Re: exposing the effective mode in a multi-mode

[Stefan Monnier <monnier@iro.umontreal.ca>]

> +(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.
> +
> +There are languages where part of the code is actually written in
> +a sub language, e.g., a Yacc/Bison or ANTLR grammar also consists
> +of plain C code.  This function returns the effective value of
> +`major-mode' at point."
> +  (or (if prog-effective-mode-function
> +          (funcall 'prog-effective-mode-function))
> +      major-mode))

As usual, I recommend to avoid the "when non-nil, ..." for function
variables (e.g. because it prevents use of add-function on it, and
because removing the nil case simplifies the code, and because removing
the nil case makes sure that the default behavior is not special).
[ Also the doc shouldn't talk about who calls it, but what it does.  ]

    (defvar-local prog-effective-mode-function #'ignore
      "Function to get the effective mode at point for embedded code chunks.
    Called with no argument, returns 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 ()
      "Return the effective mode at point.
    There are languages where part of the code is actually written in
    a sub language, e.g., a Yacc/Bison or ANTLR grammar also consists
    of plain C code.  This function returns the effective value of
    `major-mode' at point."
      (or (funcall 'prog-effective-mode-function)
          major-mode))

Tho we can simplify it yet further:

    (defvar-local prog-effective-mode-function (lambda () major-mode)
      "Function to get the effective mode at point for embedded code chunks.
    Called with no argument, returns the effective major mode at point.
    This function should return a symbol naming a major mode, e.g. `css-mode'.")
    
    (defun prog-effective-mode ()
      "Return the effective mode at point.
    There are languages where part of the code is actually written in
    a sub language, e.g., a Yacc/Bison or ANTLR grammar also consists
    of plain C code.  This function returns the effective value of
    `major-mode' at point."
       (funcall 'prog-effective-mode-function))

at which point we may as well drop `prog-effective-mode`.


        Stefan