  Note : run in R5RS
(define false #f)
(define true #t)

(define (eval. exp env)
  (cond ((self-evaluating? exp) exp)
        ((variable? exp) (lookup-variable-value exp env))
        ((quoted? exp) (text-of-quotation exp))
        ((assignment? exp) (eval-assignment exp env))
        ((definition? exp) (eval-definition exp env))
        ((if? exp) (eval-if exp env))
        ((lambda? exp)
         (make-procedure (lambda-parameters exp) (lambda-body exp) env))
        ((begin? exp) (eval-sequence
                       (begin-actions exp)
                       env))
        ((cond? exp) (eval (cond->if exp) env))
        ((application? exp) (apply. (eval. (operator exp) env)
                                   (list-of-values (operands exp) env)))
        (else (error "Unknown expression type: EVAL" exp))))

(define (apply. procedure arguments)
  (cond ((primitive-procedure? procedure)
         (apply-primitive-procedure 
          procedure 
          arguments))
        ((compound-procedure? procedure)
         (eval-sequence
           (procedure-body procedure)
           (extend-environment
             (procedure-parameters 
              procedure)
             arguments
             (procedure-environment 
              procedure))))
        (else
         (error "Unknown procedure type: APPLY" procedure))))

(define (list-of-values exps env)
  (if (no-operands? exps)
      '()
      (cons (eval. (first-operand exps) env)
            (list-of-values 
             (rest-operands exps) 
             env))))

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

; called by apply. to eval the seq of expressions in a proc body
; and called by eval to evaluate the seq of expressions in a begin
; block:
(define (eval-sequence exps env)
  (cond ((last-exp? exps) 
         (eval. (first-exp exps) env))
        (else 
         (eval. (first-exp exps) env)
         (eval-sequence (rest-exps exps) 
                        env))))


(define (eval-assignment exp env)
  (set-variable-value!
   (assignment-variable exp)
   (eval. (assignment-value exp) env)
   env)
  'ok)
(define (eval-definition exp env)
  (define-variable!
    (definition-variable exp)
    (eval. (definition-value exp) env)
    env)
  'ok)

(define (tagged-list? exp tag)
  (if (pair? exp)
      (eq? (car exp) tag)
      false))

; Language syntax
; ===============
(define (self-evaluating? exp)
  (cond ((number? exp) true)
        ((string? exp) true)
        (else #f)))

(define (variable? exp) (symbol? exp))

(define (quoted? exp) (tagged-list? exp 'quote))

(define (text-of-quotation exp)
  (cadr exp))

(define (assignment? exp)
  (tagged-list? exp 'set!))

(define (assignment-variable exp) 
  (cadr exp))

(define (assignment-value exp) (caddr exp))

; definitions are either: (define ⟨var⟩ ⟨value⟩) or (define (⟨var⟩ ⟨param1⟩ ... ⟨paramn⟩) ⟨body⟩)
(define (definition? exp)
  (tagged-list? exp 'define))

(define (definition-variable exp)
  (if (symbol? (cadr exp))
      (cadr exp)
      (caadr exp)))

(define (definition-value exp)
  (if (symbol? (cadr exp))
      (caddr exp)
      (make-lambda 
       (cdadr exp)   ; formal parameters
       (cddr exp)))) ; body

(define (lambda? exp) 
  (tagged-list? exp 'lambda))
(define (lambda-parameters exp) (cadr exp))
(define (lambda-body exp) (cddr exp))
(define (make-lambda parameters body)
  (cons 'lambda (cons parameters body)))

(define (if? exp) (tagged-list? exp 'if))
(define (if-predicate exp) (cadr exp))
(define (if-consequent exp) (caddr exp))
(define (if-alternative exp)
  (if (not (null? (cdddr exp)))
      (cadddr exp)
      'false))
(define (make-if predicate consequent alternative)
  (list 'if predicate consequent alternative))

