SICP 問題 5.41

翻訳時環境に対する変数の文面アドレスを返す手続きfind-variableの実装

(define (find-variable var ct-env)
  (define (env-loop frame-address env)
    (define (scan variable-address frame)
      (cond ((null? frame)
             (env-loop (+ frame-address 1) (enclosing-environment env)))
            ((eq? (car frame) var)
             (list frame-address variable-address))
            (else
             (scan (+ variable-address 1) (cdr frame)))))
    (if (null? env)
        'not-found
        (let ((frame (first-frame env)))
          (scan 0 frame))))
  (env-loop 0 ct-env))

test

gosh> (find-variable 'c '((y z) (a b c d e) (x y)))
(1 2)
gosh> (find-variable 'x '((y z) (a b c d e) (x y)))
(2 0)
gosh> (find-variable 'w '((y z) (a b c d e) (x y)))
not-found