maybe-example.hs

-- taken from https://www.youtube.com/watch?v=fg24wrRKE8g
import Prelude hiding (Maybe(..), lookup)

data Maybe a = Just a | Nothing
  deriving Show

favoriteFoods :: [(String, String)]
favoriteFoods = [("David", "Green Papaya Salad")
                 ,("Becky", "Sorbet")]
                 
lookup key [] = Nothing
lookup key ((k, v) : rest) = if key == k then Just v else lookup key rest

fromMaybe def (Just a) = a
fromMaybe def Nothing = def

main = do
  let favorite = lookup "Johan" favoriteFoods
  print (fromMaybe "Ice Cream" favorite)