learning about Haskell type classes

type-classes.hs

import qualified Data.Map as Map

-- A tuple of three Ints
type RGB = (Int, Int, Int)

-- Colors can be specified in three ways.
data Color =
  ColorName String |
  ColorHex String |
  ColorRgb Int Int Int

-- This will be used convert from names to RGB values.
colorNameToRgbMap :: Map.Map String RGB
colorNameToRgbMap = Map.fromList [
  ("red", (255, 0, 0)),
  ("green", (0, 255, 0)),
  ("blue", (0, 0, 255))]

-- A Wagon is one kind of thing that has a color.
data Wagon = Wagon {
  length :: Int,
  width :: Int,
  color :: Color
}

-- This type class specifies the function signatures
-- for functions that should be implemented
-- for everything that has a color.
class HasColor a where
  getColorName :: a -> String
  getColorHex :: a -> String
  getColorRgb :: a -> RGB

-- This implements the HasColor type class for the Wagon type.
instance HasColor Wagon where
  -- How can these deal with all three ways of defining a color?
  getColorName wagon = "some color name"
  getColorHex wagon = "some hex value"
  getColorRgb wagon = (1, 2, 3)

main :: IO ()
main = do
  let wagon = Wagon 36 20 (ColorName "red")
  let name = getColorName wagon
  putStrLn $ "name = " ++ name
  let hex = getColorHex wagon
  putStrLn $ "hex = " ++ hex
  let rgb = getColorRgb wagon
  putStrLn $ "rgb = " ++ show rgb