(define (begin? exp) 
  (tagged-list? exp 'begin))
(define (begin-actions exp) (cdr exp))
(define (last-exp? seq) (null? (cdr seq)))
(define (first-exp seq) (car seq))
(define (rest-exps seq) (cdr seq))
(define (sequence->exp seq)
  (cond ((null? seq) seq)
        ((last-exp? seq) (first-exp seq))
        (else (make-begin seq))))
(define (make-begin seq) (cons 'begin seq))

(define (application? exp) (pair? exp))
(define (operator exp) (car exp))
(define (operands exp) (cdr exp))
(define (no-operands? ops) (null? ops))
(define (first-operand ops) (car ops))
(define (rest-operands ops) (cdr ops))

(define (cond? exp) 
  (tagged-list? exp 'cond))
(define (cond-clauses exp) (cdr exp))
(define (cond-else-clause? clause)
  (eq? (cond-predicate clause) 'else))
(define (cond-predicate clause) 
  (car clause))
(define (cond-actions clause) 
  (cdr clause))
(define (cond->if exp)
  (expand-clauses (cond-clauses exp)))
(define (expand-clauses clauses)
  (if (null? clauses)
      'false     ; no else clause
      (let ((first (car clauses))
            (rest (cdr clauses)))
        (if (cond-else-clause? first)
            (if (null? rest)
                (sequence->exp 
                 (cond-actions first))
                (error "ELSE clause isn't last: COND->IF"
                       clauses))
            (make-if (cond-predicate first)
                     (sequence->exp 
                      (cond-actions first))
                     (expand-clauses 
                      rest))))))
; syntax fin


(define (true? x)
  (not (eq? x #f)))

(define (false? x)
  (eq? x false))

(define (make-procedure parameters body env)
  (list 'procedure parameters body env))
(define (compound-procedure? p)
  (tagged-list? p 'procedure))
(define (procedure-parameters p) (cadr p))
(define (procedure-body p) (caddr p))
(define (procedure-environment p) (cadddr p))

; the environment
; ===============

(define (enclosing-environment env) (cdr env))
(define (first-frame env) (car env))
(define the-empty-environment '())
(define (make-frame variables values)
  (cons variables values))
(define (frame-variables frame) (car frame))
(define (frame-values frame) (cdr frame))
(define (add-binding-to-frame! var val frame)
  (set-car! frame (cons var (car frame)))
  (set-cdr! frame (cons val (cdr frame))))

(define (extend-environment vars vals base-env)
  (if (= (length vars) (length vals))
      (cons (make-frame vars vals) base-env)
      (if (< (length vars) (length vals))
          (error "Too many arguments supplied" vars vals)
          (error "Too few arguments supplied" vars vals))))

(define (lookup-variable-value var env)
  (define (env-loop env)
    (define (scan vars vals)
      (cond ((null? vars)
             (env-loop 
              (enclosing-environment env)))
            ((eq? var (car vars))
             (car vals))
            (else (scan (cdr vars) 
                        (cdr vals)))))
    (if (eq? env the-empty-environment)
        (error "Unbound variable" var)
        (let ((frame (first-frame env)))
          (scan (frame-variables frame)
                (frame-values frame)))))
  (env-loop env))

(define (set-variable-value! var val env)
  (define (env-loop env)
    (define (scan vars vals)
      (cond ((null? vars)
             (env-loop 
              (enclosing-environment env)))
            ((eq? var (car vars))
             (set-car! vals val))
            (else (scan (cdr vars) 
                        (cdr vals)))))
    (if (eq? env the-empty-environment)
        (error "Unbound variable: SET!" var)
        (let ((frame (first-frame env)))
          (scan (frame-variables frame)
                (frame-values frame)))))
  (env-loop env))

