Higher-order function test suite

problem.scm

(define sum (make-aggregate + 0))
(sum (list 1 2 3))  ; 6
(define product (make-aggregate * 1))
(product (list 2 3 4))  ; 24

(define sum2 (make-aggregate (lambda (a b) (+ a b)) 0))
(sum2 (list 1 2 3))  ; 6
(define product2 (make-aggregate (lambda (a b) (* a b)) 1))
(product2 (list 2 3 4))  ; 24

solution.scm

(define (make-aggregate operate identity)
  (define (aggregator lst) 
    (if (null? lst)
        identity
        (operate (car lst) (aggregator (cdr lst)))))
  aggregator)