June 5th Tuesday (六月　五日　火曜日)

原创于 2007-07-14 11:11:00 发布 · 699 阅读

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文章标签：

#function #forms #basic #object

本文介绍了Eval函数的基本用法及其在表达式求值中的作用。通过具体的例子展示了如何使用Eval函数进行变量值的获取，并解释了在调用过程中表达式被求值两次的现象。

-- Function: eval form
     This is the basic function evaluating an expression. It evaluates
     FORM in the current environment and returns the result. How the
     evaluation proceeds depends on the type of the object (*note
     Forms::).

     Since `eval' is a function, the argument expression that appears
     in a call to `eval' is evaluated twice: once as preparation before
     `eval' is called, and again by the `eval' function itself. Here
     is an example:

          (setq foo 'bar)
               => bar
          (setq bar 'baz)
               => baz
          ;; Here `eval' receives argument `foo'
          (eval 'foo)
               => bar
          ;; Here `eval' receives argument `bar', which is the value of `foo'
          (eval foo)
               => baz