Arrows

arrow.ss

The teachpack arrow.ss implements controller for moving pictures across a canvass. It provides three operations:

control-left-right : shape number move -> true :

  
 It consumes a shape, a number, and a move program.  The
  move program consumes a shape and a number and re-draws the shape on
  some canvas and produces a shape that is translated by N pixels left or
  right.

 control-up-down : shape number move -> true :

  
 It consumes a shape, a number, and a move program.  The
  move program consumes a shape and a number and re-draws the shape on
  some canvas and produces a shape that is translated by N pixels up or down.

 control : shape number move-lr move-ud -> true :

  
 It consumes a shape, a number, and two move programs.  The
  move programs consume a shape and a number and re-draw the shape on
  some canvas and produces a shape that is translated by N pixels left or
  right.



Example: 
(define RAD 10)

(define (translate sh delta)
  (make-posn (+ (posn-x sh) delta) (posn-y sh)))

(define (move sh delta)
  (cond
    [(and (clear-solid-disk sh RAD) (draw-solid-disk (translate sh delta) RAD))
     (translate sh delta)]
    [else false]))

(start 100 100)

(control-left-right (make-posn 10 20) 10 move)