Haskell Type Class Basics

dubbler_type_class.hs

module Main where

-- essentially we're defining functions that makes sense to
-- call with things that can be doubled. it can be, well,
-- a value of any type (as long as that type instantiates
-- the Dubbleable type class). Dispatch can happen by virtue
-- of what you're going to do with the return value of the
-- function or what you're passing to the function as its
-- arguments
--
class Dubbleable t where
  dubble :: t -> t

-- this implementation of dubble will be used if you
-- call dubble with a list of whatever
--
instance Dubbleable [a] where
  dubble list = list ++ list

-- this implementation will be used if you call dubble
-- with an Integer
--
instance Dubbleable Integer where
  dubble n = n * 2

main :: IO ()
main = do
  putStrLn $ show $ dubble [1, 2, 3]         -- here, we dispatch based on the input to the function
  putStrLn $ show $ ((dubble 25) :: Integer) -- here, we dispatch based on the expected return type of the function
  return ()