SICP問題4.13
束縛を除去するunbind!式の定義。
unbind!の実装は最初のフレームからだけ結合を除去する仕様とした。
これはdefineが最初のフレームに束縛を追加するのと対応づけるためである。
(define (unbind? exp) (tagged-list? exp 'unbind!)) (define (eval-unbind! exp env) (unbind-variable! (definition-variable exp) env)) (define (unbind-variable! var env) (let ((frame (first-frame env))) (let ((result (scan var (frame-variables frame) (frame-values frame) (lambda (vars vals) (set-car! vars (cdr vars)) (set-car! vals (cdr vals)) #t)))) (if (not result) (error "Unbound variable --UNBIND!" var)))))
evalにも組み込む
(define (eval exp env) (cond ((self-evaluating? exp) exp) ((variable? exp) (lookup-variable-value exp)) ((quoted? exp) (text-of-quotation exp)) ((assignment? exp) (eval-assignment exp env)) ((definition? exp) (eval-definition exp env)) ((unbind? exp) (eval-unbind! exp env)) ; 追加 ((if? exp) (eval-if exp env)) ((lambda? exp) (make-procedure (lambda-parameters exp) (lambda-body exp) env)) ((let? exp) (eval (let->combination exp) env)) ((let*? exp) (eval (let*->nested-lets exp) env)) ((while? exp) (eval (while->named-let exp) env)) ((begin? exp) (eval-sequence (begin-actions exp) env)) ((cond? exp) (eval (cond->if exp) env)) ((and? exp) (eval-and exp env)) ((or? exp) (eval-or exp env)) ((application? exp) (apply (eval (operator exp) env) (list-of-values (operands exp) env))) (else (error "Unknown expression type -- EVAL" exp))))