Add watch_effect

observ/__init__.py

@@ -9,4 +9,4 @@
     to_raw,
 )
 from .scheduler import scheduler
-from .watcher import computed, watch
+from .watcher import computed, watch, watch_effect

observ/watcher.py

@@ -6,7 +6,7 @@
 from __future__ import annotations
 
 from collections.abc import Container
-from functools import wraps
+from functools import partial, wraps
 import inspect
 from itertools import count
 from typing import Any, Callable, Optional, TypeVar
@@ -25,7 +25,7 @@
 
 def watch(
     fn: Callable[[], Any] | Proxy | list[Proxy],
-    callback: Optional[Callable],
+    callback: Optional[Callable] = None,
     sync: bool = False,
     deep: bool | None = None,
     immediate: bool = False,
@@ -39,6 +39,9 @@ def watch(
     return watcher
 
 
+watch_effect = partial(watch, immediate=True, deep=True, callback=None)
+
+
 def computed(_fn=None, *, deep=True):
     def decorator_computed(fn: T) -> T:
         """

tests/test_watch_effect.py

@@ -0,0 +1,22 @@
+from observ import reactive
+from observ.watcher import watch_effect
+
+
+def test_watch_effect():
+    state = reactive({"count": 0})
+    count_mirror = -1
+
+    def bump():
+        nonlocal count_mirror
+        count_mirror = state["count"]
+
+    watcher = watch_effect(bump, sync=True)
+    assert watcher.lazy is False
+
+    assert state["count"] == 0
+    assert count_mirror == 0
+
+    state["count"] = 1
+
+    assert state["count"] == 1
+    assert count_mirror == 1