Simplifying Dr.Racket drawing code

Question

I am using Dr.Racket, Intermediate Student with Lambda. I was wondering if there was any way I can simplify this code using any sort of method like lambda, abstraction, map, filter, etc.

 ; DRAWING FUNCTIONS

 ; draw-rocket: rocket scene --> scene
 ; Purpose: To draw the given rocket in the given scene
 (define (draw-rocket a-rocket a-scene) 
 (place-image rocket-img a-rocket ROCKET-Y a-scene))


 ; draw-alien: alien scene --> scene
 ; Purpose: To draw the given alien in the given scene
 (define (draw-alien an-alien a-scene)
 (place-image alien-img 
   (posn-x an-alien) 
   (posn-y an-alien)
   a-scene))

    ; draw-aliens: loa scn --> scene 
    ; Purpose: To draw the aliens in the given scene 
    (define (draw-aliens a-loa scn) 
    (cond [(empty? a-loa) scn] 
    [else (draw-alien (first a-loa) (draw-aliens (rest a-loa) scn) 
                  )]))
;

; draw-shot: shot scene --> scene 
; Purpose: To draw the given shot in the given scene 
(define (draw-shot a-shot scn) 
(place-image SHOT-IMG (posn-x a-shot) (posn-y a-shot) scn))

; draw-aliens: loa scn --> scene 
; Purpose: To draw the aliens in the given scene 
(define (draw-shots a-los scn) 
(cond [(empty? a-los) scn] 
[else (draw-shot (first a-los) (draw-shots (rest a-los) scn) 
             )])) 

; draw-world: world --> scene
; Purpose: Draw the world in the empty scene
(define (draw-world a-world)
(draw-rocket (world-rocket a-world) 
(draw-aliens (world-aliens a-world)

Answer 1

There are significant issues here:

; draw-rocket: rocket scene --> scene
; Purpose: To draw the given rocket in the given scene
(define (draw-rocket a-rocket a-scene) 
(place-image rocket-img a-rocket ROCKET-Y a-scene))

Why break the pattern from drawing code for the others? Use (pos-x a-rocket), (pos-y a-rocket) instead of a-rocket ROCKET-Y. That will help you abstract the code later on.

; draw-aliens: loa scn --> scene 
; Purpose: To draw the aliens in the given scene 
(define (draw-shots a-los scn) 
(cond [(empty? a-los) scn] 
[else (draw-shot (first a-los) (draw-shots (rest a-los) scn) 
         )])) 

This method has the wrong purpose and signature. This should be:

; draw_shots: los scn --> scn
; Purpose: To draw the shots in the given scene

---

This code seems to have multiple issues:

; draw-world: world --> scene
; Purpose: Draw the world in the empty scene
(define (draw-world a-world)
(draw-rocket (world-rocket a-world) 
(draw-aliens (world-aliens a-world)

1. The parentheses are mismatched, as mentioned in the comments.
2. Why are the draw-shots methods not called here?

I also think that you want to nest the calls so that the results are cumulative. This is done like so:

; draw-world: world --> scene
; Purpose: Draw the world in the scene (why would does it have to be empty?)
(define (draw-world a-world)
(draw-shots (world-shots 
(draw-rocket (world-rocket 
(draw-aliens (world-aliens a-world)))))))

---

As for using higher-order functions:

Try using foldr like so:

(define (draw-shots a-los scn)
(foldr draw-shot (first a-los) (rest a-los)))

Similarly, you can apply this to all the functions on lists.

---

Finally, using abstraction is very easy here. Create a new type, called Sprite or Actor or something similar, and have Aliens, Shots and Rocket all extend it. That way you only need 2 functions: 1 for singular, 1 for plural.