Re: Recent updates to tree-sitter branch

[Ihor Radchenko]

Yuan Fu <casouri@gmail.com> writes:

>> treesit-font-lock-rules rules take a form of
>> (MATCHER FACENAME) or (MATCHER FUNCTION)
>> 
>> where MATCHER can only be a query.
>> 
>> Is there any reason why MATCHER in treesit-font-lock-rules cannot be a
>> function with access to the fontified node?
>
> Hmm, I’m not sure what do you mean. The whole thing passed to 
> treesit-font-lock-rules is a single query, and we can’t really change the 
> query syntax, that’s defined by tree-sitter. Basically in a query you have 
> patterns paired with capture names, if the pattern matches to a node, that 
> node is returned with corresponding capture name tagged on it. For font-lock, 
> we just use face names as capture names, and when a query returns captured 
> nodes, fontify the node with its capture name, aka a face (or a function).

What I am asking is an extra dynamic condition in addition to the query.
For example:
1. Only apply FACENAME for nodes matching QUERY, but only when Elisp
   variable is non-nil

2. Only apply FACENAME for nodes matching QUERY, which are in the second
   half of the buffer

3. Only apply FACENAME for notes matching QUERY, which also have a field
   matching a dynamically assigned regexp.

Essentially any condition that is not covered by the QUERY, but can be
checked in Elisp given that node object is passed to the test function.

>> Further, can OVERRIDE FLAG of the MATCH-HIGHLIGHT as in
>> font-lock-keywords be supported?
>> 
>>   "If OVERRIDE is t, existing fontification can be overwritten. If
>>    keep, only parts not already fontified are highlighted. If prepend or
>>    append, existing fontification is merged with the new, in which the new
>>    or existing fontification, respectively, takes precedence.”
>
> I can do that, but would it be really useful? Unlike regex font-lock which is 
> used for so many different things, tree-sitter font-lock is, IMO, only used 
> to apply a base layer of language-specific highlight. How would one use the 
> override feature in this scenario?

For example, consider a function definition with docstring field.
Imagine that you want the function definition to have gray background,
but the docstring to have yellow background. OVERRIDE t is how this is
usually implemented in font-lock-keywords.

-- 
Ihor Radchenko,
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