How can I set a default path for interactive directory selection to start with in a elisp defun?

Question

I want a function to interactively prompt for an existing directory, but instead of starting from default-directory, I would like a function local default path like '~/should/start/here/always/in/this/function' to start at when using (interactive "D") how can I achieve this? My first thought is to create another function which first sets default-dir and then calls my original function, but that doesn't seem right, and I am unsure of how interactive would be prompted in that case.

Answer 1

Since you're writing this yourself, you can do something like this:

(defun choose-directory (directory)
  "sample that uses interactive to get a directory"
  (interactive (list (read-directory-name "What directory? " 
                                          choose-directory-default-directory)))
  (message "You chose %s." directory))

(defvar choose-directory-default-directory "/home/tjackson/work/data"
  "Initial starting point.")

Which uses interactive with a lisp expression to call read-directory to get a directory name (you might want to add additional arguments, check the link/docs).

Your original hunch would work as well, though, as you thought, isn't quite as clean. But, it does work well when you don't want to, or cannot, modify the function whose behavior you want to change. I've included that solution below to show you how you'd achieve it (the only piece of the puzzle you didn't mention was call-interactively):

;; original version of choose-directory, calling (interactive "D")
(defun choose-directory (directory)
  "sample that uses interactive to get a directory"
  (interactive "DWhat directory? ")
  (message "You chose %s." directory))

(defun wrap-choose-directory ()
  (interactive)
  (let ((default-directory choose-directory-default-directory))
    (call-interactively 'choose-directory)))