reducer replaces merge_strategy.

tests/core/test_nested_state_paths.py

@@ -4,9 +4,9 @@ from wf_core import SchemaRef, StateField, StateSchema, Workflow
 from wf_core.runtime.ops.state import write_state_value
 
 
-def test_exact_nested_state_path_uses_declared_merge_strategy() -> None:
+def test_exact_nested_state_path_uses_declared_reducer() -> None:
     workflow = _workflow(
-        fields={"person.tags": StateField(type="array", merge_strategy="append")}
+        fields={"person.tags": StateField(type="array", reducer="wf.std.append")}
     )
     state = {"person": {"tags": ["seed"]}}
 
@@ -17,7 +17,7 @@ def test_exact_nested_state_path_uses_declared_merge_strategy() -> None:
 
 def test_parent_state_declaration_does_not_apply_to_nested_write() -> None:
     workflow = _workflow(
-        fields={"person": StateField(type="object", merge_strategy="merge_object")}
+        fields={"person": StateField(type="object", reducer="wf.std.merge_object")}
     )
     state = {"person": {"tags": ["seed"]}}
 
@@ -35,6 +35,22 @@ def test_undeclared_nested_state_path_defaults_to_replace() -> None:
     assert state["person"]["tags"] == ["next"]
 
 
+def test_state_field_defaults_to_replace_reducer() -> None:
+    assert StateField(type="string").reducer == "wf.std.replace"
+
+
+def test_unknown_state_reducer_fails_clearly() -> None:
+    workflow = _workflow(fields={"person.tags": StateField(type="array", reducer="x.nope")})
+    state = {"person": {"tags": ["seed"]}}
+
+    try:
+        write_state_value(workflow, state, "state.person.tags", ["next"])
+    except Exception as exc:
+        assert "unknown reducer 'x.nope'" in str(exc)
+    else:
+        raise AssertionError("expected unknown reducer to fail")
+
+
 def _workflow(*, fields: dict[str, StateField]) -> Workflow:
     return Workflow(
         name="nested_state_paths",