chapter 26 - EitherT monad transformer implementation

Author: mukeshsoni

chapter26/eithert_monad_transformer.hs

@@ -0,0 +1,20 @@
+newtype EitherT e m a =
+    EitherT { runEitherT :: m (Either e a) }
+
+instance Functor m => Functor (EitherT e m)  where
+    fmap f (EitherT etm) = EitherT $ (fmap . fmap) f etm
+
+instance Applicative m => Applicative (EitherT e m) where
+    pure x = EitherT $ (pure . pure) x
+    (EitherT f) <*> (EitherT g) =
+        EitherT $ (<*>) <$> f <*> g
+
+instance Monad m => Monad (EitherT e m) where
+    return = pure
+    (EitherT etm) >>= f =
+        EitherT $ do
+            m <- etm
+            case m of
+                Left e -> pure (Left e)
+                Right a ->
+                    runEitherT (f a)