(define-struct ae (fn args))
(define testcase1 (make-ae '+ (list 'x (make-ae '* (list 'y 2)))))
(define ass-lst1 '((x 2) (y 3) (z 4)))

(define (find-val key ass-lst)
  (cond [(empty? ass-lst) #f]
        [(symbol=? key (first (first ass-lst)))
         (second (first ass-lst))]
        [else (find-val key (rest ass-lst))]))

(define (eval1 expr ass-lst)
  (cond [(number? expr) expr]
        [else (apply1 (ae-fn expr) (ae-args expr) ass-lst)]))

(define (apply1 fn expr-lst ass-lst)
  (cond [(empty? expr-lst)
         (cond [(symbol=? fn '*) 1]
               [(symbol=? fn '+) 0])]
        [else
            ((cond [(symbol=? fn '*) *]
                   [(symbol=? fn '+) +])
             (eval1
              (cond [(or (number? (first expr-lst))
                       (ae? (first expr-lst)))
                     (first expr-lst)]
                    [(symbol? (first expr-lst))
                     (find-val (first expr-lst) ass-lst)]) ass-lst)
             (apply1 fn (rest expr-lst) ass-lst))]))

(eval1 testcase1 ass-lst1)