\tikztostart)\relax
        \pgf@xa\pgf@x
        \pgf@ya\pgf@y
        \tikz@scan@one@point\pgfutil@firstofone(\tikztotarget)\relax
        \ifnum#1=2
          \pgfmathifthenelse{abs(\pgf@xa-\pgf@x)<abs(\pgf@ya-\pgf@y)}{0}{1}% 0 = ver
                                                                           % 1 = hor
        \else
          \def\pgfmathresult{#1}%
        \fi
        \ifnum\pgfmathresult=0
          \ifdim\pgf@ya<\pgf@y
            \def\tikz@to@out{90}\def\tikz@to@in{270}%
          \else
            \def\tikz@to@out{270}\def\tikz@to@in{90}%
          \fi
        \else
          \ifdim\pgf@xa<\pgf@x
            \def\tikz@to@out{0}\def\tikz@to@in{180}%
          \else
            \def\tikz@to@out{180}\def\tikz@to@in{0}%
          \fi
        \fi
      \endpgfextra
      \tikz@to@curve@path
    }
  },
  curve -/.style={curve +=1},
  curve |/.style={curve +=0},
}
\makeatother

\begin{document}
\foreach \a in {0,2,...,359}{% Warning: Will typeset 180 pages!
\begin{tikzpicture}
\useasboundingbox (-2.2,-2.2) rectangle (2.2,2.2);
\node (A) at (0,0) {A};
\node (B) at (\a:2cm) {B};
\path[->] (A) edge [black!50, curve |] (B)
              edge [black!50, curve -] (B)
              edge [black,    curve +] (B);
\end{tikzpicture}
}
\end{document}

Output

Answer 2

You can create your custom .is choice family. It will also complain if you mistype, or use an undesignated option that doesn't belong to the choice family which is helpful to debug.

\documentclass[tikz]{standalone}

\tikzset{inout/.is choice,
inout/horizo/.style={in=180,out=0},
inout/horizorev/.style={out=180,in=0}
}

\begin{document}
\begin{tikzpicture}
\node (A) at (0,0) {A};
\node (B) at (3,2) {B};
\node (C) at (3,-2) {C};
\path[->] (A) edge [inout=horizo] (B);
\path[->] (A) edge [inout=horizorev] (C);
\end{tikzpicture}
\end{document}