Fix chapter 23

23-state/23-state.md

@@ -0,0 +1,55 @@
+# 23 State
+
+## 23.5 Exercises: Roll Your Own
+
+```haskell
+repeat :: a -> [a]
+repeat a = a : repeat a
+
+instance Functor (State s) where
+fmap f st = state $ \s -> let (a, s') = runState st s
+                          in (f a, s')
+
+-- Applying this to infiniteDie means that we advance the StdGen once and
+-- repeat the rolled die. I.e. we only call randomR once and then repeat
+-- the result.
+
+replicateM :: Applicative f => Int -> f a -> f [a]
+replicateM n fa
+    | n <= 0 = pure []
+    | otherwise = (:) <$> fa <*> replicateM (n - 1) fa
+
+instance Applicative (State s) where
+pure a = state $ \s -> (a, s)
+stf <*> sta = state $ \s -> let (f, s') = runState stf s
+                                (a, s'') = runState sta s'
+                            in (f a, s'')
+
+-- randomR will be called for every fmap statement in replicateM. This is
+-- because we now from previous statement that fmap will call randomR and the
+-- resulting State object will no longer call randomR. The second argument
+-- in <*> will eventually reduce to pure [] which won't change the state and
+-- call a randomR once for each fmap.
+-- Hence we get an array with a call to randomR for each element, propagating
+-- the StdGen forward along it.
+```
+
+[src/RandomExample2.hs](./src/RandomExample2.hs)
+
+To run in ghci:
+
+```shell
+Prelude> :l src/RandomExample2.hs src/RandomExample.hs
+```
+
+## 23.6 Write `State` for yourself
+
+[src/Moi.hs](./src/Moi.hs)
+
+## 23.7 Fizzbuzz differently
+
+[src/fizzbuzz.hs](./src/fizzbuzz.hs)
+
+## 23.8 Chapter Exercises
+
+[src/chapter.hs](./src/chapter.hs)

23-state/23.5-roll-your-own.md

@@ -1,2 +0,0 @@
-# Exercises: Roll Your Own
-see src/RandomExample2.hs

23-state/23.8-chapter-exercises.md

@@ -1,2 +0,0 @@
-# Chapter Exercises
-see src/chapter.hs

23-state/src/Moi.hs

@@ -13,8 +13,8 @@ instance Applicative (Moi s) where
     pure a = Moi $ \s -> (a, s)
 
     (<*>) :: Moi s (a -> b) -> Moi s a -> Moi s b
-    (Moi f) <*> (Moi g) = Moi $ \s -> let (a, s') = g s
+    (Moi f) <*> (Moi g) = Moi $ \s -> let (f', s') = f s
-                                          (f', s'') = f s'
+                                          (a, s'') = g s'
                                       in (f' a, s'')
 
 instance Monad (Moi s) where

23-state/src/chapter.hs

@@ -6,8 +6,8 @@ instance Functor (State s) where
                              in (f a, s')
 instance Applicative (State s) where
     pure a = State $ \s -> (a, s)
-    (State f) <*> k = State $ \s -> let (a, s') = runState k s
+    (State f) <*> k = State $ \s -> let (f', s') = f s
-                                        (f', s'') = f s'
+                                        (a, s'') = runState k s'
                                     in (f' a, s'')
 instance Monad (State s) where
     return = pure