Order of evaluation in scheme

Question

This is what works:

(define obj1 (maak-object (coord 1 1) #f #f #t))
(set! karaktersenobjectenlijst (append karaktersenobjectenlijst
                                  (list (list 'object obj1)))))

> (cadar karaktersenobjectenlijst)
obj1
> (positie obj1)
{1 . 1}

This doesn't work:

> (positie (cadar karaktersenobjectenlijst))
. . vector-ref: expects type <vector> as 1st argument, 
. .   given: obj1; other arguments were: 0

How can I make it use the value obj1 when (cadar karaktersobjectenlijst) is evaluated?

(define (maak-object positie blokkerend? breekbaar? water?) (vector object-tag positie blokkerend? breekbaar? water?)) — Vincent, Feb 11 '11 at 15:11
(define (positie k) (cond ((karakter? k) (vector-ref k 6)) ((object? k) (vector-ref k 1)))) — Vincent, Feb 11 '11 at 15:12
Can you be more specific about the program you're writing? There are a lot of undefined variables here: `object-tag`, `karakter?`, `coord`, `karaktersenobjectenlijst`. Providing what seems like reasonable values for them doesn't produce results like what you're seeing. — Sam Tobin-Hochstadt, Feb 11 '11 at 16:17
What scheme system is this? I'm most suspicious about the value that prints as 'obj1'. Also, 'cadar' is going to select from the first element of the karaktersenobjectenlijst; if it's not empty to begin with, then the stuff you're adding makes no difference. Sam's right; there are too many dangly bits here to make sense of this request. — John Clements, Feb 11 '11 at 16:54
@Vincent, I can't try running your program without those variables. If they're too complicated to include here, try to provide a smaller example of your question. — Sam Tobin-Hochstadt, Feb 15 '11 at 14:08

score 1 · Answer 1 · answered May 02 '11 at 16:18

The code is correct. I have replaced your functions with some dummys and it evaluates fine:

(define coord cons)
(define maak-object list)
(define positie car)

(define obj1 (maak-object (coord 1 1) #f #f #t))
(define karaktersenobjectenlijst '())
(set! karaktersenobjectenlijst (append karaktersenobjectenlijst
                                       (list (list 'object obj1))))
(cadar karaktersenobjectenlijst)

(positie obj1) #-> (1 . 1)
(positie (cadar karaktersenobjectenlijst)) #-> (1 . 1)

The problem must be in your library code or the way you use it. The Scheme evaluation works fine.

Order of evaluation in scheme

1 Answers1