Merge pull request #84 from mlang/alternativeRWST

Add missing Plus and Alternative instances for RWST

Author: garyb

src/Control/Monad/RWS/Trans.purs

@@ -9,13 +9,15 @@ module Control.Monad.RWS.Trans
 import Prelude
 
 import Control.Alt (class Alt, (<|>))
+import Control.Alternative (class Alternative)
 import Control.Monad.Eff.Class (class MonadEff, liftEff)
 import Control.Monad.Error.Class (class MonadError, throwError, catchError)
 import Control.Monad.Reader.Class (class MonadAsk, class MonadReader)
 import Control.Monad.Rec.Class (class MonadRec, tailRecM, Step(..))
 import Control.Monad.State.Class (class MonadState)
 import Control.Monad.Trans.Class (class MonadTrans, lift)
 import Control.Monad.Writer.Class (class MonadWriter, class MonadTell)
+import Control.Plus (class Plus, empty)
 
 import Data.Monoid (class Monoid, mempty)
 import Data.Newtype (class Newtype)
@@ -62,6 +64,8 @@ instance applyRWST :: (Bind m, Monoid w) => Apply (RWST r w s m) where
 instance altRWST :: Alt m => Alt (RWST r w s m) where
   alt (RWST m) (RWST n) = RWST $ \ r s -> m r s <|> n r s
 
+instance alternativeRWST :: (Monoid w, Alternative m, Monad m) => Alternative (RWST r w s m)
+
 instance bindRWST :: (Bind m, Monoid w) => Bind (RWST r w s m) where
   bind (RWST m) f = RWST \r s ->
     m r s >>= \(RWSResult s' a w) ->
@@ -119,3 +123,6 @@ instance monadRecRWST :: (MonadRec m, Monoid w) => MonadRec (RWST r w s m) where
           pure case result' of
             Loop x -> Loop (RWSResult state' x (writer <> writer'))
             Done y -> Done (RWSResult state' y (writer <> writer'))
+
+instance plusRWST :: Plus m => Plus (RWST r w s m) where
+  empty = RWST \ _ _ -> empty