Metacircular Interpreter From SICP

meta.scm

(define apply-in-underlying-scheme apply)

(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 (self-evaluating? exp)
  (or (number? exp) (string? exp)))

(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)
      #f))

(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)
       (cddr exp))))

(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
      (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 #f))

(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 (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 'display display)
        ))

(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)
  (apply-in-underlying-scheme
   (primitive-implementation proc) args))

(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 (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 input-prompt ";;; Lightning Talk input:")
(define output-prompt ";;; Lightning Talk 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)))

(driver-loop)