Unable to bind a key in ledger-mode-map

Question

Having written a function (say, my-func), I wanted to map it in ledger-mode-map to the keypress that I will call "Control dollar," that is, the dollar sign $, typed while the Ctrl key is held down.

The expression:

(define-key ledger-mode-map (kbd "C-$") 'my-func)

when executed does not indicate a failure, but a subsequent C-h my-func indicates no key binding.

Executing C-h k C-$ produces the output "C-$ is undefined".

I need a way to express "Control dollar" so that Emacs can recognize it as a keypress, if that is even possible. Here are a few of the alternatives I have tried to (kbd "C-$"):

Expression	Result when evaluated
[?\C-4]	67108916
[?\C-\$]	67108900
(kbd "C-\$")	67108900
[C-S-4]	[C-S-4]
(kbd "[67108900]")	"[67108916]"
[$]	[$]
[\$]	[$]
(kbd "C-[$]")	C- must prefix a single character, not [$]
(kbd "C-$")	67108900
[(control $)]	[(control $)]
[(control \$)]	[(control $)]

@amitp: It does work if I use global-map instead of ledger-mode-map. It does not work if I use, say, C-! instead of C-$ with ledger-mode-map.

Answer 1

Here's a MWE:

(require 'package)
(setq package-archives nil)
(add-to-list 'package-archives '("org"   . "https://orgmode.org/elpa/"    ) t)
(add-to-list 'package-archives '("melpa" . "http://melpa.org/packages/"   ) t)
(add-to-list 'package-archives '("gnu"   . "http://elpa.gnu.org/packages/") t)
(package-initialize)
(package-refresh-contents)

(defun my-func ()
  "Say `HEY'."
  (interactive)
  (message "HEY"))

(use-package ledger-mode
  :ensure         t
  :bind           (:map ledger-mode-map
                        ("<C-f13>"   . my-func)))

(define-key ledger-mode-map (kbd "C-$") 'my-func)

If we evaluate the above expression, then edit a buffer in ledger-mode, and then press either C-$or C-f13, "HEY" shows up in the minibuffer.

But if instead of using define-key to do the mapping for C-$, we add it to the :bind section of use-package thus:

(use-package ledger-mode
  :ensure         t
  :bind           (:map ledger-mode-map
                        ("<C-f13>"   . my-func)
                        ((kbd "C-$") . my-func)))

and then evaluate the expression, the *Warnings* buffer displays this:
Error (use-package): Failed to parse package ledger-mode: use-package: ledger-mode wants arguments acceptable to the 'bind-keys' macro, or a list of such values

The trick is to change that last line from

                        ((kbd "C-$") . my-func)))

to

                        ("C-$"       . my-func)))

Then once again either C-$ or C-f13 display "HEY" in the minibuffer.