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

(defun* tofilename (directorylist &optional (filename nil))
  "concatenate directory names into a path, with an optional file name as last part"
  (concat
   (mapconcat 'directory-file-name directorylist "/")
   "/"
   filename))


(tofilename '("~/" "Temp/") "temp.txt")
;; => "~/Temp/temp.txt"

(tofilename '("~/" "Temp/"))
;; => "~/Temp/"

(tofilename '("~/" "Temp/" "test"))
;; => "~/Temp/temp/"