doStuff :: [a] -> a
doStuff = doAllTheThings

doStuff :: [a] -> a

doStuff = doAllTheThings

class Functor f where
  fmap :: (a -> b) -> f a -> f b

instance Functor [] where
  fmap = map

class Monoid a where
  mempty  :: a
  mappend :: a -> a -> a
  mconcat :: [a] -> a

ints :: [Int]
ints = [1, 2, 3]

sumInts :: [Int] -> Int
sumInts xs = mconcat xs

sumInts ints

ints :: [Int]
ints = [1, 2, 3]

sumInts :: [Int] -> Int
sumInts xs = mconcat xs

multiplyInts :: [Int] -> Int
multiplyInts xs = mconcat xs

sumInts ints
multiplyInts ints

newtype Sum = Sum Int

instance Monoid Sum where
  mempty = Sum 0
  mappend (Sum x) (Sum y) = Sum (x + y)

newtype Product = Product Int

instance Monoid Product where
  mempty = Product 1
  mappend (Product x) (Product y) = Product (x * y)

ints :: [Int]
ints = [1, 2, 3, 4]

sumInts :: [Int] -> Int
sumInts xs = getInt (mconcat (map Sum xs))

multiplyInts :: [Int] -> Int
multiplyInts xs = getProduct (mconcat (map Product xs))

sumInts ints      -- | 10
multiplyInts ints -- | 24

class Functor f where
  fmap :: (a -> b) -> f a -> f b

instance Functor [] where
  -- | :: (a -> b) -> [a] -> [b]
  fmap fn xs = map fn xs

instance Functor [] where
  fmap = map

ints :: [Int]
ints = [1, 2, 3, 4]

sumInts :: [Int] -> Int
sumInts xs = getInt (mconcat (fmap Sum xs))

multiplyInts :: [Int] -> Int
multiplyInts xs = getProduct (mconcat (fmap Product xs))

sumInts ints      -- | 10
multiplyInts ints -- | 24

fmap (+1) [1, 2, 3, 4] -- | [2, 3, 4, 5]

fmap (+1) (Just 1) -- | Just 2

fmap (+1) Nothing  -- | Nothing

data Maybe a = Nothing | Just a

fmap (+1) [1] -- | [2]

fmap (+1) []  -- | []

fmap (+1) (Just 1) -- | Just 2

fmap (+1) Nothing  -- | Nothing

instance Functor (Maybe a) where
  fmap fn (Just a) = Just (fn a)
  fmap _  Nothing  = Nothing