111.rkt~

;; The first three lines of this file were inserted by DrRacket. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-beginner-reader.ss" "lang")((modname |111|) (read-case-sensitive #t) (teachpacks ((lib "image.rkt" "teachpack" "2htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "image.rkt" "teachpack" "2htdp")) #f)))
(define-struct vec [x y])
; A vec is
;   (make-vec PositiveNumber PositiveNumber)
; interpretation represents a velocity vector

(define (checked-make-vec x y)
  (cond
    [(number? x) (if (x >= 0) x (error "x shold be more than or equal than 0"))]
    [(number? y) (if (y >= 0) y (error "y should be more than or equal than 0"))]
   [else (error "x and y should be numbers")]))