How can I set the default value for reading a directory name?

Question

I have a folder hardcoded in variable:

(defvar folder-start-mine "/path/to/start/folder/")

Then I want to interactively define new non-existing subfolder name:

(defun my-new-defun (sel-subdir)  
     (interactive "GNew folder: " folder-start-mine)
     (mkdir sel-subdir)
     ... do some stuff with sel-subdir ...
)

The file navigator in minibuffer starts in home folder and I want it to start in folder-start-mine.

Answer 1

The built-in support for prompting a user for a directory name, via the G flag as you have used, does not support setting the default directory. You can do this explicitly with a lisp expression instead of the usual interactive codes.

(defvar folder-start-mine "/path/to/start/folder/")

(defun my-new-defun (sel-subdir)  
  (interactive
   (list (read-directory-name "New Folder: " folder-start-mine)))
  (mkdir sel-subdir))

This will prompt the user for a directory name, starting from the value of folder-start-mine, and then use that as the value of the variable sel-subdir.