ch16.hs

module Ch16 where

data Sum a b = First a | Second b deriving (Eq, Show)
instance Functor (Sum e) where
  fmap _ (First a) = First a
  fmap f (Second b) = Second (f b)

data Company a b c = DeepBlue a c | Something b deriving (Eq, Show)
instance Functor (Company e e') where
  fmap _ (Something b) = Something b
  fmap f (DeepBlue a c) = DeepBlue a (f c)

data More a b = L a b a | R b a b deriving (Eq, Show)
instance Functor (More x) where
  fmap f (L a b a') = L a (f b) a
  fmap f (R b a b') = R (f b) a (f b)

data Notorious g o a t = Notorious (g o) (g a) (g t)
instance Functor g => Functor (Notorious g o a) where
  fmap f (Notorious x y z) = Notorious x y $ fmap f z

data MyList a = Nil | Cons a (MyList a) deriving (Eq, Show)
instance Functor MyList where
  fmap _ Nil = Nil
  fmap f (Cons x y) = Cons (f x) (fmap f y)

data GoatLord a = NoGoat | OneGoat a | MoreGoats (GoatLord a) (GoatLord a) (GoatLord a)
instance Functor GoatLord where
  fmap _ NoGoat            = NoGoat
  fmap f (OneGoat x)       = OneGoat (f x)
  fmap f (MoreGoats x y z) = MoreGoats (fmap f x) (fmap f y) (fmap f z)

data TalkToMe a = Halt | Print String a | Read (String -> a)
instance Functor TalkToMe where
  fmap _ Halt        = Halt
  fmap f (Print x y) = Print x (f y)
  fmap f (Read g)    = Read $ fmap f g