Re: Low level trickery for changing character syntax?

[Thorsten Jolitz]

Andreas Röhler <andreas.roehler@easy-emacs.de> writes:

> 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?
>>
>
> Possible is a lot, the easiest IMO would be switching the regexp
>
> if A-type:
>
> ...
>
> if B-type:

The question assumes the restriction that the functions of gro.el, that
work for files of type-A, should be used with files of type-B, but I
cannot change (the hundreds of hardcoded regexp strings scattered all
over the giant file) gro.el. 

When it becomes clear that nothing can be done under this restriction, I
start thinking about how to circumvent it, but for now I'm interested in
some fancy trickery at the syntax-table level - do not touch the
hard-coded regexp strings, but give a few characters a different meaning
when called from a file of type-B.

-- 
cheers,
Thorsten