SCHEME Mutable Functions

Question

I've been self-teaching myself Scheme R5RS for the past few months and have just started learning about mutable functions. I've did a couple of functions like this, but seem to find my mistake for this one.

(define (lst-functions)
  (let ((lst '()))
    (define (sum lst)     
      (cond ((null? lst) 0)
            (else
             (+ (car lst) (sum (cdr lst))))))
    (define (length? lst)
      (cond ((null? lst) 0)
            (else
             (+ 1 (length? (cdr lst))))))
    (define (average)
      (/ (sum lst) (length? lst)))
    (define (insert x)
      (set! lst (cons x lst)))
    (lambda (function)
      (cond ((eq? function 'sum) sum)
            ((eq? function 'length) length?)
            ((eq? function 'average) average)
            ((eq? function 'insert) insert)
            (else
             'undefined)))))

(define func (lst-functions))
((func 'insert) 2)
((func 'average))

Answer 1

You're not declaring the lst parameter in the procedures that use it, but you're passing it when invoking them. I marked the lines that were modified, try this:

(define (lst-functions)
  (let ((lst '()))
    (define (sum lst)     ; modified
      (cond ((null? lst) 0)
            (else
             (+ (car lst) (sum (cdr lst))))))
    (define (length? lst) ; modified
      (cond ((null? lst) 0)
            (else
             (+ 1 (length? (cdr lst))))))
    (define (average)
      (/ (sum lst) (length? lst)))
    (define (insert x)
      (set! lst (cons x lst)))
    (lambda (function)
      (cond ((eq? function 'sum) (lambda () (sum lst)))        ; modified
            ((eq? function 'length) (lambda () (length? lst))) ; modified
            ((eq? function 'average) average)
            ((eq? function 'insert) insert)
            (else
             'undefined)))))

Now it works as expected:

(define func (lst-functions))
((func 'insert) 2)

((func 'average)) 
=> 2
((func 'sum))
=> 2
((func 'length))
=> 1