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.

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 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.

The historical diversity of file systems in existence during the 70s and 80s can be easy to forget. Kent Pitman, one of the principal technical editors of the Common Lisp standard, described the situation once in comp.lang.lisp (Message-ID: sfwzo74np6w.fsf@world.std.com) thusly:

If you look at the pathname abstraction from the point of view of any single file system, it seems baroque. However, if you take even two such similar file systems as Windows and Unix, you can already begin to see differences the pathname system can help abstract away—Windows filenames contain a drive letter, for instance, while Unix filenames don’t. The other advantage of having the pathname abstraction designed to handle the wide variety of file systems that existed in the past is that it’s more likely to be able to handle file systems that may exist in the future. If, say, versioning file systems come back into vogue, Common Lisp will be ready.