Add remaining haskell book exercises

Haskell-book/20/Exercises.hs

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+module Exercises where
+
+-- 1
+data Constant a b = Constant b deriving (Show)
+
+instance Foldable (Constant a) where
+    foldr f acc (Constant x) = f x acc
+
+-- 2
+data Two a b = Two a b deriving (Show)
+
+instance Foldable (Two a) where
+    foldr f acc (Two _ x) = f x acc
+
+-- 3
+data Three a b c = Three a b c deriving (Show)
+
+instance Foldable (Three a b) where
+    foldr f acc (Three _ _ x) = f x acc
+
+-- 4
+data Three' a b = Three' a b b deriving (Show)
+
+instance Foldable (Three' a) where
+    foldr f acc (Three' _ x y) = f y $ f x acc
+
+-- 5
+data Four' a b = Four' a b b b deriving (Show)
+
+instance Foldable (Four' a) where
+    foldr f acc (Four' _ x y z) = f z $ f y $ f x acc
+
+filterF :: ( Applicative f
+           , Foldable t
+           , Monoid (f a))
+        => (a -> Bool) -> t a -> f a
+filterF f x = foldMap y x
+    where y k = if f k then pure k else mempty