bug#74963: Ambiguous treesit named and anonymous nodes in ruby-ts-mode

[Juri Linkov <juri@linkov.net>]

[This is a separate bug report from bug#73404]

>> While testing treesit-forward-sexp-list, I discovered that
>> thing-navigation functions are not restricted to named nodes.
>> 
>> I wonder if there a reason to find anonymous nodes as things?
>
> We should rather ask is there any reason to not find anonymous nodes
> as things? Even ruby-ts-mode defines a bunch of anonymous nodes as
> sexp, no? In any case, excluding anonymous nodes from things doesn’t
> sound right.

Indeed, there are many anonymous nodes used in ruby-ts-mode.

>> The problem was found with the node "unless" in Ruby:
>> 
>>  unless cond
>>    a += 1
>>  else
>>    b -= 1
>>  end
>> 
>> Here the named node 'unless' has exactly the same name
>> as the anonymous node with the text "unless":
>> 
>>  (unless "unless" condition: (identifier)
>
> I feel like Ruby’s grammar should call the named node something else,
> like unless_statement.

Agreed, the problem is that nodes defined in Ruby’s grammar
are too ambiguous.  There are more such nodes with the same name
for named and anonymous: "if", "while", "until", etc.

>> Finding anonymous nodes breaks forward-sexp when point is on "unless":
>> 
>>  un-!-less cond
>>    a += 1
>>  else
>>    b -= 1
>>  end
>> 
>> because (treesit-thing-at (point) 'sexp t) finds
>> 
>>  #<treesit-node "unless" in 156-162>
>> 
>> instead of
>> 
>>  #<treesit-node unless in 156-203>
>> 
>> Also this breaks backward-sexp and backward-up-list
>> because treesit--thing-sibling finds
>> the anonymous node "unless" as a previous sibling
>> instead of the named node 'unless' as a parent.
>> 
>> Would the right solution be to check if the found thing
>> is a named node?  With something like:
>> 
>> diff --git a/lisp/treesit.el b/lisp/treesit.el
>> index 18200acf53f..9ad879ee40c 100644
>> --- a/lisp/treesit.el
>> +++ b/lisp/treesit.el
>> @@ -2711,6 +2774,7 @@ treesit--thing-sibling
>>                      (lambda (n) (>= (treesit-node-start n) pos))))
>>          (iter-pred (lambda (node)
>>                       (and (treesit-node-match-p node thing t)
>> +                           (treesit-node-check node 'named)
>>                            (funcall pos-pred node))))
>>          (sibling nil))
>>     (when cursor
>> @@ -2760,6 +2824,7 @@ treesit-thing-at
>>   (let* ((cursor (treesit-node-at pos))
>>          (iter-pred (lambda (node)
>>                       (and (treesit-node-match-p node thing t)
>> +                           (treesit-node-check node 'named)
>>                            (if strict
>>                                (< (treesit-node-start node) pos)
>>                              (<= (treesit-node-start node) pos))
>
> A better solution IMO is to add some way to distinguish between named and
> anonymous nodes. I can think of two ways, either add “and” and
> “named/anonymous” predicate, so (and named “unless”) only matches the named
> “unless” node; or we add a special syntax such that “(unless)” only matches
> named nodes, and “\”unless\”” only matches anonymous nodes.

Either predicate or a special syntax is welcome.

This would be more handy than writing a lambda with implicit calls
of treesit-node-check.