Chapter 4, Metalinguistic Abstraction
Exercise 4.9
I implemented for. Let’s first check the output:
Note-0: User can use any names for variables - there won’t be any conflict.
Note-1: i installed few more premitive procedures like display, error etc.
Note-2: for some reason when i try to display a procedure defined(eg check proc here in the output) using the evaluator, it goes into a infinite loop. since it does not concern in this problem, i am not not wearing my debugging cap :)
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;;; M-Eval input:
(define proc (lambda () '((1 a) (2 b))))
;;; M-Eval value:
metacircular-evaluator-loaded
;;; M-Eval input:
;;; M-Eval value:
ok
;;; M-Eval input:
(for item (proc) (display item))
(1 a)(2 b)
;;; M-Eval value:
#!unspecific
;;; M-Eval input:
(for item (proc) (display item) 'done)
(1 a)(2 b)
;;; M-Eval value:
done
;;; M-Eval input:
(define sum 0)
;;; M-Eval value:
ok
;;; M-Eval input:
(for i '(1 2 3 4 5) (set! sum (+ i sum)) sum)
;;; M-Eval value:
15
;;; M-Eval input:
(for iter '(a b) iter)
;;; M-Eval value:
b
;;; M-Eval input:
(for body '(a b) body)
;;; M-Eval value:
b
;;; M-Eval input:
(for l '(a b) l)
;;; M-Eval value:
b
;;; M-Eval input:
exit
;Value: exiting..
It works my converting the for into a let expression for eg:
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(for x '(1 2)
(define it 100)
(define body 200)
(newline)
(display x)
(newline)
(+ it x))
;; this for loop gets converted to:
(let iter ((l '(1 2)))
(define body
(lambda (x)
(define l 100)
(define body 200)
(newline)
(display x)
(newline)
(+ l x body)))
(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)))))
Now, here comes the code:
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(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)))