(define (define-variable! var val env)
  (let ((frame (first-frame env)))
    (define (scan vars vals)
      (cond ((null? vars)
             (add-binding-to-frame! 
              var val frame))
            ((eq? var (car vars))
             (set-car! vals val))
            (else (scan (cdr vars) 
                        (cdr vals)))))
    (scan (frame-variables frame)
          (frame-values frame))))
; REPL
; ====
(define primitive-procedures
  (list (list 'car car)
        (list 'cdr cdr)
        (list 'cons cons)
        (list 'null? null?)
        (list '= =)
        (list '+ +)
        (list '- -)
        (list '* *)
        (list '/ /)))

(define (primitive-procedure-names)
  (map car primitive-procedures))

(define (primitive-procedure-objects)
  (map (lambda (proc) 
         (list 'primitive (cadr proc)))
       primitive-procedures))

(define (setup-environment)
  (let ((initial-env
         (extend-environment 
          (primitive-procedure-names)
          (primitive-procedure-objects)
          the-empty-environment)))
    (define-variable! 'true #t initial-env)
    (define-variable! 'false #f initial-env)
    initial-env))

(define the-global-environment 
  (setup-environment))
(define (primitive-procedure? proc)
  (tagged-list? proc 'primitive))
(define (primitive-implementation proc) 
  (cadr proc))



(define (apply-primitive-procedure proc args)
  (apply (primitive-implementation proc) args))
(define input-prompt  ";;; M-Eval input:")
(define output-prompt ";;; M-Eval value:")


(define (driver-loop)
  (prompt-for-input input-prompt)
  (let ((input (read)))
    (let ((output (eval. input the-global-environment)))
      (announce-output output-prompt)
      (user-print output)))
  (driver-loop))

(define (prompt-for-input string)
  (newline) (newline) 
  (display string) (newline))

(define (announce-output string)
  (newline) (display string) (newline))

(define (user-print object)
  (if (compound-procedure? object)
      (display 
       (list 'compound-procedure
             (procedure-parameters object)
             (procedure-body object)
             '<procedure-env>))
      (display object)))

(define the-global-environment 
  (setup-environment))
(driver-loop)

 called by apply. to eval the seq of expressions in a proc body
 and called by eval to evaluate the seq of expressions in a begin
 block:
 Language syntax
 ===============
 definitions are either: (define ⟨var⟩ ⟨value⟩) or (define (⟨var⟩ ⟨param1⟩ ... ⟨paramn⟩) ⟨body⟩)
 formal parameters
 body
 no else clause
 syntax fin
 the environment
 ===============
 REPL
 ====
  Note : run in R5RS
(define false #f)
(define true #t)

(define (eval. exp env)
  (cond ((self-evaluating? exp) exp)
        ((variable? exp) (lookup-variable-value exp env))
        ((quoted? exp) (text-of-quotation exp))
        ((assignment? exp) (eval-assignment exp env))
        ((definition? exp) (eval-definition exp env))
        ((if? exp) (eval-if exp env))
        ((lambda? exp)
         (make-procedure (lambda-parameters exp) (lambda-body exp) env))
        ((begin? exp) (eval-sequence
                       (begin-actions exp)
                       env))
        ((cond? exp) (eval (cond->if exp) env))
        ((application? exp) (apply. (eval. (operator exp) env)
                                   (list-of-values (operands exp) env)))
        (else (error "Unknown expression type: EVAL" exp))))

(define (apply. procedure arguments)
  (cond ((primitive-procedure? procedure)
         (apply-primitive-procedure 
          procedure 
          arguments))
        ((compound-procedure? procedure)
         (eval-sequence
           (procedure-body procedure)
           (extend-environment
             (procedure-parameters 
              procedure)
             arguments
             (procedure-environment 
              procedure))))
        (else
         (error "Unknown procedure type: APPLY" procedure))))

(define (list-of-values exps env)
  (if (no-operands? exps)
      '()
      (cons (eval. (first-operand exps) env)
            (list-of-values 
             (rest-operands exps) 
             env))))

