uiop:pathname-parent-directory-pathname

(uiop:pathname-parent-directory-pathname pathname)

One directory level up from the pathname's directory: /foo/bar/baz/file.type answers #P"/foo/bar/". The root's parent is the root, and the parent of a single-level relative directory is the empty pathname.

Backend support

Works on all four backends (Lisp source, uiop-pathname.lisp).