Describe #145 and Callback in CHANGELOG

Author: japgolly

doc/CHANGELOG-0.10.md

@@ -8,9 +8,104 @@
   please copy Router v1 into your own codebase.
   Maintaining two Routers in scalajs-react is not good.
 
+* [#145](https://github.com/japgolly/scalajs-react/issues/145): Unify `-->` `==>` `~~>`, remove Scalaz `IO`, add `Callback`.
+
+  **Problem:**<br>
+  A gap in the community and library was growing because there were two separate ways of writing callbacks.
+  An example of which, numerous users using the built-in `Router` experienced confusion and were forced to
+  discover Scalaz's (and Haskell's) `unsafePerformIO()` without understanding the reasons or benefit.
+  Another example, writing mixins or shared libraries, one would have to create duplicate methods with differing
+  type signatures in order to accomodate both styles.
+  More examples exist, but it should be clear that this please-everyone approach has failed.
+
+  **Solution:**<br>
+  A new data type `Callback` has been added
+  (which is actually an alias to the real type `CallbackTo[A]` where `Callback = CallbackTo[Unit]`).
+  It replaces the existing two competing methods of `() => Unit` and Scalaz `IO[Unit]`.
+  A basic rule of thumb is if a function performs an effect (changing DOM, using network, changing React state),
+  it should be wrapped in a `Callback`.
+  
+  * When creating HTML, `-->` and `==>` only accept `Callback`s. 
+  * `setState()`, `modState()` etc now return `Callback`s.
+  * Core now has  `_setState()`, `_modState()`.
+  * Component lifecycle methods (like `componentWillMount`) now accept `Callback`.
+  * `Router` no longer uses Scalaz `IO`.
+  * Mixins in `extra` no longer have duplicate methods for Scalaz `IO`.
+
+  For Scalaz users:<br>
+  Either use `CallbackTo` instead of `IO`, or else change your `IO`s to `CallbackTo`s when you pass them to React.
+  * `~~>` is removed. Use `-->` and `==>` instead.
+  * `{,_}{set,mod}StateIO` methods all removed and in core without the `IO`.
+  * Isomorphism between `IO` and `CallbackTo` with convenience extension methods `.toCallback` and `.toIO`.
+  * `OpCallbackIO` has been removed in favour of plain `Callback`.
+  * New state-monad type-inference convenience: `FixCB[S] = FixT[CallbackTo, S]`
+
+  **Example migrations**<br>
+  
+  Example #1
+  ```scala
+  // Before
+  val Example = ReactComponentB[Unit]("Example")
+    .initialState(0)
+    .render { $ =>
+
+      def clickHandler = $.modState(_ + 1)
+      <.div(
+        <.div("Button pressed ", $.state, " times."),
+        <.button("CLICK ME", ^.onClick --> clickHandler))
+
+    }.buildU
+
+  // After
+  // Surprise! No change needed.
+  // modState() now returns a Callback which is exactly what --> expects.
+  ```
+
+  Example #2<br>
+  ```scala
+  // Before (excerpt from example above)
+  def clickHandler: Unit = {
+    println("Updating state.")
+    $.modState(_ + 1)
+  }
+  
+  // Callbacks can compose in many different ways.
+  // The following examples all do the same thing in the same order.
+
+  // After, Method #1: precedeWith
+  def clickHandler: Callback =
+    $.modState(_ + 1).precedeWith {
+      println("Updating state.")
+    }
+  
+  // After, Method #2: >>
+  def clickHandler: Callback =
+    Callback(println("Updating state.")) >>
+    $.modState(_ + 1)
+
+  // After, Method #3: <<
+  def clickHandler: Callback =
+    $.modState(_ + 1) << Callback(println("Updating state."))
+
+  // After, Method #4: runNow()
+  def clickHandler = Callback {
+    println("Updating state.")
+    $.modState(_ + 1).runNow()
+  }
+  ```
+
+
 <br>
 Migration commands:
 ```sh
 # extra.{router2 ⇒ router}
 find . -name '*.scala' -type f -exec perl -pi -e 's/(?<=extra\.router)2//g' {} +
+
+# Unfortunately the migration to use Callback is mostly manual.
+# Here are some commands I used to help but they'll only get you halfway there.
+find . -name '*.scala' -type f -exec perl -pi -e 's/(?<=forceUpdate)\(\)//g' {} +
+find . -name '*.scala' -type f -exec perl -pi -e 's/(?<=tryFocus)\(\)//g' {} +
+find . -name '*.scala' -type f -exec perl -pi -e 's/(?<=(set|mod)State)IO//g' {} +
+find . -name '*.scala' -type f -exec perl -pi -e 's/FixT\[IO *, */FixCB[/' {} +
+find . -name '*.scala' -type f -exec perl -pi -e 's/ReactST\[IO *,/ReactST[CallbackTo,/' {} +
 ```