Fix transformState to use current state

According to React's docs:

  setState() does not immediately mutate this.state but creates a
  pending state transition. Accessing this.state after calling this
  method can potentially return the existing value.

To make sure that transformState gets the latest value, we use
setState's callback mode.

transformState now returns Unit instead of the state type.

Author: spencerjanssen

src/React.js

@@ -53,6 +53,17 @@ function readState(this_) {
 }
 exports.readState = readState;
 
+function transformState(this_){
+  return function(update){
+    return function(){
+      this_.setState(function(old, props){
+        return {state: update(old.state)};
+      });
+    };
+  };
+}
+exports.transformState = transformState;
+
 function createClass(spec) {
   var result = {
     displayName: spec.displayName,

src/React.purs

@@ -284,10 +284,7 @@ foreign import writeState :: forall props state access eff. ReactThis props stat
 foreign import readState :: forall props state access eff. ReactThis props state -> Eff (state :: ReactState (read :: Read | access) | eff) state
 
 -- | Transform the component state by applying a function.
-transformState :: forall props state eff. ReactThis props state -> (state -> state) -> Eff (state :: ReactState ReadWrite | eff) state
-transformState ctx f = do
-  state <- readState ctx
-  writeState ctx $ f state
+foreign import transformState :: forall props state eff. ReactThis props state -> (state -> state) -> Eff (state :: ReactState ReadWrite | eff) Unit
 
 -- | Create a React class from a specification.
 foreign import createClass :: forall props state eff. ReactSpec props state eff -> ReactClass props