前言

本文是6. From Substitution to Environments的实验记录。

计划

• Define a environment
• Implement lookup in environment

codes

1. Define a environment as list.

(define-type Binding
  [bind (name : symbol) (val : number)])
(define-type-alias Env (listof Binding))

2. Implement lookup in environment

; DONE : Define lookup
(define (lookup [for : symbol] [env : Env]) : number
  (cond
    [(empty? env) (error 'lookup "name not found")]
    [(equal? for (bind-name (first env))) (bind-val (first env))]
    [else (lookup for (rest env))]))

3. Test cases

(test (lookup 'a (list (bind 'a 5))) => 5)
(test (lookup 'b (list (bind 'a 5) (bind 'b 6))) => 6)
(test (lookup 'a (list (bind 'a 5) (bind 'b 6))) => 5)

总结

通过实现环境的定义和查找功能，我们为解释器添加了状态管理的能力。环境作为绑定的列表，允许我们在运行时存储和检索变量值。