Книга: Practical Common Lisp

Filenames

Filenames

So far you've used strings to represent filenames. However, using strings as filenames ties your code to a particular operating system and file system. Likewise, if you programmatically construct names according to the rules of a particular naming scheme (separating directories with /, say), you also tie your code to a particular file system.

To avoid this kind of nonportability, Common Lisp provides another representation of filenames: pathname objects. Pathnames represent filenames in a structured way that makes them easy to manipulate without tying them to a particular filename syntax. And the burden of translating back and forth between strings in the local syntax—called namestrings—and pathnames is placed on the Lisp implementation.

Unfortunately, as with many abstractions designed to hide the details of fundamentally different underlying systems, the pathname abstraction introduces its own complications. When pathnames were designed, the set of file systems in general use was quite a bit more variegated than those in common use today. Consequently, some nooks and crannies of the pathname abstraction make little sense if all you're concerned about is representing Unix or Windows filenames. However, once you understand which parts of the pathname abstraction you can ignore as artifacts of pathnames' evolutionary history, they do provide a convenient way to manipulate filenames.[156]

Most places a filename is called for, you can use either a namestring or a pathname. Which to use depends mostly on where the name originated. Filenames provided by the user—for example, as arguments or as values in configuration files—will typically be namestrings, since the user knows what operating system they're running on and shouldn't be expected to care about the details of how Lisp represents filenames. But programmatically generated filenames will be pathnames because you can create them portably. A stream returned by OPEN also represents a filename, namely, the filename that was originally used to open the stream. Together these three types are collectively referred to as pathname designators. All the built-in functions that expect a filename argument accept all three types of pathname designator. For instance, all the places in the previous section where you used a string to represent a filename, you could also have passed a pathname object or a stream.

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