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

(define (eval-sequence exps env)
  (cond ((last-exp? exps) 
         (eval. (first-exp exps) env))
        (else 
         (eval. (first-exp exps) env)
         (eval-sequence (rest-exps exps) 
                        env))))


(define (eval-assignment exp env)
  (set-variable-value!
   (assignment-variable exp)
   (eval. (assignment-value exp) env)
   env)
  'ok)
(define (eval-definition exp env)
  (define-variable!
    (definition-variable exp)
    (eval. (definition-value exp) env)
    env)
  'ok)

(define (tagged-list? exp tag)
  (if (pair? exp)
      (eq? (car exp) tag)
      false))

(define (self-evaluating? exp)
  (cond ((number? exp) true)
        ((string? exp) true)
        (else #f)))

(define (variable? exp) (symbol? exp))

(define (quoted? exp) (tagged-list? exp 'quote))

(define (text-of-quotation exp)
  (cadr exp))

(define (assignment? exp)
  (tagged-list? exp 'set!))

(define (assignment-variable exp) 
  (cadr exp))

(define (assignment-value exp) (caddr exp))

(define (definition? exp)
  (tagged-list? exp 'define))

(define (definition-variable exp)
  (if (symbol? (cadr exp))
      (cadr exp)
      (caadr exp)))

(define (definition-value exp)
  (if (symbol? (cadr exp))
      (caddr exp)
      (make-lambda 

(define (lambda? exp) 
  (tagged-list? exp 'lambda))
(define (lambda-parameters exp) (cadr exp))
(define (lambda-body exp) (cddr exp))
(define (make-lambda parameters body)
  (cons 'lambda (cons parameters body)))

(define (if? exp) (tagged-list? exp 'if))
(define (if-predicate exp) (cadr exp))
(define (if-consequent exp) (caddr exp))
(define (if-alternative exp)
  (if (not (null? (cdddr exp)))
      (cadddr exp)
      'false))
(define (make-if predicate consequent alternative)
  (list 'if predicate consequent alternative))

(define (begin? exp) 
  (tagged-list? exp 'begin))
(define (begin-actions exp) (cdr exp))
(define (last-exp? seq) (null? (cdr seq)))
(define (first-exp seq) (car seq))
(define (rest-exps seq) (cdr seq))
(define (sequence->exp seq)
  (cond ((null? seq) seq)
        ((last-exp? seq) (first-exp seq))
        (else (make-begin seq))))
(define (make-begin seq) (cons 'begin seq))

(define (application? exp) (pair? exp))
(define (operator exp) (car exp))
(define (operands exp) (cdr exp))
(define (no-operands? ops) (null? ops))
(define (first-operand ops) (car ops))
(define (rest-operands ops) (cdr ops))

(define (cond? exp) 
  (tagged-list? exp 'cond))
(define (cond-clauses exp) (cdr exp))
(define (cond-else-clause? clause)
  (eq? (cond-predicate clause) 'else))
(define (cond-predicate clause) 
  (car clause))
(define (cond-actions clause) 
  (cdr clause))
(define (cond->if exp)
  (expand-clauses (cond-clauses exp)))
(define (expand-clauses clauses)
  (if (null? clauses)
      (let ((first (car clauses))
            (rest (cdr clauses)))
        (if (cond-else-clause? first)
            (if (null? rest)
                (sequence->exp 
                 (cond-actions first))
                (error "ELSE clause isn't last: COND->IF"
                       clauses))
            (make-if (cond-predicate first)
                     (sequence->exp 
                      (cond-actions first))
                     (expand-clauses 
                      rest))))))


(define (true? x)
  (not (eq? x #f)))

(define (false? x)
  (eq? x false))

(define (make-procedure parameters body env)
  (list 'procedure parameters body env))
(define (compound-procedure? p)
  (tagged-list? p 'procedure))
(define (procedure-parameters p) (cadr p))
(define (procedure-body p) (caddr p))
(define (procedure-environment p) (cadddr p))


