Re: Low level trickery for changing character syntax?

[Andreas Röhler]

Am 08.04.2014 19:00, schrieb Thorsten Jolitz:
> Hi List,
>
> assume an imaginary elisp library gro.el I cannot (or don't want to)
> change that is used on files of type A, with functions matching these
> kinds of strings:
>
> #+begin_src emacs-lisp
>    (defconst rgxp-1 "^[*] [*]Fat[*]$")
>
>    (defun foo (strg)
>      (and (string-match "^\\*+[ \t]* \\*.+\\*" strg)
>           (string-match rgxp-1 strg)))
> #+end_src
>
> #+results:
> : foo
>
> #+begin_src emacs-lisp
> (foo "* *Fat*")
> #+end_src
>
> #+results:
> : 0
>
> #+begin_src emacs-lisp
> (foo "+ *Fat*")
> #+end_src
>
> #+results:
>
> Now assume I want to use gro.el functionality on files of type B
> such that it matches strings likes this:
>
> #+begin_src emacs-lisp
> (foo "// # *Fat*//" )
> #+end_src
>
> In short, when called from file.type-A, I want foo to match "// #
> *Fat*//", while it should only match "* *Fat*" when called from
> file.type-B (without changing foo or rgxp-1).
>
> Thus in rgxp-1 and in foo, "^" would need to be replaced with "^// ",
> the first "*" would need to be replaced with "#" (the other occurences
> not), and "$" would need to be replaced with "//$".
>
> Now I wonder what would be the best way (or at least a possible way) to
> achieve this with Emacs low-level trickery (almost) without touching
> gro.el. I don't enough know about syntax table low-level stuff besides
> reading the manual, so these are only vague speculations:
>
>   1. Change the syntax-table of gro.el whenever it is applied to files of
>   type B such that "^" is seen as "^// ", "*" as "#" etc.?
>
>   2. Define new categories and put "^" "*" and "$" in them, and somehow
>   load/activate these categories conditional on the type of file gro.el
>   functionality is called upon. These categories should then achieve that
>   "^" is seen as "^// " etc when the categories are loaded?
>
>   3. Define "^" and "$", when found at beg/end of a string, as 'generic
>   comment delimiter, and define "/" as generic comment delimiter too, such
>   that "^//" and "//$" are matched by "^" and "$"?
>
> I know that these ideas do not and cannot work as described, but I'm
> looking for a hint which idea could possibly work? What would be the way
> to go?
>
> Or is this completely unrealistic and the only way to achieve it is to
> change the hardcoded regexps in (imaginary) library gro.el?

You could define different syntax-tables and than call functions

if type-A
(with-syntax-table type-A ...