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CprS376 Class 15
CprS376 Class 17

Metacircular Evaluator

Class 16 - Section 4.1



Contents

mceval

;; ch4-mceval.ss

;;;;METACIRCULAR EVALUATOR FROM CHAPTER 4 (SECTIONS 4.1.1-4.1.4) of
;;;; STRUCTURE AND INTERPRETATION OF COMPUTER PROGRAMS

;;;;Matches code in ch4.scm

;;;;This file can be loaded into Scheme as a whole.
;;;;Then you can initialize and start the evaluator by evaluating
;;;; the two commented-out lines at the end of the file (setting up the
;;;; global environment and starting the driver loop).

;;;;**WARNING: Don't load this file twice (or you'll lose the primitives
;;;;  interface, due to renamings of apply).

;;;from section 4.1.4 -- must precede def of metacircular apply
(define apply-in-underlying-scheme apply)

;;===========================================

;; Modifications by Chris Parrish are tagged <cp: comment>

;; eval renamed to mceval to avoid redefining an important system primitive
;; apply renamed to mcapply to avoid redefining an important system primitive
;; a few more primitives are added to the interpreter's initial environment
;; ... and a few other things

;;===========================================

;;;SECTION 4.1.1

(define (mceval 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) (mceval (cond->if exp) env))
        ((application? exp)
         (mcapply (mceval (operator exp) env)
                (list-of-values (operands exp) env)))
        (else
         (error "Unknown expression type -- EVAL" exp))))

;; <cp: rename apply to mcapply to avoid name clash with an important system primitive>

(define (mcapply 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 (mceval (first-operand exps) env)
            (list-of-values (rest-operands exps) env))))

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

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

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

(define (eval-definition exp env)
  (define-variable! (definition-variable exp)
                    (mceval (definition-value exp) env)
                    env)
  'ok)

;;;SECTION 4.1.2

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

(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 (variable? exp) (symbol? 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 (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                          ; 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))))))

;;;SECTION 4.1.3

(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))


(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))))

;;;SECTION 4.1.4

;; <cp: define boolean values for compatibility with MIT scheme>

(define true #t)
(define false #f)

(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! 'the-empty-list '() initial-env) ; <cp: we need an empty list
    initial-env))

;[do later] (define the-global-environment (setup-environment))

(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?)
;;      more primitives  <cp: let's add a few more primitives>
        (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))

;[moved to start of file] (define apply-in-underlying-scheme apply)

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


;; <cp: changed prompt from "M-Eval input:" to "MC-Eval input:">

(define input-prompt ";;; MC-Eval input:") 

;; <cp: changed prompt from "M-Eval value:" to "MC-Eval value:">

(define output-prompt ";;; MC-Eval value:")

(define (driver-loop)
  (prompt-for-input input-prompt)
  (let ((input (read)))
    (if (equal? input 'bye)
      'chao
      (let ((output (mceval 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)
                     '))
      (display object)))

;;;Following are commented out so as not to be evaluated when
;;; the file is loaded.
;;(define the-global-environment (setup-environment))
;;(driver-loop)

'METACIRCULAR-EVALUATOR-LOADED


mceval Transcript

;; ch4-mceval.transcript

;; =============================

;; load mceval source code

(load "ch4-mceval.ss")

;; =============================

;; setup the global environment

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

;; enter the interpreter
;; type bye to quit

(driver-loop)

12
'hi-there
(quote hi-there)
(define a 2)
a
(set! a 9)
a
(define (add5 x) (+ 5 x))
(add5 5)
add5
(define add6 (lambda (x) (+ 6 x)))
(add6 6)
add6
(add6 (add5 4))
(define mult (lambda (x y) (* x y)))
(mult 3 4)
(= 0 1)
(= 1 1)
'()
(cond (true 'true) (else 'false))
(cond (false 'false) (else 'true))
(cond ((= 0 1) 'true) (else 'false))
(cond ((= 1 1) 'true) (else 'false))
bye

writeln

You might consider using the writeln utility to let your interpreter display useful information as it interprets its input. For instance, at an appropriate place in the interpreter you might type (if *debug* (writeln "MCeval: " exp)) and then set the global flag *debug* to #t or #f according to your whims.

; writeln.ss

(define writeln
      (lambda args
        (let ((display-arg (lambda (arg)
                             (display arg)
                             (display " "))))
          (for-each display-arg args)
          (newline))))

; testing

; >>> (writeln "hi there!" 123 'abc)
; hi there! 123 abc 
; #t

SICP Source Code

Programming Exercises

In class today we will begin to work on the following exercises from chapter 4: 1, 4, 5, 6, 7, 8, 11, 13, 14, 15, 16, 20, 23, 24, 25, 27, 29, 31, 33, 34, 38, 39, 42, 43, 45, 50, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 69.