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Re: When is a syntax-propertize-function called when parse-sexp-lookup-p

From: Pierre Rouleau
Subject: Re: When is a syntax-propertize-function called when parse-sexp-lookup-properties is t for a current buffer?
Date: Tue, 5 Oct 2021 14:45:43 -0400

Well, it never hurts to get the latest Emacs! I'm on GNU Emacs
> 29.0.50 so that's quite a difference ...

I want to make it work on all these platforms and 26.1 is a necessary target
imposed by non-technical constraints out of my control.

> But ... I don't understand 100% what it is you'd like to
> happen? Can you describe it in human terms rather? "When I do
> ... I'd like this to happen ... and not this ..." ?

No problem.
I know the following is long, but I think I need to give some context...

I’m trying to add navigation with forward-sexp and backward-sexp
inside erlang-mode buffers to support the Erlang Bit Syntax expressions

Currently, with the latest version of erlang.el, the erlang-mode activates
the ability to move to the >> matching the << just after point, but
*only* after you just write the Erlang code.

So let's say you just wrote the following Erlang statement inside an
erlang-mode buffer:

Abc = << <<Bin>> || Bin <- [<<3,7,5,4,7>>] >>

The statement is not complete, the full-stop has not yet been typed.
At this point you can use the backward-sexp command to move the point just
before the first < character on the line.  And you can also use
to come back where you were.

Let's say you just type the full-stop (period) to terminate the statement
and then hit RET to move point to the next line.  From then on, if you
the point just before the first < or after the last >, forward-sexp and
backward-sexp do not work: they do not move the point to the matching
If you save and re-open the file you also lose the matching ability.

My understanding of the reason why it stops working is because the
erlang.el syntax
table, modified by erlang.el `erlang-ensure-syntax-table-is-initialized'
does not identify
the < and > characters as parens that match another one.  The erlang.el
code does this:

(modify-syntax-entry ?< "." table)
(modify-syntax-entry ?> "." table)

It assigns punctuation syntax to these characters instead of doing the

(modify-syntax-entry ?< "(>" table)
(modify-syntax-entry ?> ")<" table)

The reason the last 2 statements are *not* used is because, > and < are
also used, like
many other programming languages as comparison operators and in erlang you
have '->', '<-' and '<=' operators.  Giving open and closing delimiter
syntax to the < and >
characters would break operations and cause a lot of unbalanced issues.

So, I am trying to syntax propertize << and >> to provide ability to
navigate across the
bit syntax expression by setting the syntax-propertize-function to activate
properties to the outer characters of << and >>.

To do that I added the following at top level, just before the
(defun erlang-ensure-syntax-table-is-initialized  ... ) form:

 (defconst erlang-mode-syntax-propertize-function
     ("\\(<\\)<" (1 "(>"))
     (">\\(>\\)" (2 ")<")))
    "Syntax properties to activate << >> pairing.")

And I added the following inside of
after the code that sets up the syntax table for Erlang :

  (setq-local parse-sexp-lookup-properties t)
  (setq-local syntax-propertize-function

After byte compilation and testing I was expecting that I'd see the
syntax-table text
property of all outer characters of << and >> bit syntax inside the erlang
buffer to be
activated allowing me to navigate across their edges with forward-sexp and

But that does not work: I still cannot navigate with forward-sexp and
and the outer << and >> characters do not have the text properties I
thought they would have.

I check inside the buffer if parse-sexp-lookup-properies is t and it is.
I also check in the buffer if syntax-propertize-function is set and it is
set to what I expect.
Yet I do not see a change of behaviour from what the non-modified erlang.el
code provides.

Therefore my question as to *when* the syntax-propertize-function should
take effect
and how I can see if it is applied.


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