Feature: Allow update to take a reducer.

A reducer is a function that takes the state and returns a new state
or a reducer.

  update(state => {
    return { value: state.value + 1 }
  }

This allows you to get the current & latest state inside the
callback of some async call.

The alternative is using the state that was passed to the
action that originated the async call. The problem is that
this reference to the state may have gone out-of-date if
other actions run and finished during the async process
and changed the state.

src/app.js

@@ -36,7 +36,10 @@ export function app(props) {
       if (typeof action === "function") {
         actions[key] = function(data) {
           emit("action", { name: name, data: data })
-          return update(emit("resolve", action(appState, appActions, data)))
+
+          var result = emit("resolve", action(appState, appActions, data))
+
+          return typeof result === "function" ? result(update) : update(result)
         }
       } else {
         initialize(actions[key] || (actions[key] = {}), action, name)
@@ -63,7 +66,7 @@ export function app(props) {
 
   function update(withState) {
     if (typeof withState === "function") {
-      return withState(update)
+      return update(withState(appState))
     }
     if (withState && (withState = emit("update", merge(appState, withState)))) {
       requestRender((appState = withState))