Filling CDs（录入 CD）

A single record, however, does not a database make. You need some larger construct to hold the records. Again, for simplicity’s sake, a list seems like a good choice. Also for simplicity you can use a global variable, *db*, which you can define with the DEFVAR macro. The asterisks (*) in the name are a Lisp naming convention for global variables.

只有单一记录还不能算是一个数据库，需要一些更大的结构来保存记录。出于简化目的，使用列表似乎也还不错。同样出于简化目的，也可以使用一个全局变量 *db*，它可以用 DEFVAR 宏来定义。名字中的星号是 Lisp 的全局变量命名约定。

  1. (defvar *db* nil)

You can use the PUSH macro to add items to db. But it’s probably a good idea to abstract things a tiny bit, so you should define a function add-record that adds a record to the database.

可以使用 PUSH 宏为 *db* 添加新的项。但稍微做得抽象一些可能更好，因此可以定义一个函数 add-record 来给数据库增加一条记录。

  1. (defun add-record (cd) (push cd *db*))

Now you can use add-record and make-cd together to add CDs to the database.

现在可以将 add-record 和 make-cd 一起使用，来为数据库添加新的 CD 记录了。

  1. CL-USER> (add-record (make-cd "Roses" "Kathy Mattea" 7 t))
  2. ((:TITLE "Roses" :ARTIST "Kathy Mattea" :RATING 7 :RIPPED T))
  3. CL-USER> (add-record (make-cd "Fly" "Dixie Chicks" 8 t))
  4. ((:TITLE "Fly" :ARTIST "Dixie Chicks" :RATING 8 :RIPPED T)
  5.  (:TITLE "Roses" :ARTIST "Kathy Mattea" :RATING 7 :RIPPED T))
  6. CL-USER> (add-record (make-cd "Home" "Dixie Chicks" 9 t))
  7. ((:TITLE "Home" :ARTIST "Dixie Chicks" :RATING 9 :RIPPED T)
  8.  (:TITLE "Fly" :ARTIST "Dixie Chicks" :RATING 8 :RIPPED T)
  9.  (:TITLE "Roses" :ARTIST "Kathy Mattea" :RATING 7 :RIPPED T))

The stuff printed by the REPL after each call to add-record is the return value, which is the value returned by the last expression in the function body, the PUSH. And PUSH returns the new value of the variable it’s modifying. So what you’re actually seeing is the value of the database after the record has been added.

那些每次调用 add-record 以后 REPL 所打印出来的东西是返回值，也就是函数体中最后一个表达式 PUSH 所返回的值，并且 PUSH 返回它正在修改的变量的新值。因此你看到的其实是每次新记录被添加以后整个数据库的值。