In theory, file-exists-p is quite similar to the standard function **PROBE-FILE**; indeed, in several implementations—SBCL, LispWorks, and OpenMCL—**PROBE-FILE** already gives you the behavior you want for file-exists-p. But not all implementations of **PROBE-FILE** behave quite the same.

CLISP, on the other hand, once again has its own way of doing things. Its **PROBE-FILE** immediately signals an error if passed a name in directory form, regardless of whether a file or directory exists with that name. It also signals an error if passed a name in file form that’s actually the name of a directory. For testing whether a directory exists, CLISP provides its own function: probe-directory (in the ext package). This is almost the mirror image of **PROBE-FILE**: it signals an error if passed a name in file form or if passed a name in directory form that happens to name a file. The only difference is it returns **T** rather than a pathname when the named directory exists.

The function pathname-as-file that you need for the CLISP implementation of file-exists-p is the inverse of the previously defined , returning a pathname that’s the file form equivalent of its argument. This function, despite being needed here only by CLISP, is generally useful, so define it for all implementations and make it part of the library.

(defun pathname-as-file (name)
  (let ((pathname (pathname name)))
    (when (wild-pathname-p pathname)
    (if (directory-pathname-p name)
      (let* ((directory (pathname-directory pathname))
             (name-and-type (pathname (first (last directory)))))
         :directory (butlast directory)
         :name (pathname-name name-and-type)
         :type (pathname-type name-and-type)
      pathname)))