Variable Basics（变量的基础知识）

As in other languages, in Common Lisp variables are named places that can hold a value. However, in Common Lisp, variables aren’t typed the way they are in languages such as Java or C++. That is, you don’t need to declare the type of object that each variable can hold. Instead, a variable can hold values of any type and the values carry type information that can be used to check types at runtime. Thus, Common Lisp is dynamically typed—type errors are detected dynamically. For instance, if you pass something other than a number to the + function, Common Lisp will signal a type error. On the other hand, Common Lisp is a strongly typed language in the sense that all type errors will be detected—there’s no way to treat an object as an instance of a class that it’s not.

和其他语言一样，Common Lisp 中的变量是一些可以保存值的具名位置。但在 Common Lisp 中，变量并非像在 Java 和 C++ 等语言中那样带有确定的类型，也就是说不需要为每一个变量声明其可以保存对象的类型。相反，一个变量可以保存任何类型的值，并且这些值带有可用于运行期类型检查的类型信息。因此，Common Lisp 是动态类型的——类型错误会被动态地检测到。举个例子，假如这些值某个并非数字的对象传给了 + 函数，那么 Common Lisp 将会报类型错误。而另一方面，Common Lisp 是一种强类型语言，因为所有的类型错误都将被检测到——无法将一个对象作为其不属于的类型的实例来对待。

All values in Common Lisp are, conceptually at least, references to objects. Consequently, assigning a variable a new value changes what object the variable refers to but has no effect on the previously referenced object. However, if a variable holds a reference to a mutable object, you can use that reference to modify the object, and the modification will be visible to any code that has a reference to the same object.

至少从概念上来说，Common Lisp 中所有的值都是对象的引用。 因此，将一个变量赋予新值就会改变该变量所指向的对象，而对之前被引用的对象却没有影响。尽管如此，如果一个变量保存了对一个可变对象的引用，那么就可以用该引用来修改此对象，而这种改动将应用于任何带有相同对象引用的代码。

One way to introduce new variables you’ve already used is to define function parameters. As you saw in the previous chapter, when you define a function with DEFUN, the parameter list defines the variables that will hold the function’s arguments when it’s called. For example, this function defines three variables—x, y, and z--to hold its arguments.

而另一种已经用到的引入新变量的方式是定义函数形参。正如前一章所示，在用 DEFUN 来定义函数时，形参列表定义了当函数被调用时用来保存实参的变量，例如，下列函数定义了三个变量 x、y 和 z，用来保存其实参：

(defun foo (x y z) (+ x y z))

Each time a function is called, Lisp creates new bindings to hold the arguments passed by the function’s caller. A binding is the runtime manifestation of a variable. A single variable—the thing you can point to in the program’s source code—can have many different bindings during a run of the program. A single variable can even have multiple bindings at the same time; parameters to a recursive function, for example, are rebound for each call to the function.

每当函数被调用时，Lisp 就会创建新的绑定来保存由函数调用者所传递的实参。绑定代表了变量在运行期的存在。单个变量即可以在程序源代码中所指出的那种东西。在程序运行过程中可以有多个不同的绑定，单个变量甚至可以同时带有多重绑定，例如，一个递归函数的形参会在每一次函数调用中被重新绑定。

As with all Common Lisp variables, function parameters hold object references. Thus, you can assign a new value to a function parameter within the body of the function, and it will not affect the bindings created for another call to the same function. But if the object passed to a function is mutable and you change it in the function, the changes will be visible to the caller since both the caller and the callee will be referencing the same object.

和所有 Common Lisp 变量一样，函数形参也可以保存对象引用。 因此，可以在函数体内为一个函数形参赋予新值，而这却并不会影响到同样函数的 另一个调用所创建的绑定。但如果改变了传递给函数的可变对象，则这些改 动将会被调用者看到，因为无论调用者还是被调用者都在引用同一个对象。

Another form that introduces new variables is the LET special operator. The skeleton of a LET form looks like this:

引入新变量的另一种方式是使用 LET 特别操作符。下面就是一个 LET 形式的结构：

(let (variable*)
  body-form*)

where each variable is a variable initialization form. Each initialization form is either a list containing a variable name and an initial value form or—as a shorthand for initializing the variable to NIL--a plain variable name. The following LET form, for example, binds the three variables x, y, and z with initial values 10, 20, and NIL:

其中每一个 variable都是一个变量的初始化形式。每一个初始化形式要么是一个含有变量名和初值形式的列表，要么就是一个简单的变量名——作为将变量初始化到 NIL 的简略写法。例如，下面的 LET 形式会将三个变量 x、y 和 z 绑定到初始值 10、20 和 NIL 上：

(let ((x 10) (y 20) z)
  ...)

When the LET form is evaluated, all the initial value forms are first evaluated. Then new bindings are created and initialized to the appropriate initial values before the body forms are executed. Within the body of the LET, the variable names refer to the newly created bindings. After the LET, the names refer to whatever, if anything, they referred to before the LET.

