Chapter 4, Metalinguistic Abstraction

Exercise 4.21


This is interesting way to solve the of simultaneous definition.

I am first writing solution for part(b)

(b)

Let’s start from Louis’s suggestion in the previous exercise. Why his suggestion was not working?

The reason is if we evaluate lambda expression for even? then odd? can not be found. Similarly evaluating lambda expression for odd? then even? is not found.

Why? As we saw in prev. exercise: the scope in which lambda expression for even? or odd? is evaluated does not contain the bindings even? and odd?.

So, how can we fix this situation?

What if instead of even?’s expression calling odd? and odd?’s expression calling even?, there is a way we can make these invocations by passing the lambda expression of these procedures to each other!

What I mean is, the problem was we don’t have name binding of even? to its definiton and odd? to its definition. So why not we instead pass the procedure to be invoked!

We pass lambda expression of even? to odd? and similarly we pass the lambda expression of odd? to even?.

In this way we are not dependent on binding of variable names even? and odd? to their respective lambda expressions!

Well thats it! Here is our procedure:

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(define (f x)
  ((lambda (even? odd?)
     (even? even? odd? x))
   (lambda (ev? od? n)
     (if (= n 0) true (od? ev? od? (- n 1))))
   (lambda (ev? od? n)
     (if (= n 0) false (ev? ev? od? (- n 1))))))

Well, it indeed solved the problem but it has decreased the readablity of the code!

(a)

This is simple now after doing part(a)

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;;; M-Eval input:
((lambda (n)
    ((lambda (fib)
        (fib fib n))
     (lambda (fb n)
        (cond ((= n 0) 0)
              ((= n 1) 1)
              (else (+ (fb fb (- n 1)) (fb fb (- n 2))))))))
 10)

;;; M-Eval value:
metacircular-evaluator-loaded

;;; M-Eval input:

;;; M-Eval value:
55

Complete Code

Since in next section, there will be quite a number of changes in code. Putting the complete code here till now.

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;;;;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).

;;;; resetting the environment so that we can reload the file
(ge (make-top-level-environment))

;;;;This warning here now not applicable in our case because of the statement above.
;;;;**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)

;;;SECTION 4.1.1

(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))
        ((unbound!? exp) (eval-unbound! 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 exp env))
		((logical? exp) (eval-logical exp env))
		((let? exp) (eval (let->combination exp) env))
		((let*? exp) (eval (let*->let exp) env))
		((letrec? exp) (eval (letrec->let exp) env))
		((for? exp) (eval (for->let 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-cond exp env)
  (define (iter clauses)
	(if (null? clauses)
		false
		(let ((first (car clauses))
			  (rest (cdr clauses)))
		  (cond ((cond-else-clause? first)
				 (if (null? rest)
					 (eval (sequence->exp (cond-actions first)) env)
					 (error "Else not last clause"
							(cond-clauses exp))))
				((cond-=>-clause? first)
				 (let ((pred (eval (cond-=>-predicate first) env)))
				   (if (true? pred)
					   (apply (eval (cond-=>-recipient first) env) (list pred))
					   (iter rest))))
				(else (if (true? (eval (cond-predicate first) env))
						  (eval (sequence->exp (cond-actions first)) env)
						  (iter rest)))))))
  (iter (cond-clauses exp)))
	 
(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 (boolean-impl val)
  (if (true? val) val false))

(define (eval-logical exp env)
  (define (iter predicates test?)
	(let ((first (eval (first-exp predicates) env)))
	  (if (test? first)
		  (if (last-exp? predicates)
			  (boolean-impl first)
			  (iter (rest-exps predicates) test?))
		  (boolean-impl first))))
  
  (let ((test? (if (and? exp) true? false?))
		(predicates (logical-predicates exp)))
	(if (null? predicates)
		(boolean-impl (test? '()))
		(iter predicates test?))))
	
;;;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 (unbound!? exp)
  (tagged-list? exp 'unbound!))

(define (unbound!-variable-name exp)
  (if (symbol? (cadr exp))
	  (cadr exp)
	  (error "Unbound variable name is not a symbol" exp)))

(define (eval-unbound! exp env)
  (unbound-variable! (unbound!-variable-name exp) env))

(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-=>-clause? clause)
  (eq? '=> (cadr clause)))

(define (cond-=>-predicate clause) (car clause))

(define (cond-=>-recipient clause)
  (let ((recp (cddr clause)))
	(if (last-exp? recp)
		(first-exp recp)
		(error "recipient can not have multiple expressions" 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))))))

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

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

(define (logical? exp) (or (and? exp) (or? exp)))

(define (logical-predicates exp) (cdr exp))

(define (logical->if exp)
  (expand-predicates (and? exp) (logical-predicates exp)))

(define (expand-predicates and-type preds)
  (if (null? preds)
	  (if and-type 'true 'false)
	  (let ((first (car preds))
			(rest (cdr preds)))
		(let ((rest-exp (if (null? rest)
							first
							(expand-predicates and-type rest))))
		  (if and-type
			  (make-if first rest-exp 'false)
			  (make-if first first rest-exp))))))

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

(define (let-has-name? exp) (symbol? (cadr exp)))

(define (let-name exp) (cadr exp))

(define (let-vardefs exp)
  (if (let-has-name? exp)
	  (caddr exp)
	  (cadr exp)))

(define (let-body exp)
  (if (let-has-name? exp)
	  (cdddr exp)
	  (cddr exp)))

