Question

Can you see what is wrong with this:

(define (box d x1 y1 x2 y2)  (
                              (graphics-draw-line d x1 y1 x1 y2)
                              (graphics-draw-line d x1 y2 x2 y2)
                              (graphics-draw-line d x2 y2 x2 y1)
                              (graphics-draw-line d x2 y1 x1 y1) ))

When I call it like this:

( begin 
     (define w (make-graphics-device 'x))
     (box  w .10 .10 .20 .20) )

I get an errer:

;The object #!unspecific is not applicable.
;To continue, call RESTART with an option number:
; (RESTART 2) => Specify a procedure to use in its place.
; (RESTART 1) => Return to read-eval-print level 1.

2 error>

This works:

(begin
    (define w (make-graphics-device 'x))
    (graphics-draw-line w .1 .1 .1 .2)
    (graphics-draw-line w .1 .2 .2 .2)
    (graphics-draw-line w .2 .2 .2 .1)
    (graphics-draw-line w .2 .1 .1 .1) )

I can’t see the difference!

Answer 1

Don’t just group expressions with ()s — that will try to use the result of the first as a function, but the value is #!unspecific — definitely not a function.

Use this:

(define (box d x1 y1 x2 y2)
  (graphics-draw-line d x1 y1 x1 y2)
  (graphics-draw-line d x1 y2 x2 y2)
  (graphics-draw-line d x2 y2 x2 y1)
  (graphics-draw-line d x2 y1 x1 y1))