The only required argument to **OPEN** is the name of the file to read. As you’ll see in the section “Filenames,” Common Lisp provides a couple of ways to represent a filename, but the simplest is to use a string containing the name in the local file-naming syntax. So assuming that /some/file/name.txt is a file, you can open it like this:

    You can use the object returned as the first argument to any of the read functions. For instance, to print the first line of the file, you can combine **OPEN**, **READ-LINE**, and **CLOSE** as follows:

    1. (let ((in (open "/some/file/name.txt")))
    2.   (format t "~a~%" (read-line in))
    3.   (close in))

    If you want to open a possibly nonexistent file without signaling an error, you can use the keyword argument :if-does-not-exist to specify a different behavior. The three possible values are :error, the default; :create, which tells it to go ahead and create the file and then proceed as if it had already existed; and **NIL**, which tells it to return **NIL** instead of a stream. Thus, you can change the previous example to deal with the possibility that the file may not exist.

    The reading functions—**READ-CHAR**, **READ-LINE**, and **READ**--all take an optional argument, which defaults to true, that specifies whether they should signal an error if they’re called at the end of the file. If that argument is **NIL**, they instead return the value of their third argument, which defaults to **NIL**. Thus, you could print all the lines in a file like this:

    1. (let ((in (open "/some/file/name.txt" :if-does-not-exist nil)))
    2.   (when in
    3.     (loop for line = (read-line in nil)
    4.     (close in)))

    In other words, it contains four s-expressions: a list of numbers, a number, a string, and a list of lists. You can read those expressions like this:

    1. CL-USER> (defparameter *s* (open "/some/file/name.txt"))
    2. *S*
    3. CL-USER> (read *s*)
    4. (1 2 3)
    5. CL-USER> (read *s*)
    6. CL-USER> (read *s*)
    7. "a string"
    8. CL-USER> (read *s*)
    9. ((A B) (C D))
    10. CL-USER> (close *s*)
    11. T

    As you saw in Chapter 3, you can use **PRINT** to print Lisp objects in “readable” form. Thus, whenever you need to store a bit of data in a file, **PRINT** and provide an easy way to do it without having to design a data format or write a parser. They even—as the previous example demonstrated—give you comments for free. And because s-expressions were designed to be human editable, it’s also a fine format for things like configuration files.1