Add @freebroccolo's Distributive instance

From purescript/purescript-distributive#4

Author: garyb

bower.json

@@ -27,10 +27,10 @@
     "output",
     "test",
     "bower.json",
-    "gulpfile.js",
     "package.json"
   ],
   "dependencies": {
+    "purescript-distributive": "^0.5.0",
     "purescript-lazy": "^0.4.0",
     "purescript-tailrec": "^0.3.0",
     "purescript-unfoldable": "^0.4.0"

src/Control/Monad/Reader/Trans.purs

@@ -6,10 +6,11 @@ import Prelude
 
 import Control.Alt
 import Control.Alternative
-import Control.Plus
+import Control.Monad.Eff.Class
 import Control.Monad.Trans
 import Control.MonadPlus
-import Control.Monad.Eff.Class
+import Control.Plus
+import Data.Distributive (Distributive, distribute, collect)
 
 -- | The reader monad transformer.
 -- |
@@ -63,6 +64,10 @@ instance monadTransReaderT :: MonadTrans (ReaderT r) where
 instance monadEffReader :: (Monad m, MonadEff eff m) => MonadEff eff (ReaderT r m) where
   liftEff = lift <<< liftEff
 
+instance distributiveReaderT :: (Distributive g) => Distributive (ReaderT e g) where
+  distribute a = ReaderT \e -> collect (flip runReaderT e) a
+  collect f = distribute <<< map f
+
 liftReaderT :: forall r m a. m a -> ReaderT r m a
 liftReaderT m = ReaderT (const m)