Define new key binding for new minor mode

Question

I am writing my first minor mode, and would like to understand how to bind a key to an action in that mode.

(define-minor-mode hello-mode
  "My Mode"
  :init-value nil
  :lighter " Hello"
  :keymap (let ((map (make-sparse-keymap)))
            (define-key map (kbd "M-p")
              (lambda ()
                (interactive)
                (message "HI!!!")))
            map)
  :group 'hello)

After running this followed by M-x hello-mode (or (hello-mode t)), I would hope that hitting M-p would display "HI!!!" in the echo area, but Emacs tells me M-p is still undefined.

What else is needed to bind a key to a function when my minor mode is active?

Answer 1

Your code works perfectly, for me. Did you try it in emacs -Q (no init file)?

And if I load library help-fns+.el and then use M-k hello-mode-map it shows me that M-p is bound to an (anonymous) command.

However, I recommend that you give your keymap a name (a symbol). That helps users (and yourself) more.

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(If you have an Emacs 28 preview then you also have that command, describe-keymap - it's been added to Emacs itself.)