tikz arrows of the form =-> and -=>

Question

I am trying to use tikz to draw a quiver and I need to draw arrows of the form =-> and -=> between two nodes. What is the best way to do it? Any help is appreciated.

Answer 1

Is this close to what you want?

\documentclass[tikz, border=5pt]{standalone}
\usetikzlibrary{decorations.pathreplacing, arrows}

\tikzset{
  -z>/.style={
    decoration={
      show path construction,
      lineto code={
        \path (\tikzinputsegmentfirst) -- (\tikzinputsegmentlast) coordinate[pos=#1] (mid);
        \draw (\tikzinputsegmentfirst) -- (mid);
        \draw[double, -implies] (mid) -- (\tikzinputsegmentlast);      }
    },decorate
  }, -z>/.default=.5,
  z->/.style={
    decoration={
      show path construction,
      lineto code={
          \path (\tikzinputsegmentfirst) -- (\tikzinputsegmentlast) coordinate[pos=#1] (mid);
                \draw[double] (\tikzinputsegmentfirst) -- (mid);
                \draw[->] (mid) -- (\tikzinputsegmentlast);
      }
    },decorate
  }, z->/.default=.5,
}

\begin{document}
\begin{tikzpicture}
  \draw [-z>] (0,0) -- (1,0);
  \begin{scope}[red, thick]
    \draw [z->=.3] (0,-.2) -- (1,-.4);
  \end{scope}
\end{tikzpicture}
\end{document}

I don't think you can have = as part of a style name, so I've denoted a double line with z. Note that the arrowheads aren't quite the same for the two styles, although this can probably be fixed. For some reason, styles directly on the \draw commands don't get reliably passed to the decorations, but if you need to style it further you can use a scope.

Answer 2

You can do something like this, which uses an edge to draw the second line with the arrow tip.

\documentclass[tikz,border=10pt]{standalone}
\usetikzlibrary{arrows.meta}

\begin{document}
\begin{tikzpicture}
  \draw  (0,0) -- (1,0) edge [double, -{Implies}] ++(10pt,0);
  \draw [double]  (0,-.25) -- (1,-.25) edge [double=none, ->] ++(10pt,0);
\end{tikzpicture}
\end{document}

But I'm not convinced I understand what you're trying to do.

Answer 3

You can use the midway key (or pos key) to coordinate or node to find the midpoint of an edge, and then draw the two halves, or redraw the double half on top of the single half. Here are some basic examples of doing this. (Note that cfr's use of the arrows.meta tikz library would produce better arrowheads.)

\documentclass[tikz,border=10pt]{standalone}
\begin{document}
\begin{tikzpicture}
\draw[black!10] (-1,0) grid (3,3);
\path (0,0) node (A)[draw,circle] {A} -- (3,1) node[draw,circle](B){B}  (A)--(B) coordinate[midway](mAB);
\path (A) -- (-1,2) node[draw,circle] (C) {C}  (A)--(C) coordinate[pos=0.5](mAC);
\draw[double,line cap=round] (A)--(mAB); \draw[->] (mAB)--(B);
\draw (A)--(mAC); \draw[double,->] (mAC)--(C);
\draw[thick] (3,3)coordinate(A)--(1,1)coordinate(Z) coordinate[midway](M); \draw[double,->] (M)--(Z);
\end{tikzpicture}
\end{document}