[PATCH] RFC: eldoc-documentation-functions hook

[Mark Oteiza <mvoteiza@udel.edu>]

This is a draft patch changing eldoc-documentation-function into a hook
variable, so instead of using add-function, one can instead use add-hook
to control the behavior of eldoc.  It is backwards compatible.

Perhaps the main motivation for doing this is that I don't like the
unreadable mess I see when looking at the value of
eldoc-documentation-function.  For example, the value in
*scratch* from emacs -Q.  This also exists in other places, like lambdas
as defcustom values in eshell, but that's another matter.

diff --git a/lisp/emacs-lisp/eldoc.el b/lisp/emacs-lisp/eldoc.el
index 6c2f869..bfbfd41 100644
--- a/lisp/emacs-lisp/eldoc.el
+++ b/lisp/emacs-lisp/eldoc.el
@@ -43,7 +43,7 @@
 
 ;; Major modes for other languages may use ElDoc by defining an
 ;; appropriate function as the buffer-local value of
-;; `eldoc-documentation-function'.
+;; `eldoc-documentation-functions'.
 
 ;;; Code:
 
@@ -80,7 +80,7 @@ eldoc-argument-case
 returns another string is acceptable.
 
 Note that this variable has no effect, unless
-`eldoc-documentation-function' handles it explicitly."
+`eldoc-documentation-functions' handles it explicitly."
   :type '(radio (function-item upcase)
 		(function-item downcase)
                 function)
@@ -103,7 +103,7 @@ eldoc-echo-area-use-multiline-p
 truncated to make more of the arglist or documentation string visible.
 
 Note that this variable has no effect, unless
-`eldoc-documentation-function' handles it explicitly."
+`eldoc-documentation-functions' handles it explicitly."
   :type '(radio (const :tag "Always" t)
                 (const :tag "Never" nil)
                 (const :tag "Yes, but truncate symbol names if it will\
@@ -113,7 +113,7 @@ eldoc-echo-area-use-multiline-p
 (defface eldoc-highlight-function-argument
   '((t (:inherit bold)))
   "Face used for the argument at point in a function's argument list.
-Note that this face has no effect unless the `eldoc-documentation-function'
+Note that this face has no effect unless the `eldoc-documentation-functions'
 handles it explicitly."
   :group 'eldoc)
 
@@ -186,7 +186,7 @@ eldoc-mode
   :group 'eldoc :lighter eldoc-minor-mode-string
   (setq eldoc-last-message nil)
   (cond
-   ((memq eldoc-documentation-function '(nil ignore))
+   ((not (eldoc-supported-p))
     (message "There is no ElDoc support in this buffer")
     (setq eldoc-mode nil))
    (eldoc-mode
@@ -211,7 +211,7 @@ global-eldoc-mode
 
 If Global Eldoc mode is on, `eldoc-mode' will be enabled in all
 buffers where it's applicable.  These are buffers that have modes
-that have enabled eldoc support.  See `eldoc-documentation-function'."
+that have enabled eldoc support.  See `eldoc-documentation-functions'."
   :group 'eldoc
   :global t
   :initialize 'custom-initialize-delay
@@ -236,9 +236,7 @@ eldoc-schedule-timer
 	     eldoc-idle-delay nil
 	     (lambda ()
                (when (or eldoc-mode
-                         (and global-eldoc-mode
-                              (not (memq eldoc-documentation-function
-                                         '(nil ignore)))))
+                         (and global-eldoc-mode (eldoc-supported-p)))
                  (eldoc-print-current-symbol-info))))))
 
   ;; If user has changed the idle delay, update the timer.
@@ -334,26 +332,29 @@ eldoc-display-message-no-interference-p
 
 \f
 ;;;###autoload
-(defvar eldoc-documentation-function #'ignore
-  "Function to call to return doc string.
-The function of no args should return a one-line string for displaying
-doc about a function etc. appropriate to the context around point.
-It should return nil if there's no doc appropriate for the context.
-Typically doc is returned if point is on a function-like name or in its
-arg list.
+(defvar eldoc-documentation-functions #'ignore
+  "Hook to run to obtain doc string.
+Each element of this variable should be a function of no args
+that should return a one-line string for displaying doc about a
+function etc. appropriate to the context around point.  It should
+return nil if there is no doc appropriate for the context.
+Typically, doc is returned if point is on a function-like name or
+in its arg list.
 
 The result is used as is, so the function must explicitly handle
 the variables `eldoc-argument-case' and `eldoc-echo-area-use-multiline-p',
 and the face `eldoc-highlight-function-argument', if they are to have any
 effect.
 
-Major modes should modify this variable using `add-function', for example:
-  (add-function :before-until (local \\='eldoc-documentation-function)
-                #\\='foo-mode-eldoc-function)
+Major modes should modify this variable using `add-hook', for example:
+  (add-hook \\='eldoc-documentation-functions #\\='foo-eldoc nil t)
 so that the global documentation function (i.e. the default value of the
 variable) is taken into account if the major mode specific function does not
 return any documentation.")
 
+(define-obsolete-variable-alias 'eldoc-documentation-function
+  'eldoc-documentation-functions "25.2")
+
 (defun eldoc-print-current-symbol-info ()
   ;; This is run from post-command-hook or some idle timer thing,
   ;; so we need to be careful that errors aren't ignored.
@@ -363,7 +364,19 @@ eldoc-print-current-symbol-info
              (when eldoc-last-message
                (eldoc-message nil)
                nil))
-	 (eldoc-message (funcall eldoc-documentation-function)))))
+	 (eldoc-message
+          (run-hook-with-args-until-success 'eldoc-documentation-functions)))))
+
+(defun eldoc-supported-p ()
+  "Return t if `eldoc-documentation-functions' has non-null elements."
+  (when eldoc-documentation-functions 
+    (catch :eldoc-supported
+      (mapc
+       (lambda (fun)
+         (when (not (memq fun '(nil ignore)))
+           (throw :eldoc-supported t)))
+       eldoc-documentation-functions)
+      nil)))
 
 ;; If the entire line cannot fit in the echo area, the symbol name may be
 ;; truncated or eliminated entirely from the output to make room for the