我正在阅读SICP ，并且出现了绑定和自由变量的主题。 但是，我很困惑。 术语“绑定变量”是否仅适用于作为形式参数的变量？ 此外，文本说过程定义“绑定”其形式参数。 这让我感到困惑的是，有些人说我们将一个值“绑定”到一个变量上。 显然，当我们讨论不同类型的变量时，这个术语似乎意味着不同的东西。 有人可以清除绑定变量是什么以及绑定意味着什么？ 最后，与绑定变量相比，什么是自由变量？ 所有这些与范围有何关系？
I'm reading SICP, and the topic of bound and free variables has come up. However, I am confused about it. Does the term "bound variables" only apply to variables that are formal parameters? In addition, the text says that a procedure definition "binds" its formal parameters. This confuses me with the fact that some people say that we "bind" a value to a variable. Obviously, the term seems to mean different things when we're talking about different types of variables. Could someone clear up what a bound variable is and what binding means? Finally, in contrast to a bound variable, what is free variable? How does all of this relate to scope?
最满意答案
只有两种类型的变量。 全球性和词汇性。 实际上，您可以将全局视为词法范围的可变根，并且在这种情况下，只有一种类型的变量。
绑定变量是当前过程的形式参数，其他一切都是全局的，或者是从先前的嵌套调用绑定的，是自由变量。
例：
(lambda (x) (let ((y (+ x x))) ; + is free x is bound (+ x y))) ; + and x is free, y is bound记住， let是语法糖，所以它真的是这样的：
(lambda (x) ((lambda (y) (+ x y)) ; + and x is free, y is bound (+ x x))) ; + is free x is bound在内部lambda中， y作为绑定变量+和x是自由的。 在外部lambda x绑定， +是自由的。 +可能是一个全球性的。
There are only two types of variables. Global and lexical. You can actually think of global as a changeable root of the lexical scope and in that case there is only one type of variables.
A bound variable is the formal parameters of the current procedure and everything else, which either is global or bound from previous nested calls, are free variables.
Example:
(lambda (x) (let ((y (+ x x))) ; + is free x is bound (+ x y))) ; + and x is free, y is boundRemember let is ust syntactic sugar so it's really the same as this:
(lambda (x) ((lambda (y) (+ x y)) ; + and x is free, y is bound (+ x x))) ; + is free x is boundIn the inner lambda with y as bound variable + and x are free. In the outer lambda x is bound and + is free. + might be a global.
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