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From: Dmitry Gutov <dgutov@yandex.ru>
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Subject: Re: exposing the effective mode in a multi-mode
Date: Mon, 18 Sep 2017 12:01:09 +0300
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On 9/18/17 2:13 AM, Tom Tromey wrote:
> I noticed that company couldn't do CSS completion in a <style> element
> in mhtml-mode.
> 
> company-css understands how web-mode handles sub-modes, by calling a
> web-mode function ("web-mode-language-at-pos") directly, but it doesn't
> understand mhtml-mode.
> 
> So, I'd like to propose this patch for Emacs 26.  It adds a new function
> to prog-mode.el so that multi-modes can expose the sub-mode at point.
> It also changes mhtml-mode to set the new variable to make this work.

Looks good to me, but consider this: sometimes (not too often, though) 
we have several major modes corresponding to a language. and they don't 
inherit from one another. Example: perl-mode and cperl-mode.

So I thought it might be useful to return the name of the language 
instead of the mode. The default implementation could look like this:

(defun prog-effective-lang ()
   (intern (replace-regexp-in-string "-mode\\'" ""
                                     (symbol-name major-mode))))

The "extra" major modes would have to provide their own implementations, 
though. Although define-derived-mode could grow a new keyword parameter, 
to make this easier.

> +(defvar-local prog-effective-mode-function nil
> +  "When non-nil, provides the effective mode at point for embedded code chunks.
> +When non-nil, this is a function that will be called by
> +`prog-effective-mode' to find the effective major mode at point.
> +This function should return a symbol naming a major mode, e.g. `css-mode'.
> +It may return nil to mean the \"outer\" major mode.")
> +
> +(defun prog-effective-mode ()
> +  "When non-nil, returns the effective mode at point.

Can it return nil? When?