20 Intermediate Haskell Solutions

Intermediate.hs

class Fluffy f where
  furry :: (a -> b) -> f a -> f b

-- Exercise 1
-- Relative Difficulty: 1
instance Fluffy [] where
  furry _ [] = []
  furry f (x:xs) = (f x):(furry f xs)

-- Exercise 2
-- Relative Difficulty: 1
instance Fluffy Maybe where
  furry _ Nothing  = Nothing
  furry f (Just x) = Just (f x)

-- Exercise 3
-- Relative Difficulty: 5
instance Fluffy ((->) t) where
  furry = (.)

newtype EitherLeft b a = EitherLeft (Either a b) deriving (Show)
newtype EitherRight a b = EitherRight (Either a b) deriving (Show)

-- -- Exercise 4
-- -- Relative Difficulty: 5
instance Fluffy (EitherLeft t) where
  furry _ (EitherLeft (Right x)) = EitherLeft (Right x)
  furry f (EitherLeft (Left x))  = EitherLeft (Left (f x))
    
-- Exercise 5
-- Relative Difficulty: 5
instance Fluffy (EitherRight t) where
  furry _ (EitherRight (Left x)) = EitherRight (Left x)
  furry f (EitherRight (Right x))  = EitherRight (Right (f x))

class Misty m where
  banana :: (a -> m b) -> m a -> m b
  unicorn :: a -> m a
  -- Exercise 6
  -- Relative Difficulty: 3
  -- (use banana and/or unicorn)
  furry' :: (a -> b) -> m a -> m b
  furry' f xs = banana (unicorn . f) xs

-- -- Exercise 7
-- -- Relative Difficulty: 2
instance Misty [] where
  banana f [x] = (f x)
  unicorn x   = [x]

-- Exercise 8
-- Relative Difficulty: 2
instance Misty Maybe where
  banana _ Nothing  = Nothing
  banana f (Just x) = (f x)
  unicorn x         = Just x