Merge pull request #55 from ethul/topic/readert-monad-rec

MonadRec instance for ReaderT

Author: paf31

docs/Control/Monad/Reader/Trans.md

@@ -35,6 +35,7 @@ instance monadReaderReaderT :: (Monad m) => MonadReader r (ReaderT r m)
 instance monadStateReaderT :: (MonadState s m) => MonadState s (ReaderT r m)
 instance monadWriterReaderT :: (Monad m, MonadWriter w m) => MonadWriter w (ReaderT r m)
 instance distributiveReaderT :: (Distributive g) => Distributive (ReaderT e g)
+instance monadRecReaderT :: (MonadRec m) => MonadRec (ReaderT r m)
 ```
 
 #### `runReaderT`

src/Control/Monad/Reader/Trans.purs

@@ -16,12 +16,15 @@ import Control.Monad.Eff.Class
 import Control.Monad.Cont.Class
 import Control.Monad.Error.Class
 import Control.Monad.Reader.Class
+import Control.Monad.Rec.Class
 import Control.Monad.State.Class
 import Control.Monad.Writer.Class
 import Control.Monad.Trans
 import Control.MonadPlus
 import Control.Plus
 
+import Data.Either
+
 -- | The reader monad transformer.
 -- |
 -- | This monad transformer extends the base monad transformer with a _global context_ of
@@ -96,3 +99,10 @@ instance monadWriterReaderT :: (Monad m, MonadWriter w m) => MonadWriter w (Read
 instance distributiveReaderT :: (Distributive g) => Distributive (ReaderT e g) where
   distribute a = ReaderT \e -> collect (flip runReaderT e) a
   collect f = distribute <<< map f
+
+instance monadRecReaderT :: (MonadRec m) => MonadRec (ReaderT r m) where
+  tailRecM k a = ReaderT \r -> tailRecM (k' r) a
+    where
+    k' r a = do
+      result <- runReaderT (k a) r
+      return $ either Left Right result