Merge pull request #56 from ethul/topic/rwst-monad-error

Adding MonadError RWST instance

Author: paf31

.gitignore

@@ -4,3 +4,4 @@
 /bower_components/
 /node_modules/
 /output/
+/tmp/

docs/Control/Monad/RWS/Trans.md

@@ -37,6 +37,7 @@ instance monadReaderRWST :: (Monad m, Monoid w) => MonadReader r (RWST r w s m)
 instance monadStateRWST :: (Monad m, Monoid w) => MonadState s (RWST r w s m)
 instance monadWriterRWST :: (Monad m, Monoid w) => MonadWriter w (RWST r w s m)
 instance monadRWSRWST :: (Monad m, Monoid w) => MonadRWS r w s (RWST r w s m)
+instance monadErrorRWST :: (MonadError e m, Monoid w) => MonadError e (RWST r w s m)
 ```
 
 #### `runRWST`

src/Control/Monad/RWS/Trans.purs

@@ -14,6 +14,7 @@ import Data.Tuple
 
 import Control.Monad.Trans
 import Control.Monad.Eff.Class
+import Control.Monad.Error.Class
 import Control.Monad.Reader.Class
 import Control.Monad.Writer.Class
 import Control.Monad.State.Class
@@ -91,3 +92,7 @@ instance monadWriterRWST :: (Monad m, Monoid w) => MonadWriter w (RWST r w s m)
   pass m = RWST \r s -> runRWST m r s >>= \{ result: Tuple a f, state: s', log: w} -> pure { state: s', result: a, log: f w }
 
 instance monadRWSRWST :: (Monad m, Monoid w) => MonadRWS r w s (RWST r w s m)
+
+instance monadErrorRWST :: (MonadError e m, Monoid w) => MonadError e (RWST r w s m) where
+  throwError e = lift (throwError e)
+  catchError m h = RWST $ \r s -> catchError (runRWST m r s) (\e -> runRWST (h e) r s)