Re: When is a syntax-propertize-function called when parse-sexp-lookup-properties is t for a current buffer?

[Pierre Rouleau <prouleau001@gmail.com>]

On Tue, Oct 5, 2021 at 8:56 AM Stefan Monnier via Users list for the GNU
Emacs text editor <help-gnu-emacs@gnu.org> wrote:

> > What I'd like to understand is what calls the generated function,
> providing
> > it the 2 arguments (start & end) and when should it be called.
>
> > Are these calls protected against error so I can't see them if they
> occur?
>
> No, although some of them can occur in contexts where the error might be
> silenced or at least not displayed in a prominent way.
>
> Why do you ask?
> Are you having an actual problem, or are you asking out of curiosity?
>
>
Both actually.    I am trying to add binary pair matching in erlang-mode,
something that works only just after the << >> pair has just been typed
by the user, but later, the matching stops working as the syntax-table
attribute seems to disappear from the character.

I'm trying to make the syntax properties to 'stick' so I can use
forward-sexp, backward-sexp to navigate over Erlang code
code that uses these.  Like the following Erlang code:

Values = << <<(X+1)/integer>> || <<X>> <= <<3,7,5,4,7>> >>.

I tried to add the following after the syntax table is built by the
erlang mode function:

  (defconst erlang-mode-syntax-propertize-function
    (syntax-propertize-rules
     ("\\(<\\)<" (1 "(>"))
     (">\\(>\\)" (2 ")<")))
    "Syntax properties to activate << >> pairing.")
  (setq-local parse-sexp-lookup-properties t)
  (setq-local syntax-propertize-function
              erlang-mode-syntax-propertize-function)

But that does not seem to work.  So I am trying to understand why
it does not work.

I saw the caution note at the end of the syntax-propertize-function in
section 34.5 Syntax Properties that states:
"Caution: When this variable is non-nil, Emacs removes syntax-table
text properties arbitrarily and relies on syntax-propertize-function to
reapply them. Thus if this facility is used at all, the function must
apply all syntax-table text properties used by the major mode."

Since the erlang-mode modifies the text attribute of the << >> characters
when the user types them I was trying to see what I'm either not doing or
doing wrong.

So I wanted to see where the code was used because it
seems that my addition is just ignored or something in erlang.el prevents
it from working and/or my 'nirvana level' understanding of that mechanism is
obviously not high enough :-)

-- 
/Pierre