effect docs

README.md

@@ -26,6 +26,7 @@ Computed:
 
 - [Here is how it works](#here-is-how-it-works)
 - [A larger example](#a-larger-example)
+- [Effects](#effects)
 - [Testing](#testing)
 - [Looking at the past](#looking-at-the-past)
 - [Computed queries](#computed-queries)
@@ -65,10 +66,7 @@ Assume for the sake of example that your business logic is to multiply the recei
 Here is how you can do this using Computed:
 
 ```
-final c = $(() => s.use * 2);
-final sub = c.asStream.listen((res){
-    db.write(res);
-});
+$(() => s.use * 2).asStream.listen(db.write);
 ```
 
 That's it. Computed will take care of re-running the computation and calling the listener as needed. Note how you did not need to specify a dependency list for the computation, Computed discovers it automatically. You don't even have any mutable state in your app code.
@@ -141,6 +139,18 @@ $(() {
 }).asStream.listen(db.write);
 ```
 
+## <a name='effects'></a>Effects
+
+Effects allow you to define computations with side effects. Like `.listen` and getters which convert computations to data sources (such as `.asStream`), effects ultimately trigger the computation graph.  
+Effects are particularly useful if you wish to define side effects depending on multiple data sources or computations:
+
+```
+Stream<PageType> activePage;
+Stream<bool> isDarkMode;
+
+Computed.effect(() => sendAnalytics(activePage.use, isDarkMode.use));
+```
+
 ## <a name='testing'></a>Testing
 
 Computed lets you mock any computation or data source, allowing you to easily test downstream behaviour.

example/example.dart

@@ -17,7 +17,7 @@ void main() async {
     return maybeReversed.use.rebuild((p0) => p0.add(0));
   });
 
-  append0.listen((value) => print(value), () => print('Exception!'));
+  Computed.effect(() => print(append0.use));
 
   // ignore: unused_local_variable
   final unused = $(() {

example/temporal_accumulator.dart

@@ -5,14 +5,13 @@ import 'package:computed/computed.dart';
 void main() async {
   final cont = StreamController<int>.broadcast(sync: true);
   final s = cont.stream;
-  Computed<int>.withPrev((prev) {
+  final c = Computed<int>.withPrev((prev) {
     var res = prev;
     s.react((val) => res += val);
     return res;
-  }, initialPrev: 0)
-      .listen((event) => print(event), (e) {
-    print('Exception: $e');
-  });
+  }, initialPrev: 0);
+
+  Computed.effect(() => print(c.use));
 
   await Future.value();
   cont.add(1);

example/temporal_difference.dart

@@ -5,15 +5,14 @@ import 'package:computed/computed.dart';
 void main() {
   final cont = StreamController<int>.broadcast(sync: true);
   final s = cont.stream;
-  $(() {
+  final c = $(() {
     s.use; // Make sure it has a value
     late int res;
     s.react((val) => res = val - s.prevOr(0));
     return res;
-  }, memoized: false)
-      .listen((event) => print(event), (e) {
-    print('Exception: $e');
-  });
+  }, memoized: false);
+
+  Computed.effect(() => print(c.use));
 
   cont.add(1);
   cont.add(1);

example/todo.dart

@@ -81,10 +81,10 @@ void main() async {
       (prev) => Tuple2(prev == null, appState.use),
       initialPrev: null);
 
-  stateIsInitial.listen((state) {
+  Computed.effect(() {
     // Do not save the initial state
-    if (!state!.item1) db.saveState(state.item2);
-  }, (e) => print('Exception: ${e.toString()}'));
+    if (!stateIsInitial.use!.item1) db.saveState(stateIsInitial.use!.item2);
+  });
 
   uiCreateController.add('new todo');
   await Future.delayed(Duration.zero);