Книга: Practical Common Lisp

Reading File Data

Reading File Data

The most basic file I/O task is to read the contents of a file. You obtain a stream from which you can read a file's contents with the OPEN function. By default OPEN returns a character-based input stream you can pass to a variety of functions that read one or more characters of text: READ-CHAR reads a single character; READ-LINE reads a line of text, returning it as a string with the end-of-line character(s) removed; and READ reads a single s-expression, returning a Lisp object. When you're done with the stream, you can close it with the CLOSE function.

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:

(open "/some/file/name.txt")

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:

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

Of course, a number of things can go wrong while trying to open and read from a file. The file may not exist. Or you may unexpectedly hit the end of the file while reading. By default OPEN and the READ-* functions will signal an error in these situations. In Chapter 19, I'll discuss how to recover from such errors. For now, however, there's a lighter-weight solution: each of these functions accepts arguments that modify its behavior in these exceptional situations.

If you want to open a possibly nonexistent file without OPEN 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.

(let ((in (open "/some/file/name.txt" :if-does-not-exist nil)))
(when in
(format t "~a~%" (read-line in))
(close in)))

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:

(let ((in (open "/some/file/name.txt" :if-does-not-exist nil)))
(when in
(loop for line = (read-line in nil)
while line do (format t "~a~%" line))
(close in)))

Of the three text-reading functions, READ is unique to Lisp. This is the same function that provides the R in the REPL and that's used to read Lisp source code. Each time it's called, it reads a single s-expression, skipping whitespace and comments, and returns the Lisp object denoted by the s-expression. For instance, suppose /some/file/name.txt has the following contents:

(1 2 3)
456
"a string" ; this is a comment
((a b)
(c d))

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:

CL-USER> (defparameter *s* (open "/some/file/name.txt"))
*S*
CL-USER> (read *s*)
(1 2 3)
CL-USER> (read *s*)
456
CL-USER> (read *s*)
"a string"
CL-USER> (read *s*)
((A B) (C D))
CL-USER> (close *s*)
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 READ 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.[151]

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