Question

I’d like to create sequences using the definitions below:

(define f1 (lambda (x) #t))
(define f2 (lambda (x) #f))             
(define f3 (lambda (x) (if (null? x) #t (car x))))
(define f4 (lambda (x) (if (null? x) #t (not (car x)))))

My code is as follows:

(define (generate func n)
       (let ((mylist '()))
       (if (= n 0) mylist
        (cons (func mylist) (generate func (- n 1)))                
   )))

It does the trick for f1 f2 and f3 but when i try f4 like (generate f4 10) it produces
(#t #t #t #t #t #t #t #t #t #t) instead of (#t #f #t #f #t #f #t #f #t #f).

Thanks for any help.

Answer 1

Well it happens because each time func is called with '() which is the value of mylist in let scope. We should pass new list value to the next call of generate to do something useful with it. Something like this

(define (generate f n)
  (define (generate-int n lst)
    (if (= n 0) 
        lst
        (generate-int (- n 1) (cons (f lst) lst))))

  (generate-int n (list)))

Now (generate f4 4) produces '(#f #t #f #t) with first generated value coming last in the list. One may reverse the result to get a “natural” order or use append instead of cons which would be less effective.