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Re: tree-sitter: conceptional problem solvable at Emacs' level?

From: Po Lu
Subject: Re: tree-sitter: conceptional problem solvable at Emacs' level?
Date: Thu, 09 Feb 2023 16:17:27 +0800
User-agent: Gnus/5.13 (Gnus v5.13)

Holger Schurig <holgerschurig@gmail.com> writes:

> Hi, I run branch emacs-29 since some time with great success. And now I
> wanted to test out tree-sitter and c++-test-mode. Unfortunately, I
> stumbled into some conceptional problems and wonder if this is actually
> solvable by Emacs, or if some would need a completely new grammar.
> The issue is: tree-sitter doesn't work well with C macros.
> I program a lot in C++/Qt. So let's look at this (valid) C++ program:
> -----------------------------------------------------------------------------
> #include <QObject>
> class Test : public QObject
> {
>         Q_OBJECT
> public:
>         Test() : QObject() {};
> public slots:
>         void someSlot() {};
> };
> -----------------------------------------------------------------------------
> If have the libraries installed (e.g. qtbase5-dev on Debian), you can
> compile this perfectly.
> However, tree-sitter produces a garbage syntax tree:
> - contain some bitfield node (which isn't really there)
> - contains an error node (despite the code being compilable)
> And as a result, BOTH the indentation and the font-locking is wrong.
> Would I need to create a tree-sitter grammar in JavaScript that
> understands this macro-enhanced C++?   That would be quite difficult.
> Or will there be a method to add some kind of tiny-preprocessor to
> c++-ts-mode, so that it can substitute "Q_OBJECT", "signals" and "slots"
> with nothing before handing things over to tree-sitter?
> In comparison, I could teach the old cc-mode about this macro-enriched
> C++ just with
>   (c-add-style "qt-gnu"
>                '("gnu" (c-access-key .
>                        "\\<\\(signals\\|public\\|protected\\|private\\|public
>                slots\\|protected slots\\|private slots\\):")))
> I guess that a lot of C and C++ programs use macros. And if there is no
> simple way to aid tree-sitter in understanding this, then I fear
> tree-sitter enhanced modes will often be unusable on them.

My suggestion is simply to stay with CC Mode.

Parsers (without a full C preprocessor inside) can only work for
languages like Python, which cannot be enhanced with syntax-modifying

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