analysis-evaluator.scm

;; Testing the Evaluator
;; ============================================================
(define scheme-apply
  ;; Saves the definition of apply prior to overwriting it
  apply)
;; ============================================================

(define nil '()) ;; convenience

(define (partial f . x)
  (lambda y (scheme-apply f (append x y))))

(define (eval exp env) ((analyze exp) env))

(define (analyze exp)
  (cond ((self-evaluating? exp)
         (analyze-self-evaluating exp))
        ((quoted? exp) (analyze-quoted exp))
        ((variable? exp) (analyze-variable exp))
        ((assignment? exp) (analyze-assignment exp))
        ((definition? exp) (analyze-definition exp))
        ((if? exp) (analyze-if exp))
        ((lambda? exp) (analyze-lambda exp))
        ((begin? exp)
         (analyze-sequence (begin-actions exp)))
        ((let? exp) (analyze (let->combination exp)))
        ((cond? exp) (analyze (cond->if exp)))
        ((application? exp) (analyze-application exp))
        (else (error "Unknown expression type: ANALYZE" exp))))

(define (analyze-self-evaluating exp)
  (lambda (env) exp))

(define (analyze-quoted exp)
  (let ((qval (text-of-quotation exp)))
    (lambda (env) qval)))

(define (analyze-variable exp)
  (lambda (env) (lookup-variable-value exp env)))

(define (analyze-assignment exp)
  (let ((var (assignment-variable exp))
        (vproc (analyze (assignment-value exp))))
    (lambda (env) (set-variable-value! var (vproc env) env) 'ok)))

(define (analyze-definition exp)
  (let ((var (definition-variable exp))
        (vproc (analyze (definition-value exp))))
    (lambda (env)
      (define-variable! var (vproc env) env) 'ok)))

(define (analyze-if exp)
  (let ((pproc (analyze (if-predicate exp)))
        (cproc (analyze (if-consequent exp)))
        (aproc (analyze (if-alternative exp))))
    (lambda (env) (if (true? (pproc env))
                      (cproc env)
                      (aproc env)))))

(define (analyze-lambda exp)
  (let ((vars (lambda-parameters exp))
        (bproc (analyze-sequence (lambda-body exp))))
    (lambda (env) (make-procedure vars bproc env))))

(define (analyze-sequence exps)
  (reduce (lambda (f g) (lambda (env) (f env) (g env)))
          nil
          (map analyze exps)))

(define (analyze-application exp)
  (let ((fproc (analyze (operator exp)))
        (aprocs (map analyze (operands exp))))
    (lambda (env)
      (execute-application
       (fproc env)
       (map (lambda (aproc) (aproc env)) aprocs)))))

(define (execute-application proc args)
  (cond ((primitive-procedure? proc)
         (apply-primitive-procedure proc args))
        ((compound-procedure? proc)
         ((procedure-body proc)
          (extend-environment
           (procedure-parameters proc)
           args
           (procedure-environment proc))))
        (else (error "Unknown procedure type: EXECUTE-APPLICATION"
                     proc))))

(define (list-of-values exps env)
  ;; Takes in a list of operands and procedures a list of
  ;; their corresponding values relative to a given environment.
  (if (no-operands? exps)
      '()
      (cons (eval (first-operand exps) env)
            (list-of-values (rest-operands exps) env))))

(define (eval-if exp env)
  ;; Evaluates the predicate part of an if-expression in a given
  ;; environment. That value is then passed to true? which will
  ;; return either true or false depending on our notion of truthfulness
  ;; within our interpreted language.
  (if (true? (eval (if-predicate exp) env))
      (eval (if-consequent exp) env)
      (eval (if-alternative exp) env)))

(define (eval-sequence exps env)
  ;; uses by apply to evaluate the sequence of expression in a procedure
  ;; body and by eval to evaluate the sequence of expressions in a begin
  ;; expression. The value returned is the value of the final expression.
  (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 (self-evaluating? exp)
  (cond ((number? exp) true)
        ((string? exp) true)
        (else false)))

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

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

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

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

(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 (cdadr exp)   ;; formal parameters
                   (cddr exp)))) ;; body

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

(define (make-call proc . args)
  (cons proc args))

(define (make-lambda parameters body)
  (cons 'lambda (cons parameters body)))

(define (lambda-parameters exp) (cadr exp))

(define (lambda-body exp) (cddr exp))

(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 (arrow-clause? clause)
  (eq? (car (cond-actions clause)) '=>))

(define (make-recipient clause)
  ;; This implementation of make-recipient doesn't test for the validity
  ;; of the recipient procedure
  (make-call (cadr (cond-actions clause))
             (cond-predicate clause)))

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

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

(define (and-predicates exp) (cdr exp))

(define (first-pred preds) (car preds))

(define (rest-preds preds) (cdr preds))

(define (last-pred? preds) (null? (cadr preds)))

(define (and->if preds)
  (cond ((null? preds) 'true)
        ((last-pred? preds) (first-pred preds))
        (else (make-if (first-pred preds)
                       (and->if (rest-preds preds))
                       'false))))

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

(define (or-predicates exp) (cdr exp))

(define (or->if preds)
  (cond ((null? preds) 'false)
        (else (make-if (first-pred preds)
                       (first-pred preds)
                       (or->if (rest-preds preds))))))

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

(define (let->combination exp)
  (define (named-let? exp) (symbol? (cadr exp)))
  (define (let-bindings exp) (cadr exp))
  (define (let-body exp) (cddr exp))
  (define (binding-exps bindings)
    (if (null? bindings)
        '()
        (cons (cadar bindings)
              (binding-exps (cdr bindings)))))
  (define (binding-var binding) (car binding))
  (cons (make-lambda
         (map binding-var (let-bindings exp))
         (let-body exp))
        (binding-exps (let-bindings exp))))

(define (true? x) (not (eq? x false)))

(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))

;; Operations on Environments
(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 (traverse-env fn var env)
  (define (env-loop env)
    (define (scan vars vals)
      (cond ((null? vars)
             (env-loop (enclosing-environment env)))
            ((eq? var (car vars)) (fn vars 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 (lookup-variable-value var env)
  (traverse-env (lambda (vars vals) (car vals)) var env))

(define (set-variable-value! var val env)
  (traverse-env (lambda (vars vals) (set-car! vals val)) var 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 (setup-environment)
  (let ((initial-env
         (extend-environment (primitive-procedure-names)
                             (primitive-procedure-objects)
                             the-empty-environment)))
    (define-variable! 'true true initial-env)
    (define-variable! 'false false initial-env)
    (define-variable! 'nil '() initial-env)
    initial-env))

(define (primitive-procedure? proc)
  (tagged-list? proc 'primitive))

(define (primitive-implementation proc) (cadr proc))

(define primitive-procedures
  (list (list 'car car)
        (list 'cdr cdr)
        (list 'cons cons)
        (list 'null? null?)
        (list 'list list)
        (list '+ +)
        (list '- -)
        (list '* *)
        (list '/ /)
        (list '= =)
        (list 'eq? eq?)
        (list '> >)
        (list '< <)
        (list '<= <=)
        (list '>= >=)
        (list 'equal? equal?)))

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

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

(define (apply-primitive-procedure proc args)
  (scheme-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)))

;; READY TO RUN
(define the-global-environment (setup-environment))

(driver-loop)