use as an example effect and add it in test_readme

README.md

@@ -361,7 +361,7 @@ from requests.exceptions import ConnectionError
 status_codes_ignoring_resolution_errors: Stream[int] = (
     Stream(["https://github.com", "https://foo.bar", "https://github.com/foo/bar"])
     .map(requests.get, concurrency=2)
-    .catch(ConnectionError, when=lambda exception: "Max retries exceeded with url" in str(exception))
+    .catch(ConnectionError, when=lambda error: "Max retries exceeded with url" in str(error))
     .map(lambda response: response.status_code)
 )
 
@@ -371,21 +371,23 @@ assert list(status_codes_ignoring_resolution_errors) == [200, 404]
 > It has an optional `finally_raise: bool` parameter to raise the first catched exception when iteration ends.
 
 > [!TIP]
-> You can add side effects for exception handling by passing a `when` function that incorporates the side effect and always returns `True`:
+> To apply side effects when catching an exception, integrate them into `when`:
 
 ```python
-def side_effect(e):
-    print("Uh-oh!")
+errors: List[Exception] = []
+
+def store_error(error: Exception) -> bool:
+    errors.append(error)
     return True
 
-even_numbers: Stream[float] = (
-    Stream(["1", 1, 2, "3", 5]
-    .map(lambda n: n % 2 == 0)
-    .catch(TypeError, when=side_effect)
-)"
+integers_in_string: Stream[int] = (
+    Stream("012345foo6789")
+    .map(int)
+    .catch(ValueError, when=store_error)
+)
 
-# prints Uh-oh twice
-even_number()
+assert list(integers_in_string) == [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+assert len(errors) == len("foo")
 ```
 
 ## `.truncate`

tests/test_readme.py

@@ -199,6 +199,21 @@ def test_catch_example(self) -> None:
 
         assert list(status_codes_ignoring_resolution_errors) == [200, 404]
 
+        errors: List[Exception] = []
+
+        def store_error(error: Exception) -> bool:
+            errors.append(error)
+            return True
+
+        integers_in_string: Stream[int] = (
+            Stream("012345foo6789")
+            .map(int)
+            .catch(ValueError, when=store_error)
+        )
+
+        assert list(integers_in_string) == [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        assert len(errors) == len("foo")
+
     def test_truncate_example(self) -> None:
         five_first_integers: Stream[int] = integers.truncate(5)