From 98329e0a3dd4f78b5d815ac3896272ec70904901 Mon Sep 17 00:00:00 2001
From: Eugen Wissner <belka@caraus.de>
Date: Thu, 11 Dec 2025 10:28:11 +0100
Subject: Add remaining haskell book exercises

---
 Haskell-book/26/MaybeT/src/Either.hs | 56 ++++++++++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)
 create mode 100644 Haskell-book/26/MaybeT/src/Either.hs

(limited to 'Haskell-book/26/MaybeT/src/Either.hs')

diff --git a/Haskell-book/26/MaybeT/src/Either.hs b/Haskell-book/26/MaybeT/src/Either.hs
new file mode 100644
index 0000000..e09bfe6
--- /dev/null
+++ b/Haskell-book/26/MaybeT/src/Either.hs
@@ -0,0 +1,56 @@
+module Either where
+
+import Control.Monad (liftM)
+import MonadTrans
+import MonadIO
+
+newtype EitherT e m a =
+    EitherT { runEitherT :: m (Either e a) }
+
+-- 1
+instance Functor m => Functor (EitherT e m) where
+    fmap f (EitherT x) = EitherT $ (fmap . fmap) f x
+
+-- 2
+instance Applicative m => Applicative (EitherT e m) where
+    pure x = EitherT $ pure $ pure x
+
+    (EitherT f) <*> (EitherT a) = EitherT $ (<*>) <$> f <*> a
+
+-- 3
+instance Monad m => Monad (EitherT e m) where
+    return = pure
+
+    (EitherT em) >>= f = EitherT $ do
+        v <- em
+        case v of
+            Left y -> return $ Left y
+            Right y -> runEitherT (f y)
+
+
+-- 4
+-- transformer version of swapEither.
+-- Hint: write swapEither first, then swapEitherT in terms of the former.
+swapEither :: Either e a -> Either a e
+swapEither (Left x) = Right x
+swapEither (Right y) = Left y
+
+swapEitherT :: (Functor m)
+            => EitherT e m a
+            -> EitherT a m e
+swapEitherT (EitherT x) = EitherT $ fmap swapEither x
+
+-- 5. Write the transformer variant of the either catamorphism.
+eitherT :: Monad m
+        => (a -> m c)
+        -> (b -> m c)
+        -> EitherT a m b
+        -> m c
+eitherT f g (EitherT x) = x >>= (either f g)
+
+instance MonadTrans (EitherT e) where
+    lift = EitherT . liftM Right
+
+instance (MonadIO m)
+  => MonadIO (EitherT e m) where
+      liftIO = lift . liftIO
-- 
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