Saving persistent undo to a single directory, alist format

Question

It looks like this can be achieved by modifying the variable undo-tree-history-directory-alist, but I can't figure out how. I tried a number of lines, the last of which is

(setq undo-tree-history-directory-alist ("." ! "~/.emacs.d/undo"))

But this returns errors.

Answer 1

You most certainly want:

(setq undo-tree-history-directory-alist '(("." . "~/.emacs.d/undo")))

The docstring of this variable speaks of an alist aka association list. These are a list of pairs, where a pair is a cons cell. The short-hand syntax for a cons cell is (foo . 1) whereas the short-hand syntax for returning a list as is would be '(...), both combined result in '((foo . 1) ...)).