In Elisp, how to get path string with slash properly inserted?

Question

I am manually constructing path strings in Elisp by concatenating partial paths and directory names. Unfortunately sometimes the paths end with slash, sometimes not. Theref

Answer 1

(file-name-as-directory dir) will return directory path dir with a trailing slash, adding one if necessary, and not otherwise.

If you had your sequence of partial paths in a list, you could do something like:

(let ((directory-list '("/foo" "bar/" "p/q/" "x/y"))
      (file-name "some_file.el"))
  (concat
   (mapconcat 'file-name-as-directory directory-list "")
   file-name))

"/foo/bar/p/q/x/y/some_file.el"

or as an alternative, if you wanted to include the file name in the list, you could utilise directory-file-name which does the opposite of file-name-as-directory:

(let ((path-list '("/foo" "bar/" "p/q/" "x/y/some_file.el")))
  (mapconcat 'directory-file-name path-list "/"))

"/foo/bar/p/q/x/y/some_file.el"

(Someone please correct me if using directory-file-name on a non-directory is not portable?)