Support named match spread for records

We supported `| [...xs]` but by accident not `| {...rest}`. It's a
little finicky but seems fine.

Author: tekknolagi

compiler.py

@@ -202,10 +202,13 @@ def try_match(self, env: Env, arg: str, pattern: Object, fallthrough: str) -> En
             self._emit(f"if (!is_record({arg})) {{ goto {fallthrough}; }}")
             updates = {}
             for key, pattern_value in pattern.data.items():
+                assert not isinstance(pattern_value, Spread), "record spread not yet supported"
                 key_idx = self.record_key(key)
                 record_value = self._mktemp(f"record_get({arg}, {key_idx})")
                 self._emit(f"if ({record_value} == NULL) {{ goto {fallthrough}; }}")
                 updates.update(self.try_match(env, record_value, pattern_value, fallthrough))
+            # TODO(max): Check that there are no other fields in the record,
+            # perhaps by length check
             return updates
         raise NotImplementedError("try_match", pattern)
 

compiler_tests.py

@@ -49,9 +49,16 @@ def test_match_int(self) -> None:
     def test_match_list(self) -> None:
         self.assertEqual(self._run("f [4, 5] . f = | [1, 2] -> 3 | [4, 5] -> 6"), "6\n")
 
+    def test_match_list_spread(self) -> None:
+        self.assertEqual(self._run("f [4, 5] . f = | [_, ...xs] -> xs"), "[5]\n")
+
     def test_match_record(self) -> None:
         self.assertEqual(self._run("f {a = 4, b = 5} . f = | {a = 1, b = 2} -> 3 | {a = 4, b = 5} -> 6"), "6\n")
 
+    @unittest.skip("TODO")
+    def test_match_record_spread(self) -> None:
+        self.assertEqual(self._run("f {a=1, b=2, c=3} . f = | {a=1, ...rest} -> rest"), "[5]\n")
+
     def test_match_hole(self) -> None:
         self.assertEqual(self._run("f () . f = | 1 -> 3 | () -> 4"), "4\n")
 

scrapscript.py

@@ -1114,10 +1114,16 @@ def match(obj: Object, pattern: Object) -> Optional[Env]:
             return None
         result: Env = {}
         use_spread = False
+        seen_keys: set[str] = set()
         for key, pattern_item in pattern.data.items():
             if isinstance(pattern_item, Spread):
                 use_spread = True
+                if pattern_item.name is not None:
+                    assert isinstance(result, dict)  # for .update()
+                    rest_keys = set(obj.data.keys()) - seen_keys
+                    result.update({pattern_item.name: Record({key: obj.data[key] for key in rest_keys})})
                 break
+            seen_keys.add(key)
             obj_item = obj.data.get(key)
             if obj_item is None:
                 return None
@@ -3300,6 +3306,9 @@ def test_match_record_doubly_binds_vars(self) -> None:
             Int(3),
         )
 
+    def test_match_record_spread_binds_spread(self) -> None:
+        self.assertEqual(self._run("(| { a=1, ...rest } -> rest) {a=1, b=2, c=3}"), Record({"b": Int(2), "c": Int(3)}))
+
     def test_match_list_binds_vars(self) -> None:
         self.assertEqual(
             self._run(
@@ -3364,6 +3373,9 @@ def test_match_list_doubly_binds_vars(self) -> None:
             Int(2),
         )
 
+    def test_match_list_spread_binds_spread(self) -> None:
+        self.assertEqual(self._run("(| [x, ...xs] -> xs) [1, 2]"), List([Int(2)]))
+
     def test_pipe(self) -> None:
         self.assertEqual(self._run("1 |> (a -> a + 2)"), Int(3))