Add a new tree-sitter query predicate 'pred'

I realized that using an arbitrary function as the predicate in
queries is very helpful for some queries I'm writing for python and
javascript, and presumably most other languages[1].

Granted, we can already filter out unwanted nodes by using a function
instead of a face for the capture name, and (1) determine whether the
captured node is valid and (2) fontify that node if it's valid.

However, such approach is a bit more cumbersome and more importantly
gets in the way of another potential use of the fontification queries:
context extraction.

For example, I could use the query for the 'variable' feature to get
all the variables in a certain region.  In this use-case, we want the
filtering happen before returning the captured nodes.

Besides, the change is relatively small and straightforward: most code
are already there, I just need to add some boilerplate.

[1] For a code like aa.bb(cc), we want bb to be in function face,
because obviously its a function.  But for aa.bb, we want bb to be in
property face, because it's a property.  In the AST, bb is always a
property, the difference between the two cases is the enclosing node:
in the first case, aa.bb is in a "call_expression" node, indicating
that bb is used as a function (a method).  So we want a predicate
function that checks whether bb is used as a function or a property,
and determine whether it should be in function or property face.

* doc/lispref/parsing.texi (Pattern Matching): Update manual.
* src/treesit.c (Ftreesit_pattern_expand): Handle :pred.
(treesit_predicate_capture_name_to_node): A new function extracted
from treesit_predicate_capture_name_to_text.
(treesit_predicate_capture_name_to_text): Use the newly extracted
function.
(treesit_predicate_pred): New predicate function.
(treesit_eval_predicates): Add new predicate.  Also fix a bug: we want
to AND the results of each predicate.
* test/src/treesit-tests.el (treesit--ert-pred-last-sibling): New
helper function.
(treesit-query-api): Test #pred predicate.

1 files changed, 45 insertions, 12 deletions

diff --git a/src/treesit.c b/src/treesit.c
index ecc977745a6..813d4222f98 100644
--- a/src/treesit.c
+++ b/src/treesit.c
@@ -2170,6 +2170,8 @@ See Info node `(elisp)Pattern Matching' for detailed explanation.  */)
     return build_pure_c_string ("#equal");
   if (EQ (pattern, QCmatch))
     return build_pure_c_string ("#match");
+  if (EQ (pattern, QCpred))
+    return build_pure_c_string ("#pred");
   Lisp_Object opening_delimeter
     = build_pure_c_string (VECTORP (pattern) ? "[" : "(");
   Lisp_Object closing_delimiter
@@ -2269,10 +2271,10 @@ treesit_predicates_for_pattern (TSQuery *query, uint32_t pattern_index)
   return Fnreverse (result);
 }
 
-/* Translate a capture NAME (symbol) to the text of the captured node.
+/* Translate a capture NAME (symbol) to a node.
    Signals treesit-query-error if such node is not captured.  */
 static Lisp_Object
-treesit_predicate_capture_name_to_text (Lisp_Object name,
+treesit_predicate_capture_name_to_node (Lisp_Object name,
                                         struct capture_range captures)
 {
   Lisp_Object node = Qnil;
@@ -2292,6 +2294,16 @@ treesit_predicate_capture_name_to_text (Lisp_Object name,
               name, build_pure_c_string ("A predicate can only refer"
                                          " to captured nodes in the "
                                          "same pattern"));
+  return node;
+}
+
+/* Translate a capture NAME (symbol) to the text of the captured node.
+   Signals treesit-query-error if such node is not captured.  */
+static Lisp_Object
+treesit_predicate_capture_name_to_text (Lisp_Object name,
+                                        struct capture_range captures)
+{
+  Lisp_Object node = treesit_predicate_capture_name_to_node (name, captures);
 
   struct buffer *old_buffer = current_buffer;
   set_buffer_internal (XBUFFER (XTS_PARSER (XTS_NODE (node)->parser)->buffer));
@@ -2365,13 +2377,30 @@ treesit_predicate_match (Lisp_Object args, struct capture_range captures)
     return false;
 }
 
-/* About predicates: I decide to hard-code predicates in C instead of
-   implementing an extensible system where predicates are translated
-   to Lisp functions, and new predicates can be added by extending a
-   list of functions, because I really couldn't imagine any useful
-   predicates besides equal and match.  If we later found out that
-   such system is indeed useful and necessary, it can be easily
-   added.  */
+/* Handles predicate (#pred FN ARG...).  Return true if FN returns
+   non-nil; return false otherwise.  The arity of FN must match the
+   number of ARGs  */
+static bool
+treesit_predicate_pred (Lisp_Object args, struct capture_range captures)
+{
+  if (XFIXNUM (Flength (args)) < 2)
+    xsignal2 (Qtreesit_query_error,
+              build_pure_c_string ("Predicate `pred' requires "
+                                   "at least two arguments, "
+                                   "but was only given"),
+              Flength (args));
+
+  Lisp_Object fn = Fintern (XCAR (args), Qnil);
+  Lisp_Object nodes = Qnil;
+  Lisp_Object tail = XCDR (args);
+  FOR_EACH_TAIL (tail)
+    nodes = Fcons (treesit_predicate_capture_name_to_node (XCAR (tail),
+                                                           captures),
+                   nodes);
+  nodes = Fnreverse (nodes);
+
+  return !NILP (CALLN (Fapply, fn, nodes));
+}
 
 /* If all predicates in PREDICATES passes, return true; otherwise
    return false.  */
@@ -2387,14 +2416,17 @@ treesit_eval_predicates (struct capture_range captures, Lisp_Object predicates)
       Lisp_Object fn = XCAR (predicate);
       Lisp_Object args = XCDR (predicate);
       if (!NILP (Fstring_equal (fn, build_pure_c_string ("equal"))))
-        pass = treesit_predicate_equal (args, captures);
+        pass &= treesit_predicate_equal (args, captures);
       else if (!NILP (Fstring_equal (fn, build_pure_c_string ("match"))))
-        pass = treesit_predicate_match (args, captures);
+        pass &= treesit_predicate_match (args, captures);
+      else if (!NILP (Fstring_equal (fn, build_pure_c_string ("pred"))))
+        pass &= treesit_predicate_pred (args, captures);
       else
         xsignal3 (Qtreesit_query_error,
                   build_pure_c_string ("Invalid predicate"),
                   fn, build_pure_c_string ("Currently Emacs only supports"
-                                           " equal and match predicate"));
+                                           " equal, match, and pred"
+                                           " predicate"));
     }
   /* If all predicates passed, add captures to result list.  */
   return pass;
@@ -3217,6 +3249,7 @@ syms_of_treesit (void)
   DEFSYM (QCanchor, ":anchor");
   DEFSYM (QCequal, ":equal");
   DEFSYM (QCmatch, ":match");
+  DEFSYM (QCpred, ":pred");
 
   DEFSYM (Qnot_found, "not-found");
   DEFSYM (Qsymbol_error, "symbol-error");