Hi everyone,
I am current trying to implement a scheme interpreter in clojure following
the instruction on SICP chap4.
Where I encounter a problem with adding/modifying a def var/fn in
enviroment.
according to chap4 4.1.3 the set-variable-value! function looks like the
following in scheme:
(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))
But I found there's no set-car! in clojure, which make me have to re-create
the env while loop though the defied variables like:
*https://github.com/zacyang/sicp-in-clj/blob/master/src/sicp/ch4/core.clj
line #348*
(defn set-variable-value!
[var-looking-for val-to-be-set env]
(defn find-and-change [vars vals]
(cond
(empty? vars) '()
(= (first vars) var-looking-for) (conj (find-and-change (rest vars)
(rest vals)) val-to-be-set)
:else (conj (find-and-change (rest vars) (rest vals)) (first vals))))
(defn env-loop
[e]
(if (= @env @the-empty-environment)
:ERROR-TRY-SET-UNBOUND-VARIABLE
(let [frame (first-frame e)
frame-vars (frame-variables frame)
frame-vals (frame-values frame)]
(if (not= @(enclosing-enviroment e) @the-empty-environment)
(extend-enviroment frame-vars frame-vals (env-loop
(enclosing-enviroment e)))
(extend-enviroment
frame-vars
(find-and-change frame-vars frame-vals)
the-empty-environment)))))
(env-loop env)
)
My question is, is there any good substitution for set-car! in scenarios
where I need just *change* the car and cdr of something created by cons??
PS: tried swap! atom , but it will also need to change the whole
environment structure.
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