Chapter 5, Computing with Register Machines
Exercise 5.41
For finding variable, i just duplicated the book’s code for looking up a variable in environment(runtime).
Initially, i used cons for lexical address but from the examples shown here use list, so i changed my code to reflect that.
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(define (make-lexical-address frame-num elem-num)
(list frame-num elem-num))
(define (lex-addr-frame-num lexical-address)
(car lexical-address))
(define (lex-addr-displacement lexical-address)
(cadr lexical-address))
(define the-empty-cenv '())
(define (extend-cenv frame base-cenv)
(cons frame base-cenv))
(define (cenv-first-frame cenv) (car cenv))
(define (enclosing-cenv cenv) (cdr cenv))
(define (find-variable var cenv)
(define (env-loop frame-num cenv)
(define (scan displacement vars)
(cond ((null? vars)
(env-loop (+ frame-num 1)
(enclosing-cenv cenv)))
((eq? var (car vars))
(make-lexical-address frame-num
displacement))
(else
(scan (+ 1 displacement)
(cdr vars)))))
(if (eq? cenv the-empty-cenv)
'not-found
(scan 0 (cenv-first-frame cenv))))
(env-loop 0 cenv))
Examples:
The above code can work indenpendently without any dependencies. Just trying the examples given in the problem:
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1 ]=>
(find-variable 'c '((y z) (a b c d e) (x y)))
;Value 15: (1 2)
1 ]=>
(find-variable 'x '((y z) (a b c d e) (x y)))
;Value 16: (2 0)
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(find-variable 'w '((y z) (a b c d e) (x y)))
;Value: not-found
1 ]=>