평가는 식을 따라가고 조회는 프레임 연결을 따라간다.

(begin
  (define (make-frame variables values)
    (if (null? variables)
        '()
        (cons (cons (car variables) (car values))
              (make-frame (cdr variables) (cdr values)))))
  (define (extend-environment variables values base)
    (cons (make-frame variables values) base))
  (define (lookup-variable-value variable environment)
    (if (null? environment)
        'unbound-variable
        (let ((record (assoc variable (car environment))))
          (if record
              (cdr record)
              (lookup-variable-value variable (cdr environment))))))
  (define (sum-values values)
    (if (null? values)
        0
        (+ (car values) (sum-values (cdr values)))))
  (define (product-values values)
    (if (null? values)
        1
        (* (car values) (product-values (cdr values)))))
  (define (apply-primitive name arguments)
    (cond ((eq? name 'primitive-plus) (sum-values arguments))
          ((eq? name 'primitive-times) (product-values arguments))
          (else 'unknown-primitive)))
  (define (eval-operands expressions environment)
    (if (null? expressions)
        '()
        (cons (model-eval (car expressions) environment)
              (eval-operands (cdr expressions) environment))))
  (define (model-apply procedure arguments)
    (if (and (pair? procedure)
             (eq? (car procedure) 'compound))
        (model-eval
          (caddr procedure)
          (extend-environment
            (cadr procedure)
            arguments
            (cadddr procedure)))
        (apply-primitive procedure arguments)))
  (define (model-eval expression environment)
    (cond ((or (number? expression)
               (string? expression)
               (boolean? expression))
           expression)
          ((symbol? expression)
           (lookup-variable-value expression environment))
          ((eq? (car expression) 'quote)
           (cadr expression))
          ((eq? (car expression) 'lambda)
           (list 'compound
                 (cadr expression)
                 (caddr expression)
                 environment))
          (else
           (model-apply
             (model-eval (car expression) environment)
             (eval-operands (cdr expression) environment)))))
  (define global-environment
    (list (list (cons '+ 'primitive-plus)
                (cons '* 'primitive-times))))
  (list
    (model-eval
      '((lambda (x)
          ((lambda (y) (+ x y)) 4))
        3)
      global-environment)
    (model-eval
      '(((lambda (x)
           (lambda (y) (+ x y)))
         5)
        8)
      global-environment)))