How to use previous points of a path to specify next?

Question

Consider this test.mf:

draw (10,20)--(10+100,20)--(10+100,20+50)--
  (10+100+30,20+50)--(10+100+30,20+50-40);
showit;
end

Is there a way in metafont/metapost to draw a line of given length in a given direction without the need to specify additional variables and repeat coordinates? Something like this pseudocode for above example:

draw (10,20)--<100right>--<50up>--<30right>--<40down>;

---

EDIT

I came up with this but I can't make it work:

pair wire[][];
z0=(10,20);
xpart(wire1\1)=x0+100; ypart(wire1\1)=y0;
xpart(wire1\2)=xpart(wire1\1); ypart(wire1\2)=ypart(wire1\1)+50;
xpart(wire1\3)=xpart(wire1\2)+30; ypart(wire1\3)=ypart(wire1\2);
xpart(wire1\4)=xpart(wire1\3); ypart(wire1\4)=ypart(wire1\3)-40;
numeric n; n:=0;
draw z0 forever :
  n:=n+1;
  exitif unknown wire1[n];
  -- wire1[n]
  endfor;
end

Answer 1

There's no built-in feature to do this, but it is not hard to write your own macro to do it. If you are short of time you can use or borrow the turtle function provided by the mfpic macros.

Like this:

prologues := 3;
outputtemplate := "%j%c.eps";
input grafbase
beginfig(1);
draw turtle((10,20), 100 right, 50 up, 30 right, 40 down);
endfig;
end.

which produces:

You can use the mfpic macros with Metafont as well as Metapost. See texdoc mfpic for details.

If you want to write your own macro, you could try something like this:

vardef turtle (text t) =
    save n, nose, track;
    pair nose, track[];
    numeric n;
    nose = origin;
    n = 0;
    for @ = t:
        nose := nose + @;
        track[n] := nose;
        n := n+1;
    endfor
    track[0] for i=1 upto n-1: -- track[i] endfor
enddef;

although you might prefer to make it more robust with some error checking.

Answer 2

Here is in the spirit of your attempt (the 2nd fig)

outputtemplate := "%j-%c.mps";

beginfig(1);
z1 = (10,20);
z2 = z1 + (100,20);
z3 = z2 + (0,50);
z4 = z3 + (30,0);
z5 = z4 + (0,-40);

draw z1 -- z2 -- z3 -- z4 --z5;
endfig;


beginfig(2);
pair myz[];
myz[1] = (10,20);
myz[2] = myz[1] + (100,20);
myz[3] = myz[2] + (0,50);
myz[4] = myz[3] + (30,0);
myz[5] = myz[4] + (0,-40);

numeric n;  n:=1;
path p;     p:=myz1;

   forever :
     n:=n+1;
     exitif unknown myz[n];
     p := p -- myz[n];
   endfor;

draw p;

endfig;

end