Scheme递归定义的共享环境框架[#56]

program. Scheme 环境查找、扩展与 define 的实现[mce-env]

Defines: (extend-environment lookup define-value!)

Assembled in Scheme 解释器的装配计划, Scheme环境模块

Outputs: metacircular.scm, lp-example/mce/environment.scm

(define (extend-environment names values outer)
       (unless (= (length names) (length values)) (throw 'mce-arity names values))
       (cons (vector (map cons names values)) outer))

(define (lookup name environment)
       (if (null? environment) (throw 'mce-unbound name)
           (let ((binding (assq name (vector-ref (car environment) 0))))
             (if binding (cdr binding) (lookup name (cdr environment))))))

(define (define-value! name value environment)
       (let* ((frame (car environment)) (binding (assq name (vector-ref frame 0))))
         (if binding (set-cdr! binding value)
             (vector-set! frame 0 (cons (cons name value) (vector-ref frame 0))))))

Scheme用户环境是框架链，每个框架保存可变的名字与值关联。闭包捕获框架链，define更新同一个框架，因此递归过程能够查找到自己的binding。框架内的binding使用dotted pair；环境实现保存原文、位置和片段身份，装配到新模块时不继承本文章的词法作用域。