问题描述
我正在阅读 SICP ，绑定和自由变量的主题已经出现.但是，我对此感到困惑.术语绑定变量"仅适用于形式参数的变量吗?另外，文本说过程定义绑定"了它的形式参数.这使我感到困惑，因为有人说我们将值绑定"到变量.显然，当我们谈论不同类型的变量时，该术语似乎意味着不同的含义.有人可以弄清绑定变量是什么，绑定的含义是什么?最后，与绑定变量相反，什么是自由变量?所有这些与范围有什么关系?
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?
推荐答案
只有两种类型的变量.全局和词汇.实际上，您可以将global视为词法范围的可变根，在这种情况下，只有一种类型的变量.
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.
绑定变量是当前过程的形式参数，而其他任何全局变量或从以前的嵌套调用绑定的都是自由变量.
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.
示例:
(lambda (x)
(let ((y (+ x x))) ; + is free x is bound
(+ x y))) ; + and x is free, y is bound
请记住，let是ust语法糖，因此它实际上与此相同:
Remember 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 bound
在内部lambda中，以y作为绑定变量的+和x是自由的.在外部lambda中，绑定了x，而+是自由的. +可能是全局的.
In 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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