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/State.hs | 41 +++++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)
 create mode 100644 Haskell-book/26/MaybeT/src/State.hs

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

diff --git a/Haskell-book/26/MaybeT/src/State.hs b/Haskell-book/26/MaybeT/src/State.hs
new file mode 100644
index 0000000..d30fdd5
--- /dev/null
+++ b/Haskell-book/26/MaybeT/src/State.hs
@@ -0,0 +1,41 @@
+module State where
+
+import MonadIO
+import MonadTrans
+
+newtype StateT s m a =
+    StateT { runStateT :: s -> m (a, s) }
+
+-- 1
+instance (Functor m)
+      => Functor (StateT s m) where
+    fmap f (StateT m) = StateT $ \s -> fmap first $ m s
+            where first = uncurry (\t1 t2 -> ((f t1), t2))
+            
+-- 2
+-- Links:
+--   http://stackoverflow.com/questions/18673525/is-it-possible-to-implement-applicative-m-applicative-statet-s-m
+--   https://github.com/NICTA/course/issues/134
+instance (Monad m)
+      => Applicative (StateT s m) where
+    pure x = StateT $ (\s -> pure (x, s))
+    StateT g <*> StateT h = StateT $ \s -> keepFirst <$> g s <*> h s
+        where keepFirst (f, s') (x, _) = (f x, s')
+  
+
+-- 3
+instance (Monad m)
+      => Monad (StateT s m) where
+    return = pure
+
+    (StateT sma) >>= f =
+        StateT $ \s -> do
+            a <- sma s
+            runStateT (f $ fst a) s
+
+instance MonadTrans (StateT s) where
+    lift c = StateT $ \s -> c >>= (\x -> return (x, s))
+
+instance (MonadIO m)
+  => MonadIO (StateT s m) where
+      liftIO = lift . liftIO
-- 
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