19.2.3 Merging Pathnames

Merging takes a pathname with unfilled components and supplies values for those components from a source of defaults.

If a component's value is nil, that component is considered to be unfilled. If a component's value is any non-nil object, including :unspecific, that component is considered to be filled.

Except as explicitly specified otherwise, for functions that manipulate or inquire about files in the file system, the pathname argument to such a function is merged with *default-pathname-defaults* before accessing the file system (as if by merge-pathnames).

19.2.3.1 Examples of Merging Pathnames

Although the following examples are possible to execute only in implementations which permit :unspecific in the indicated position andwhich permit four-letter type components, they serve to illustrate the basic concept of pathname merging.

 (pathname-type
   (merge-pathnames (make-pathname :type "LISP")
                    (make-pathname :type "TEXT")))
→ "LISP"
 
 (pathname-type
   (merge-pathnames (make-pathname :type nil)
                    (make-pathname :type "LISP")))
→ "LISP"
 
 (pathname-type
   (merge-pathnames (make-pathname :type :unspecific)
                    (make-pathname :type "LISP")))
→ :UNSPECIFIC