Subject: 25.0.50; anonymous functions in `auto-mode-alist'

From: Michael Heerdegen <[email-masked]>
Date: Mon, 01 Jun 2015 17:58:25 +0200

Hello,

the doc of `auto-mode-alist' doesn't say that the mode functions
specified must be named functions.  That's good IMO, since using
anonymous functions in the alist e.g. while let-binding it can sometimes
be useful.

But this is buggy.  Try this:

--8<---------------cut here---------------start------------->8---
;; -*- lexical-binding: t -*-

(setq auto-mode-alist
      `((".*" . ,(lambda ()
                   (emacs-lisp-mode)
                   'and-do-something-in-addition))))
--8<---------------cut here---------------end--------------->8---

Save it in a file, and load it (uncompiled) so that

  auto-mode-alist
   => ((".*" closure (t) nil (emacs-lisp-mode) (quote and-do-something-in-addition)))

Now open any file.  You get:

File mode specification error: (void-function closure)

The problem is these lines in the defun of `set-auto-mode' starting from
line 2871 in files.el:

--8<---------------cut here---------------start------------->8---
(if (and mode
         (consp mode)
         (cadr mode))
    (setq mode (car mode)
         ...
--8<---------------cut here---------------end--------------->8---

i.e., the value in the alist pair is not tested whether it is a
function, and the (REGEXP FUNCTION NON-NIL) case is assumed, which is
wrong.

Thanks,

Michael.

In GNU Emacs 25.0.50.4 (x86_64-unknown-linux-gnu, GTK+ Version 3.14.5)
 of 2015-05-30 on drachen
Windowing system distributor `The X.Org Foundation', version 11.0.11701000
System Description:	Debian GNU/Linux testing (stretch)