(define (let->combination exp)
  (let ((res (fold-right
			  (lambda (new rem)
				(cons (cons (car new) (car rem))
					  (cons (cadr new) (cdr rem))))
			  (cons '() '())
			  (let-vardefs exp))))
	(let ((vars (car res))
		  (vexps (cdr res)))
	  (define proc (make-lambda vars (let-body exp)))
	  (if (let-has-name? exp)
		  (cons
		   (make-lambda '()
						(list (list 'define (let-name exp) proc)
							  (cons (let-name exp) vexps)
							  )) '())
		  (cons proc vexps)
	  ))))

;; var-defs and body should be list
(define (make-let var-defs body)
  (cons 'let (cons var-defs body)))

;; var-defs and body should be list
(define (make-named-let name var-defs body)
  (cons 'let (cons name (cons var-defs body))))

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

(define (let*->let exp)
  (car
   (fold-right (lambda(new rem)
				 (list (make-let (list new) rem)))
			   (let-body exp)
			   (let-vardefs exp))))

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

(define (for-var exp) (cadr exp))

(define (for-data exp) (caddr exp))

(define (for-body exp) (cdddr exp))

(define (for->let exp)
  (define body
	(list 'define 'body (make-lambda
						 (list (for-var exp))
						 (for-body exp))))
  (define cond-exp '(cond ((null? l) '())
		((not (pair? l)) (error "invalid data- for loop expects list"))
		((null? (cdr l)) (body (car l)))
		(else (body (car l)) (iter (cdr l)))))
  (make-named-let 'iter
				  (list (list 'l (for-data exp)))
				  (list body cond-exp)))

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

(define (letrec-vardefs exp) (cadr exp))

(define (letrec-body exp) (cddr exp))

(define (letrec->let exp)
  (define rs
	(fold-right (lambda(new rem)
				  (cons (cons (list (car new) ''*unassigned*) (car rem))
						(cons (list 'set! (car new) (cadr new)) (cdr rem))))
				(cons '() '())
				(letrec-vardefs exp)))
  (make-let (car rs) (append (cdr rs) (letrec-body exp))))

;;;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 (scan-out-defines 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 '())

;; we need cons to contain list of pairs.
;; the reason is same as when we build tables in ch-3:
;; if we add at beginning of list or pairs
;; it will change the initial point of the pairs.
;; thus we need a place to point to the head of this list. 
(define (make-frame variables values)
  (cons 'table (map cons variables values)))

;;(define (frame-variables frame) (car frame))
;;(define (frame-values frame) (cdr frame))

(define (frame-pairs frame) (cdr frame))

(define (find contains? next data)
  (if (null? data)
	  #f
	  (let ((found (contains? data)))
		(if found
			found
			(find contains? next (next data))))))

(define (assoc var table)
  (find (lambda(tbl)
		  (let ((pair (car tbl)))
			(if (eq? (car pair) var)
				pair
				#f)))
		 cdr
		 table))

(define (find-pair var env)
  (find (lambda(e)
		  (assoc var (frame-pairs (first-frame e))))
		enclosing-environment
		env))

(define (add-binding-to-frame! var val frame)
  (set-cdr! frame (cons (cons var 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)
  (let ((pair (find-pair var env)))
	(if pair 
		(if (eq? (cdr pair) '*unassigned*)
			(error "Variable is unassigned!")
			(cdr pair))
		(error "Unbound variable" var))))

(define (set-variable-value! var val env)
  (let ((pair (find-pair var env)))
	(if pair
		(set-cdr! pair val)
		(error "Unbound variable" var))))

(define (define-variable! var val env)
  (let ((pair (assoc var (frame-pairs (first-frame env)))))
	(if pair
		(set-cdr! pair val)
		(add-binding-to-frame! var val (first-frame env))
		)))

(define (unbound-variable! var env)
  (let ((pair (find-pair var env)))
	(if pair
		(begin (set-car! pair '()) (set-cdr! pair '()))
		(error "Unbound variable" var))))

(define (scan-out-defines proc-body)
  (let ((result
		 (fold-right
		  (lambda(new rem)
			(if (definition? new)
				(let ((var-def (list (definition-variable new) ''*unassigned*))
					  (var-set (list 'set! (definition-variable new) (definition-value new))))
					(list (cons var-def (car rem))
						  (cons var-set (cadr rem))))
				(list (car rem)
					  (cons new (cadr rem)))))
		  (list '() '())
		  proc-body)))
	
	  (if (eq? '() (car result))
		  proc-body ;;return original else infinite loop!
		  ;;put let in list bec apply expects it a list
		  (list (make-let (car result)
						  (cadr result))))))

;;;SECTION 4.1.4

(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)
    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?)
		(list '+ +)
		(list '* *)
		(list 'assoc assoc)
		(list 'cadr cadr)
		(list 'cddr cddr)
		(list '= =)
		(list '- -)
        (list 'pair? pair?)
        (list 'not not)
        (list 'newline newline)
        (list 'display display)
        (list 'error error)
;;      more primitives
        ))

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

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

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

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

(define input-prompt ";;; M-Eval input:")
(define output-prompt ";;; M-Eval value:")
(define exit-symbol 'exit)

(define (driver-loop)
  (prompt-for-input input-prompt)
  (let ((input (read)))
	(if (eq? input exit-symbol)
		'exiting..
		(let ((output (eval input the-global-environment)))
		  (announce-output output-prompt)
		  (user-print output)
		  (driver-loop)))))

;;[user-print moved before primitive procedure as it is used there]

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

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

;;;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