[racket] new to Scheme and functional programming- weird error I can't wrap my mind around

From: Chun How Yong (chunhow.yong at gmail.com)
Date: Tue Nov 15 01:57:27 EST 2011

Currently going through DrRacket's "draw.ss" teachpack.

There is a structure called "posn" defined in the following way.

*(struct posn (x y))*
Also a function that returns a posn defined as:

*(define (make-posn x y)*
*    (posn x y))*
Using the draw-circle function defined in draw.ss, I wanted to draw a green
circle centered at (posn 150 150), with radius 20.
> (draw-circle (make-posn 150 150) 20 'green)
. . draw-circle: expects a posn as first argument, given #<posn>

I don't understand the above error. Draw-circle expects a posn structure,
and DrRacket recognized that I gave it one. So where did the error stem

Chun How
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