;;; Section 4.1.1

(define (eval exp env) ;;lexical (usual) binding
  (cond ((self-evaluating? exp) exp)
        ((quoted? exp) (text-of-quotation exp))
        ((variable? exp) (lookup-variable-value exp env))
        ((definition? exp) (eval-definition exp env))
        ((assignment? exp) (eval-assignment exp env))
        ((lambda? exp) (make-procedure exp env))
        ((conditional? exp) (eval-cond (clauses exp) env))
        ((application? exp)
         (apply (eval (operator exp) env)
                (list-of-values (operands exp) env)))
        (else (error "Unknown expression type -- EVAL" exp))))

(define (eval exp env) ;;dynamic binding version
  (cond ;; <<<same stuff until...>>>
        ((application? exp)
         (apply (eval (operator exp) env)
                (list-of-values (operands exp) env)
                env))                    ;***
        (else (error "Unknown expression type -- EVAL" exp))))

;;; note: now apply takes a third argument, the env when EVAL is called

;;;;;;;;;;;;;;;;;;;;

(define (apply procedure arguments)  ;; lexical (usual) binding
  (cond ((primitive-procedure? procedure)
         (apply-primitive-procedure procedure arguments))
        ((compound-procedure? procedure)
         (eval-sequence (procedure-body procedure)
                        (extend-environment
                         (parameters procedure)
                         arguments
                         (procedure-environment procedure))))
        (else
         (error "Unknown procedure type -- APPLY" procedure))))


(define (apply procedure arguments env)  ;***  dynamic version
  ;; note  extra ENV argument
  ;;   <<same stuff...>>
         (eval-sequence (procedure-body procedure)
                        (extend-environment
                         (parameters procedure)
                         arguments
                         env)))          ;***

;;;; important-- we extent the current environment with the environment
;;;; that eval was given, NOT with the environment that was tacked on to
;;;; the procedure we are applying 
        (else
         (error "Unknown procedure type -- APPLY" 
                procedure))))

;;; AND THAT's ALL