当这个 LET 形式被求值时，所有的初始值形式都将首先被求值，然后创建出新的绑定，并在形式体被执行之前这些绑定将初始化到适当的初始值上。在 LET 形式体中，变量名将引用新创建的绑定。在 LET 形式体执行结束后，这些变量名将重新引用在执行 LET 之前它们所引用的内容，如果有的话。

The value of the last expression in the body is returned as the value of the LET expression. Like function parameters, variables introduced with LET are rebound each time the LET is entered.

形式体中最后一个表达式的值将作为 LET 表达式的值返回。和函数形参一样，由 LET 所引入的变量将在每次进入 LET 时被重新绑定。

The scope of function parameters and LET variables—the area of the program where the variable name can be used to refer to the variable’s binding—is delimited by the form that introduces the variable. This form—the function definition or the LET--is called the binding form. As you’ll see in a bit, the two types of variables—lexical and dynamic—use two slightly different scoping mechanisms, but in both cases the scope is delimited by the binding form.

函数形参和 LET 变量的作用域（变量名可被用来引用该绑定的程序区域）被限定在引入该变量的形式之内，该形式即函数定义或 LET，被称为绑定形式。你很快将看到，词法变量和动态变量使用两种略有不同的作用域机制，但两者的其作用域都被界定在绑定形式之内。

If you nest binding forms that introduce variables with the same name, then the bindings of the innermost variable shadows the outer bindings. For instance, when the following function is called, a binding is created for the parameter x to hold the function’s argument. Then the first LET creates a new binding with the initial value 2, and the inner LET creates yet another binding, this one with the initial value 3. The bars on the right mark the scope of each binding.

如果嵌套了引入同名变量的绑定形式，那么最内层的变量绑定将覆盖外层的绑定。例如，在调用下面的函数时，将创建一个形参 x 的绑定来保存函数的参数。第一个 LET 创建了一个带有初始值 2 的新绑定，而内层的 LET 创建了另外一个绑定，其初始值为 3。右边的竖线标记出了每一个绑定的作用域。

(defun foo (x)
  (format t "Parameter: ~a~%" x)      ; |<------ x is argument 
  (let ((x 2))                        ; |
    (format t "Outer LET: ~a~%" x)    ; | |<---- x is 2
    (let ((x 3))                      ; | |
      (format t "Inner LET: ~a~%" x)) ; | | |<-- x is 3
    (format t "Outer LET: ~a~%" x))   ; | |
  (format t "Parameter: ~a~%" x))     ; |

Each reference to x will refer to the binding with the smallest enclosing scope. Once control leaves the scope of one binding form, the binding from the immediately enclosing scope is unshadowed and x refers to it instead. Thus, calling foo results in this output:

每一次对 x 的引用都将指向最小封闭作用域中的绑定。一旦程序控制离开了一个绑定形式的作用域，其最近的闭合作用域中的绑定就被解除覆盖，并且 x 将转而指向它。因此，调用 foo 将得到这样的输出：

CL-USER> (foo 1)
Parameter: 1
Outer LET: 2
Inner LET: 3
Outer LET: 2
Parameter: 1
NIL

In future chapters I’ll discuss other constructs that also serve as binding forms—any construct that introduces a new variable name that’s usable only within the construct is a binding form.

后面的章节将讨论其他可作为绑定形式使用的程序构造，其特点在于所引入的新变量名只能用于该构造。

For instance, in Chapter 7 you’ll meet the DOTIMES loop, a basic counting loop. It introduces a variable that holds the value of a counter that’s incremented each time through the loop. The following loop, for example, which prints the numbers from 0 to 9, binds the variable x:

例如，你将在第 7 章里遇到 DOTIMES 循环，一种基本的计数循环。它引入了一个变量用来保存每次通过循环时递增的计数器的值。例如下面这个可以打印从 0 到 9 的数字循环，它绑定了变量 x：

(dotimes (x 10) (format t "~d " x))

Another binding form is a variant of LET, LET*. The difference is that in a LET, the variable names can be used only in the body of the LET--the part of the LET after the variables list—but in a LET*, the initial value forms for each variable can refer to variables introduced earlier in the variables list. Thus, you can write the following:

另一个绑定形式是 LET 的变种：LET*。两者的区别在于，在一个 LET 中，被绑定的变量名只能被用在 LET 的形式体之内——LET 形式中变量列表之后的那部分；但在一个 LET* 中，每个变量的初始值形式，都可以引用到那些在变量列表中早先引入的变量。因此可以写成下面这样：

(let* ((x 10)
       (y (+ x 10)))
  (list x y))

but not this:

但却不能这样写：

(let ((x 10)
      (y (+ x 10)))
  (list x y))

However, you could achieve the same result with nested LETs.

不过也可以通过嵌套的 LET 来达到相同的效果：

(let ((x 10))
  (let ((y (+ x 10)))
    (list x y)))