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

begin{document}

begin{pspicture}[showgrid](5,5)

psline[arrows=->](1,1)(1,1)

psellipticarc[arrows=->](2.5,2.5)(1,2){0}{0}

psellipticarcn[arrows=->](2.5,2.5)(2,1){0}{0}

psarc[arrows=->](2.5,2.5){1}{0}{0}

psarcn[arrows=->](2.5,2.5){2}{0}{0}

end{pspicture}

end{document}

Question:

Should zero-length psline[arrows=->](1,1)(1,1) be displayed?

Why does zero-length psellipticarc get rendered while zero-length psarc does not?

With an up-to-date pstricks.tex from http://archiv.dante.de/~herbert/texnik/tex/generic/pstricks/

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

begin{document}

    begin{pspicture}[showgrid](5,5)

    psLine[arrows=->](1,1)(1,1)

    psellipticarc[arrows=->](2.5,2.5)(1,2){0}{0}

    psellipticarcn[arrows=->](2.5,2.5)(2,1){0}{0}

    psarc[arrows=->](2.5,2.5){1}{0}{0}

    psarcn[arrows=->](2.5,2.5){2}{0}{0}

    end{pspicture}

end{document}

you'll get an empty grid.

I can only reproduce the single arrow for psline[arrows=->](1,1)(1,1) because I use the latest TeX Live 2018 update. I think someone has changed the arc-related macros several days ago.

A single arrow for the zero-length line is a feature that is intentionally kept as it is because according to someone it is useful for drawing zero-length vectors. TikZ also has the same feature, right? :-)

So if you want to get nothing for a zero-length line, do modify the psline as follows.

makeatletter

defpsline{pst@object{psline}}% a special Line 

defpsline@i{pst@getarrows{begin@OpenObj pst@getcoors[psline@ii}}

defpsline@ii{%

    addto@pscode{

      ifPst@noCurrentPointelsepst@cpfi% current point?

      4 copy Pyth2 psk@arrowlength ge 

        { psline@iii tx@Line }% arc and lineto type

        { pop pop pop pop } ifelse }%

  end@OpenObj}

makeatother

Here I present an example in which zero-length lines are practically useful in our life. Of course we can use conditional macro to get the same result.

documentclass[pstricks,border=1cm]{standalone}

usepackage{pstricks-add}

makeatletter

defpsline{pst@object{psline}}% a special Line 

defpsline@i{pst@getarrows{begin@OpenObj pst@getcoors[psline@ii}}

defpsline@ii{%

    addto@pscode{

      ifPst@noCurrentPointelsepst@cpfi% current point?

      4 copy Pyth2 psk@arrowlength ge 

        { psline@iii tx@Line }% arc and lineto type

        { pop pop pop pop } ifelse }%

  end@OpenObj}

makeatother



usepackage[nomessages]{fp}

newcommandconst[3][3]{%

    expandafterFPevalcsname#2endcsname{round(#3:#1)}%

    pstVerb{/#2 csname#2endcsnamespace def}%

}

newcommandConst[3][3]{begingroupedeftemp{endgroupnoexpandconst[#1]{#2}{#3}}temp}



Const{Tpeak}{1}

Const{Theta}{80/180*pi}

Const{Gravity}{10}

Const{SpeedFactor}{0.2}

Const{FPS}{11}



defX#1{Vinit*cos(Theta)*#1}

defY#1{Vinit*sin(Theta)*#1-Gravity*pow(2,#1)/2}



Const{Vinit}{Tpeak*Gravity/sin(Theta)}

Const{Xpeak}{X{Tpeak}}

Const{Ypeak}{Y{Tpeak}}



defpoint#1{%

    pnode(!Vinit Theta RadToDeg 2 copy cos mul #1 mul 3 1 roll sin mul #1 mul Gravity #1 2 exp mul 2 div sub){P}

    pscircle[linecolor=red,fillstyle=solid,fillcolor=yellow](P){3pt}

    pnode[!Vinit Theta RadToDeg cos mul SpeedFactor mul 0](P){PX}

    pnode[!0 Vinit Theta RadToDeg sin mul Gravity #1 mul sub SpeedFactor mul](P){PY}

    %

    psline[linecolor=blue]{->}(P)(PX)

    psline[linecolor=magenta]{->}(P)(PX|PY)

    psline[linecolor=blue]{->}(P)(PY)

}





Const{DeltaTime}{1/FPS}

Const[0]{TotalFrames}{FPS*2*Tpeak}

Const[0]{TotalFrames}{TotalFrames+1}



begin{document}

multido{nt=0.000+DeltaTime}{TotalFrames}{%

begin{pspicture}[showgrid=false](0,-35pt)(2dimexprXpeakpsxunitrelax,dimexprYpeakpsyunit+7ptrelax)

    parabola[linewidth=0.5pslinewidth,linestyle=dashed](0,0)(Xpeak,Ypeak)

    point{nt}

end{pspicture}}

end{document}

Try out this:

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

begin{document}

begin{pspicture}[showgrid](5,5)

psline[arrows=->](1,1)(1,1)

psline[arrows=->](0,1)(1,0)

psellipticarc[arrows=->](2.5,2.5)(1,2){0}{360}

psellipticarcn[arrows=->](2.5,2.5)(2,1){0}{360}

psarc[arrows=->](2.5,2.5){1}{0}{360}

psarcn[arrows=->](2.5,2.5){2}{0}{360}

end{pspicture}

end{document}

What do you want to get when letting start a vector (psline) at the same point where it ends? Just produce an arrowhead with a given direction?

Note: psarc[arrows=->](2.5,2.5){1}{0}{360}

The parentheses give the coordinates of the center of the arc of the circle.
The first mandatory argument gives the radius of the arc of the circle.
The second and third mandatory arguments give the starting and ending angle of the arc.

When both angles are chosen to 0, there is no arc to be drawn, as well as with the ellipses ... I cannot reproduce that the elliptic arcs are drawn.

psellipticarc[arrows=->](2.5,2.5)(1,2){0}{0}

psellipticarcn[arrows=->](2.5,2.5)(2,1){0}{0}