algorithm - 方案功能

Question

我对方案功能有疑问:

> (let ((x 2) (y 3))
>         (let ((x 7) (z (+ x y)))
>               (* z x) ) )

结果是:35。有人能解释一下为什么吗？我不明白的是，我们将 x 的值更改为 7，我认为结果应该是 70。

Answer 1

实际上，Sylwester 已经解释过了。

; I like to write `let` forms in this way:
(let ((x 2)
      (y 3))
  (let ((x 7)
        (z (+ x y))) ; since this is within a `let` definition part,
                     ; and not in a `let*` definition part, 
                     ; the definition of z - which is (+ x y) - 
                     ; cannot "see" (x 7) yet,
                     ; so it "searches" for x and "finds" it above 
                     ; in the let definition (x 2).
                     ; therefore, (z (+ x y)) will be evaluated as 
                     ; (z (+ 2 3)) thus (z 5)
    (* z x)))        ; this now is in the inner-let body,
                     ; thus it "sees" (x 7) which has "shadowed" (x 2)
                     ; therefore this will be evaluated as (* 5 7), thus 35.