Monad instance for State''

Haskell Excercises & Code/Alejandro - Book of Monads/5c6d7d7b-02fa-4e67-b84b-aac2ee3b3db7.hs

@@ -0,0 +1,41 @@
+{-# LANGUAGE FlexibleContexts #-}
+module MonadicCode where
+
+import Control.Monad.State as CMS
+
+
+-- implement `ap` yourself
+
+ap'' :: Monad m => m (a -> b) -> m a -> m b
+ap'' mf mx = do
+    f <- mf
+    x <- mx
+    return (f x)
+
+
+-- Another exercise. Write fmap with ap & pure
+
+-- fmap'' :: Functor f => (a -> b) -> f a -> f b
+fmap'' f a = pure f `ap` a
+
+-- Ex 3.3 Write ZipList functor instancce
+
+newtype ZipList a = ZipList { getZipList :: [a] } deriving (Eq, Show)
+-- so getZipList type :: ZipList a -> [a]
+
+instance Functor ZipList where
+    fmap f (ZipList gz) = ZipList $ fmap f gz
+
+newtype State'' s a = State { runState :: s -> (a,s) }
+
+-- runState :: State s a -> s -> (a,s)
+
+instance Functor (State'' s) where
+    fmap f (State rs) = State $ \s -> let (a,s') = rs s in (f a, s')
+
+instance Applicative (State'' s) where
+    -- pure :: x -> State s x
+    pure x = State $ \n -> (x, n)
+    State fz <*> State xz = State $ \s -> let a =  fst $ xz s
+                                              ab = fst $ fz s
+                                          in (ab a, s)

Haskell Excercises & Code/Alejandro - Book of Monads/monadic.hs

@@ -24,4 +24,30 @@ newtype ZipList a = ZipList { getZipList :: [a] } deriving (Eq, Show)
 -- so getZipList type :: ZipList a -> [a]
 
 instance Functor ZipList where
-    fmap f (ZipList gz) = ZipList $ fmap f gz 
+    fmap f (ZipList gz) = ZipList $ fmap f gz
+
+newtype State'' s a = State { runState' :: s -> (a,s) }
+
+-- runState :: State s a -> s -> (a,s)
+
+instance Functor (State'' s) where
+    fmap f (State rs) = State $ \s -> let (a,s') = rs s in (f a, s')
+
+instance Applicative (State'' s) where
+    -- pure :: x -> State s x
+    pure x = State $ \n -> (x, n)
+    State fz <*> State xz = State $ \s -> let a =  fst $ xz s
+                                              ab = fst $ fz s
+                                          in (ab a, s)
+
+instance Monad (State'' s) where
+    return x = State $ \s -> (x, s) 
+    rsa >>= f = State $ \input -> let (a, input')   = runState' rsa input
+                                      (a', input'') = runState' (f a) input'
+                                  in  (a', input'')
+
+
+
+    -- let (a, s) = rsa input in (f a, s)
+    -- m a -> (a -> m b) -> m b
+    -- State s a -> ( a -> State s b) -> State s b