(define (enclosing-environment env) (cdr env))
(define (first-frame env) (car env))
(define the-empty-environment '())
(define (make-frame variables values)
  (cons variables values))
(define (frame-variables frame) (car frame))
(define (frame-values frame) (cdr frame))
(define (add-binding-to-frame! var val frame)
  (set-car! frame (cons var (car frame)))
  (set-cdr! frame (cons val (cdr frame))))

(define (extend-environment vars vals base-env)
  (if (= (length vars) (length vals))
      (cons (make-frame vars vals) base-env)
      (if (< (length vars) (length vals))
          (error "Too many arguments supplied" vars vals)
          (error "Too few arguments supplied" vars vals))))

(define (lookup-variable-value var env)
  (define (env-loop env)
    (define (scan vars vals)
      (cond ((null? vars)
             (env-loop 
              (enclosing-environment env)))
            ((eq? var (car vars))
             (car vals))
            (else (scan (cdr vars) 
                        (cdr vals)))))
    (if (eq? env the-empty-environment)
        (error "Unbound variable" var)
        (let ((frame (first-frame env)))
          (scan (frame-variables frame)
                (frame-values frame)))))
  (env-loop env))

(define (set-variable-value! var val env)
  (define (env-loop env)
    (define (scan vars vals)
      (cond ((null? vars)
             (env-loop 
              (enclosing-environment env)))
            ((eq? var (car vars))
             (set-car! vals val))
            (else (scan (cdr vars) 
                        (cdr vals)))))
    (if (eq? env the-empty-environment)
        (error "Unbound variable: SET!" var)
        (let ((frame (first-frame env)))
          (scan (frame-variables frame)
                (frame-values frame)))))
  (env-loop env))

(define (define-variable! var val env)
  (let ((frame (first-frame env)))
    (define (scan vars vals)
      (cond ((null? vars)
             (add-binding-to-frame! 
              var val frame))
            ((eq? var (car vars))
             (set-car! vals val))
            (else (scan (cdr vars) 
                        (cdr vals)))))
    (scan (frame-variables frame)
          (frame-values frame))))
(define primitive-procedures
  (list (list 'car car)
        (list 'cdr cdr)
        (list 'cons cons)
        (list 'null? null?)
        (list '= =)
        (list '+ +)
        (list '- -)
        (list '* *)
        (list '/ /)))

(define (primitive-procedure-names)
  (map car primitive-procedures))

(define (primitive-procedure-objects)
  (map (lambda (proc) 
         (list 'primitive (cadr proc)))
       primitive-procedures))

(define (setup-environment)
  (let ((initial-env
         (extend-environment 
          (primitive-procedure-names)
          (primitive-procedure-objects)
          the-empty-environment)))
    (define-variable! 'true #t initial-env)
    (define-variable! 'false #f initial-env)
    initial-env))

(define the-global-environment 
  (setup-environment))
(define (primitive-procedure? proc)
  (tagged-list? proc 'primitive))
(define (primitive-implementation proc) 
  (cadr proc))



(define (apply-primitive-procedure proc args)
  (apply (primitive-implementation proc) args))
(define input-prompt  ";;; M-Eval input:")
(define output-prompt ";;; M-Eval value:")


(define (driver-loop)
  (prompt-for-input input-prompt)
  (let ((input (read)))
    (let ((output (eval. input the-global-environment)))
      (announce-output output-prompt)
      (user-print output)))
  (driver-loop))

(define (prompt-for-input string)
  (newline) (newline) 
  (display string) (newline))

(define (announce-output string)
  (newline) (display string) (newline))

(define (user-print object)
  (if (compound-procedure? object)
      (display 
       (list 'compound-procedure
             (procedure-parameters object)
             (procedure-body object)
             '<procedure-env>))
      (display object)))

(define the-global-environment 
  (setup-environment))
(driver-loop)
