bulleted-list.hs

import Data.Monoid

data List a = List | List a :• a

xs • x = xs :• x
infixl •

instance Show a => Show (List a) where
  show List = "List"
  show (xs :• x) = show xs ++ " •" ++ show x

ex1 = List • 1 • 2 • 3 • 4 • 5

ex2 = List
      • "Eggs"
      • "Milk"
      • "Flour"
      • "Cabbage"

instance Monoid (List a) where
  mempty = List
  mappend List ys = ys
  mappend xs List = xs
  mappend xs (ys :• y) = mappend xs ys :• y

instance Functor List where
  fmap f List = List
  fmap f (xs :• x) = (fmap f xs) • f x

ex3 = (++ "!") <$> ex2 <> fmap show ex1
-- List •"Eggs!" •"Milk!" •"Flour!" •"Cabbage!" •"1!" •"2!" •"3!" •"4!" •"5!"