Arrows

arrow.ss

A shape is a class of data for which move and draw operations can be drawn.

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

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

  
 It consumes a shape, a number, a move function and a
  draw function.  The
  move function 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 is like control-left-right but controls movements by N pixels up or down.

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

  
 It consumes a shape, a number, two move functions, and a
  draw function.  The move functions 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 and up or down, respectively.



Example: 
;; A shape is a structure: 
;;   (make-posn num num)

;; RAD : the radius of the simple disk moving across a canvas
(define RAD 10)

;; move : shape number -> shape or false 
;; to move a shape by delta according to translate 
;; effect: to redraw it 
(define (move sh delta)
  (cond
    [(and (clear-solid-disk sh RAD)
          (draw-solid-disk (translate sh delta) RAD))
     (translate sh delta)]
    [else false]))

;; translate : shape number -> shape 
;; to translate a shape by delta in the x direction 
(define (translate sh delta)
  (make-posn (+ (posn-x sh) delta) (posn-y sh)))

;; draw-it : shape -> true
;; to draw a shape on the canvas: a disk with radius
(define (draw-it sh)
  (draw-solid-disk sh RAD))

;; TESTS: 

;; this creates the canvas
(start 100 50)

;; this creates the controller GUI
(control-left-right (make-posn 10 20) 10 move draw-it)