bug#19080: 24.4; wish: enable show-paren-mode to highlight matching same-level keywords

[Eli Zaretskii]

> From: Yuan Fu <casouri@gmail.com>
> Date: Thu, 12 May 2022 15:52:14 -0700
> Cc: Lars Ingebrigtsen <larsi@gnus.org>,
>  josh@berdine.net,
>  19080@debbugs.gnu.org
> 
> > We could perhaps add a new feature, based on tree-sitter, to highlight
> > a block of code, where the block is defined by PL rules, not by
> > parens, braces, and similar simple syntactic elements.
> > 
> > Yuan, WDYT?  Can this be added to the tree-sitter branch perhaps?
> 
> That sounds interesting, and it should be easy. But what do we want the API 
> be? First of all, should we make a separate mode or extend show-paren-mode? 
> Extending show-paren-mode entails doing some non-trivial refactoring to 
> paren.el.

IMO, it should be a separate minor mode; show-paren-mode is not
necessarily about program blocks, since parentheses appear not only
(and mostly not) as block delimiters.

The API should be probably: given a buffer POSITION, return the 2
positions of the innermost enclosing block, or nil if POSITION is not
in any block.  A possible extension could be to return a list of
position pairs that provide also the blocks enclosed inside the
POSITION's block.

> I think, that adding tree-sitter “backend” needs refactoring existing code 
> non-trivially.

If show-paren-mode would want to use tree-sitter, it will need
refactoring, yes.  Which other packages you envision could benefit
from using tree-sitter?