I'm looking for something similar to Eldoc mode (hover over a function and the parameter list appears in the minibuffer), only I'd like to have a help buffer appear, as if I had just run describe-function. Is there a minor mode that will do this?
Having the help buffer be in a separate frame would be ideal, but I can figure that out at a later point. Funnily enough, I think I saw something like this for SLIME, but I haven't been able to find it again.
For years I've used the rgr/context-help solution from the EmacsWiki for this, with some slight tweaks. A few aspects still bothered me, though, so this question inspired me to write a new variant of that as a global minor mode. Among other differences, this utilises help-xref-interned to display all (variable, function, face) kinds of help for a symbol instead of arbitrarily choosing between describe-function and describe-variable when both cells are defined.
(define-minor-mode my-contextual-help-mode
"Show help for the elisp symbol at point in the current *Help* buffer.
Advises `eldoc-print-current-symbol-info'."
:lighter " C-h"
:global t
(require 'help-mode) ;; for `help-xref-interned'
(when (eq this-command 'my-contextual-help-mode)
(message "Contextual help is %s" (if my-contextual-help-mode "on" "off")))
(and my-contextual-help-mode
(eldoc-mode 1)
(if (fboundp 'eldoc-current-symbol)
(eldoc-current-symbol)
(elisp--current-symbol))
(my-contextual-help :force)))
(defadvice eldoc-print-current-symbol-info (before my-contextual-help activate)
"Triggers contextual elisp *Help*. Enabled by `my-contextual-help-mode'."
(and my-contextual-help-mode
(derived-mode-p 'emacs-lisp-mode)
(my-contextual-help)))
(defvar-local my-contextual-help-last-symbol nil
;; Using a buffer-local variable for this means that we can't
;; trigger changes to the help buffer simply by switching windows,
;; which seems generally preferable to the alternative.
"The last symbol processed by `my-contextual-help' in this buffer.")
(defun my-contextual-help (&optional force)
"Describe function, variable, or face at point, if *Help* buffer is visible."
(let ((help-visible-p (get-buffer-window (help-buffer))))
(when (or help-visible-p force)
(let ((sym (if (fboundp 'eldoc-current-symbol)
(eldoc-current-symbol)
(elisp--current-symbol))))
;; We ignore keyword symbols, as their help is redundant.
;; If something else changes the help buffer contents, ensure we
;; don't immediately revert back to the current symbol's help.
(and (not (keywordp sym))
(or (not (eq sym my-contextual-help-last-symbol))
(and force (not help-visible-p)))
(setq my-contextual-help-last-symbol sym)
sym
(save-selected-window
(help-xref-interned sym)))))))
(defun my-contextual-help-toggle ()
"Intelligently enable or disable `my-contextual-help-mode'."
(interactive)
(if (get-buffer-window (help-buffer))
(my-contextual-help-mode 'toggle)
(my-contextual-help-mode 1)))
(my-contextual-help-mode 1)
