;;;;LAZY EVALUATOR FROM SECTION 4.2 OF
;;;; STRUCTURE AND INTERPRETATION OF COMPUTER PROGRAMS

;;;;Matches code in ch4.scm
;;;; Also includes enlarged primitive-procedures list

;;;;This file can be loaded into Scheme as a whole.
;;;;**NOTE**This file loads the metacircular evaluator of
;;;;  sections 4.1.1-4.1.4, since it uses the expression representation,
;;;;  environment representation, etc.
;;;;  You may need to change the (load ...) expression to work in your
;;;;  version of Scheme.
;;;;**WARNING: Don't load mceval twice (or you'll lose the primitives
;;;;  interface, due to renamings of apply).

;;;;Then you can initialize and start the evaluator by evaluating
;;;; the two lines at the end of the file ch4-mceval.scm
;;;; (setting up the global environment and starting the driver loop).


;;;;  To run without memoization, reload the first version of force-it below


;;**implementation-dependent loading of evaluator file
;;Note: It is loaded first so that the section 4.2 definition
;; of eval overrides the definition from 4.1.1

;;;SECTION 4.2.2

;;; Modifying the evaluator

(define (actual-value exp env)
  (force-it (eval exp env)))

(define (list-of-arg-values exps env)
  (if (no-operands? exps)
      '()
      (cons (actual-value (first-operand exps) env)
            (list-of-arg-values (rest-operands exps)
                                env))))

(define (list-of-delayed-args exps env)
  (if (no-operands? exps)
      '()
      (cons (delay-it (first-operand exps) env)
            (list-of-delayed-args (rest-operands exps)
                                  env))))

(define (eval-if exp env)
  (if (true? (actual-value (if-predicate exp) env))
      (eval (if-consequent exp) env)
      (eval (if-alternative exp) env)))

(define (force-it obj)
  (if (thunk? obj)
      (actual-value (thunk-exp obj) (thunk-env obj))
      obj))

;; thunks

(define (delay-it exp env)
  (list 'thunk exp env))

(define (thunk? obj)
  (and (pair? obj)
       (= (length obj) 3)
       (tagged-list? obj 'thunk)))

(define (thunk-exp thunk) (cadr thunk))
(define (thunk-env thunk) (caddr thunk))

;; "thunk" that has been forced and is storing its (memoized) value
(define (evaluated-thunk? obj)
  (tagged-list? obj 'evaluated-thunk))

(define (thunk-value evaluated-thunk) (cadr evaluated-thunk))

;; memoizing version of force-it
(define (force-it obj)
  (cond ((thunk? obj)
         (let ((result (actual-value
                        (thunk-exp obj)
                        (thunk-env obj))))
           (set-car! obj 'evaluated-thunk)
           (set-car! (cdr obj) result)  ; replace exp with its value
           (set-cdr! (cdr obj) '())     ; forget unneeded env
           result))
        ((evaluated-thunk? obj)
         (thunk-value obj))
        (else obj)))

'LAZY-EVALUATOR-LOADED