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Re: [ELPA] New package: bnf-mode

From: Stefan Monnier
Subject: Re: [ELPA] New package: bnf-mode
Date: Sat, 04 May 2019 14:35:21 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

> I created a major mode for editing BNF grammars [1] and would like to
> add this package to the ELPA [2].  Currently it provides basic syntax
> and font-locking for BNF files.  BNF notation is supported exactly
> form as it was first announced in the ALGOL 60 report.  EBNF and ABNF
> are not supported but in my plans for the near future.

In what kind of circumstances would this be used?
I've seen BNF-like syntax used by various tools (like bison and
friends, for example), but never with this particular syntax.

    ;; Basically `syntax-propertize-function' is a construct which belongs
    ;; to `font-lock'.

No.  It's used by font-lock (as well as various other things), but it
does not belong to it.

    ;; But correct indentation depends on
    ;; syntax properties of the text, and that should ideally be
    ;; independent of font-lock being activated or not.

Which is why syntax-propertize was created (as opposed to the previous
font-lock-syntactic-keywords, which *did* belong to font-lock and
suffered from the problem you describe).

    ;; For `bnf-mode', this means that with `font-lock' disabled, we wont
    ;; have our syntax properties set correctly, and indentation will
    ;; suffer.

That's not true.

    ;; To patch our way around this, we issue a `syntax-propertize' call
    ;; manually, `font-lock' enabled or not.
      (when bnf-mode-algol-comments-style
        (funcall syntax-propertize-function (point-min) (point-max))))
Don't do that: syntax-propertize-function will be called when needed
(e.g. by indent-according-to-mode).


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