Btukfyl

documentclass[pstricks,border=12pt]{standalone}

usepackage{pstricks-add}

begin{document}

multido{i=0+30}{12}{%

begin{pspicture}(-2,-2)(2,2)

    pscircle{1}

    pnode(1;i){R}

    psset{arrows=->}

    psline(R)

    psline[linecolor=blue](R|0,0)

    psline[linecolor=red](0,0|R)

    psset{linecolor=gray,linestyle=dashed,linewidth=0.5pslinewidth,arrows=-,dash=2pt 2pt}

    psline(R)(R|0,0)

    psline(R)(0,0|R)

end{pspicture}}

end{document}

Does a zero-length line with an arrow make sense? Is it a bug? I hope it is a bug.
If it is not a bug, what is the best way to handle this case using a conditional macro in either TeX or PS level?

Apparently, this feature is also adopted by TikZ.

documentclass[tikz,border=0pt]{standalone}

usepackage{tikz}

begin{document}

    tikzpicture

        fill[yellow] (-1,-1) -- (-1,1) -- (1,1) -- (1,-1) -- cycle;

        draw[->] (0,0)--(0,0);

    endtikzpicture

end{document}

With pstricks.tex from http://archiv.dante.de/~herbert/texnik/tex/generic/pstricks/ psLine can have only one or two pairs of coordinates. Will later be on CTAN.

documentclass[pstricks,border=12pt]{standalone}

usepackage{pst-node,multido}

begin{document}

multido{i=0+30}{12}{%

begin{pspicture}(-2,-2)(2,2)

    pscircle{1}

    pnode(1;i){R}

    psset{arrows=->}

    psLine(R)

    psLine[linecolor=blue](R|0,0)

    psLine[linecolor=red](0,0|R)

    psset{linecolor=gray,linestyle=dashed,linewidth=0.5pslinewidth,arrows=-,dash=2pt 2pt}

    psLine(R)(R|0,0)

    psLine(R)(0,0|R)

end{pspicture}}

end{document}

In TikZ, you can avoid drawing arrow heads if the last segment of a path has a length of zero by patching the internal pgf@check@for@arrows macro:

makeatletter

defpgf@check@for@arrows{%

  pgf@drawarrowsfalse%

  ifxpgf@startarrowpgfutil@emptyelsepgf@drawarrowstruefi%

  ifxpgf@endarrowpgfutil@emptyelsepgf@drawarrowstruefi%

  ifdimpgf@shorten@end@additional=0ptrelaxelsepgf@drawarrowstruefi%

  ifdimpgf@shorten@start@additional=0ptrelaxelsepgf@drawarrowstruefi%

  ifpgf@drawarrows%

    pgfsyssoftpath@getcurrentpathpgf@arrowpath%

    ifxpgf@arrowpathpgfutil@empty%

      pgf@drawarrowsfalse%

    else%

      pgfprocesscheckclosed{pgf@arrowpath}{pgf@drawarrowsfalse}%

      %%% New stuff starts here

      % Extract first, second, second last and last points

      pgfprocesspathextractpoints{pgf@arrowpath}%

      % If the second last and last points are identical ...

      ifxpgfpointsecondlastonpathpgfpointlastonpath%

        % ... disable the arrow head

        pgf@drawarrowsfalse%

      fi%

      %%% New stuff ends here

    fi%

  fi%

}

makeatother

documentclass[tikz,border=0pt]{standalone}

usepackage{tikz}



makeatletter

defpgf@check@for@arrows{%

  pgf@drawarrowsfalse%

  ifxpgf@startarrowpgfutil@emptyelsepgf@drawarrowstruefi%

  ifxpgf@endarrowpgfutil@emptyelsepgf@drawarrowstruefi%

  ifdimpgf@shorten@end@additional=0ptrelaxelsepgf@drawarrowstruefi%

  ifdimpgf@shorten@start@additional=0ptrelaxelsepgf@drawarrowstruefi%

  ifpgf@drawarrows%

    pgfsyssoftpath@getcurrentpathpgf@arrowpath%

    ifxpgf@arrowpathpgfutil@empty%

      pgf@drawarrowsfalse%

    else%

      pgfprocesscheckclosed{pgf@arrowpath}{pgf@drawarrowsfalse}%

      pgfprocesspathextractpoints{pgf@arrowpath}%

      ifxpgfpointsecondlastonpathpgfpointlastonpath%

        pgf@drawarrowsfalse%

      fi%

    fi%

  fi%

}

makeatother



begin{document}

foreach angle in {0,5,...,355}{%

    begin{tikzpicture}[

        scale=1.5,

        arrow/.style={

            -stealth, thick, line cap=round 

        }

    ]

        fill [white] (-1.02,-1.02) rectangle (1.02, 1.02);

        draw [gray, densely dashed] (0:{cos(angle)}) |- (90:{sin(angle)});

        draw (0,0) circle [radius=1];

        draw [blue, arrow] (0,0) -- (0:{cos(angle)});

        draw [red, arrow] (0,0) -- (90:{sin(angle)});

        draw [black, arrow] (0,0) -- (angle:1);

    end{tikzpicture}%

}

end{document}