Electric pair mode's delete adjacent pairs functionality not working in Slime REPL

Question

Part of Emacs's electric-pair-mode functionality (controlled by a variable called electric-pair-delete-adjacent-pairs) is that when the point is between an empty set of delimiters and you delete the opening one, it also deletes the closing one for you: if the point is at the pipe character in (|), pressing backspace will delete both the opening and closing parenthesis.

I like this functionality very much but annoyingly it doesn't work in the Slime REPL. I do have electric-pair-mode enabled in the REPL, and it does do the other things it is supposed to, like insert the closing parenthesis automatically when you insert the opening one, or wrap the region in parenthesis when the mark is active and you press (. Just deleting adjacent pairs doesn't work. How do I get that functionality in the Slime REPL? (I think the answer has to do with keymap precedence. Maybe...)

EDIT: Here is how I'm testing this: I'm using Emacs 26.1 (on both GNU/Linux and on Windows 10, the same problem appears) and Slime 2.23. To test I run emacs -Q and then evaluate this in the scratch buffer:

(package-initialize)
(setq slime-contribs '(slime-repl slime-indentation))
(setq slime-lisp-implementations '((sbcl ("sbcl"))))
(require 'slime)

Then I run M-x slime and when the REPL appears, I run M-x electric-pair-mode. Then typing a ( inserts () leaving the point between the parenthesis. Typing backspace then deletes only the opening parenthesis insetad of both.

Answer 1

slime-repl-mode overrides in slime-repl-mode-map the key-binding for electric-pair-delete-pair. The override is not very surprising. It is just backward-delete-char-untabify. The binding in global-map is delete-backward-char. If you do not really need the untabify part in the repl removing the override shouldn't do any harm.

Just put the following Elisp snippet into your init file.

(eval-after-load "slime" '(define-key slime-repl-mode-map (kbd "DEL") nil))

EDIT: After some background check it turns out that you can even make sure that backward-delete-char-untabify is called by the backspace key if you bind it in the local map of slime-repl-mode.

Just add also the following Elisp snippet into your init file:

(add-hook 'slime-repl-mode-hook
      (lambda ()
        (local-set-key (kbd "DEL") #'backward-delete-char-untabify)))

Background:

The reason for the strange behavior are peculiarities of slime-repl-mode and electric-pair-mode.

1. slime-repl-mode is not defined via define-derived-mode. It does not use its local map as keymap but uses the minor mode map of the minor mode slime-repl-map-mode.

2. electric-pair-mode is defined as an autoloaded minor mode. Its keymap is put into minor-mode-map-alist as soon as the autoloads are executed. That means the keymap of electric-pair-mode is always already in minor-mode-map-alist when slime-repl-map-mode is defined through (require 'slime). Note, (define-minor-mode slime-repl-map-mode ...) is not autoloaded.

We have here two concurrent minor mode maps. slime-repl-map-mode is defined last and wins.

Note that minor mode maps always override local keymaps. Furthermore electric-pair-mode uses a filter for its DEL entry in electric-pair-mode-map. The filter lets through the original binding (e.g., the local binding) if point is not between two delimiters handled by electric-pair-mode. That means: At points where electric-pair-mode does not kick in you can define the behavior of the backspace key in the local map of slime-repl-mode.