Книга: Practical Common Lisp

Filing CDs

Filing CDs

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.[25]

(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.

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

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

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

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