Fix chapter 25

25-composing-types/25-composing-types.md

@@ -0,0 +1,10 @@
+# 25 Composing Types
+
+## 25.4 Twinplicative
+
+see [src/gotcha.hs](./src/gotcha.hs)
+
+## 25.6 Compose Instances
+
+- see [src/gotcha.hs](./src/gotcha.hs)
+- see [src/bifunctor.hs](./src/bifunctor.hs)

25-composing-types/25.4-twinplicative.md

@@ -1,2 +0,0 @@
-# GOTCHA! Exercise time
-see src/gotcha.hs

25-composing-types/25.6-compose-instances.md

@@ -1,5 +0,0 @@
-# Exercises: Compose Instances
-see src/gotcha.hs
-
-## Bifunctor
-see src/bifunctor.hs

25-composing-types/src/gotcha.hs

@@ -16,6 +16,11 @@ newtype Compose f g a = Compose { getCompose :: f (g a) } deriving (Eq, Show)
 instance (Functor f, Functor g) => Functor (Compose f g) where
     fmap f (Compose fga) = Compose $ (fmap . fmap) f fga
 
+myApply :: (Applicative f, Applicative g)
+        => Compose f g (a -> b) -> Compose f g a -> Compose f g b
+myApply (Compose h) (Compose c) =
+    Compose $ (fmap (<*>) h) <*> c
+
 instance (Applicative f, Applicative g) => Applicative (Compose f g) where
     pure :: a -> Compose f g a
     pure a = Compose $ pure (pure a)
@@ -39,7 +44,7 @@ instance (Applicative f, Applicative g) => Applicative (Compose f g) where
     -- Which brings us to:
     -- :: (<*>) f a = (fmap (<*>) f) <*> a
     (<*>) :: Compose f g (a -> b) -> Compose f g a -> Compose f g b
-    (Compose h) <*> (Compose c) = Compose $ 
+    (Compose h) <*> (Compose c) = Compose $
         (fmap (<*>) h) <*> c
 
 instance (Foldable f, Foldable g) => Foldable (Compose f g) where
@@ -53,7 +58,7 @@ instance (Foldable f, Foldable g) => Foldable (Compose f g) where
 -- (foldMap . foldMap) :: (a -> m) -> f (g a) -> m
 
 instance (Traversable f, Traversable g) => Traversable (Compose f g) where
-    traverse :: Applicative f' => (a -> f' b) -> Compose f g a 
+    traverse :: Applicative f' => (a -> f' b) -> Compose f g a
                                             -> f' (Compose f g b)
     traverse f (Compose fga) = Compose <$> ((traverse . traverse) f fga)