Postscript Programming Question

The following Postscript program draws a rectangular box.

Code:

---

/mybox {
  /Yh exch def
  /Xw exch def
  /Y0 exch def
  /X0 exch def
 
  newpath
    X0 Y0 moveto
    0 Yh rlineto
    Xw 0 rlineto
  % 0 Yh neg rlineto
  closepath
} def

50 50 300 500 mybox stroke

---

It works, but I don't understand how the following line is executed.

Code:

---

/Yh exch def

---

I've read a bunch of tutorials, and apparently exch exchanges the top 2 operands in the stack, while def is used to define a variable. But I don't see how the combination of the two operands sets Yh = 500. Maybe somebody can enlighten me. :)

Edit!

I think I understand it now.

What exch does is to change

500 /Yh exch def

into

/Yh 500 def

Which sets Yh = 500.

I came to this conclusion after Googling how "exch def" works and found this PDF which explains it. :)

You can see now why only Sun was crazy enough to actually write a window manager using this ancient gibberish, can't you? :rolleyes:

I didn't know that. Searching on Wikipedia found this article about NeWs (Network extensible Window System). Is this the one you're talking about? :)

Yep. It was a good idea at the time, I suppose...