Add example for Hash-based containers (#12)

hash-based-containers.hs

@@ -0,0 +1,43 @@
+{-# LANGUAGE DeriveAnyClass #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE DerivingStrategies #-}
+{-# LANGUAGE RecordWildCards #-}
+
+import Data.Hashable (Hashable)
+import Data.HashMap.Strict (HashMap)
+import qualified Data.HashMap.Strict as HashMap
+import Data.Maybe
+import GHC.Generics (Generic)
+
+data Movie
+  = Movie
+  { title :: String
+  , director :: String
+  , imdbRating :: Int
+  }
+  deriving stock (Show, Eq, Generic)
+  deriving anyclass (Hashable)
+
+scores :: HashMap Movie Int
+scores = HashMap.fromList $ mapMaybe rankingForMovie movies
+  where
+    rankingForMovie m@Movie{..} = case HashMap.lookup title numRaters of
+      Just raters -> Just $ (m, raters * imdbRating)
+      Nothing -> Nothing
+
+numRaters :: HashMap String Int
+numRaters = HashMap.fromList
+  [ ("Pulp Fiction", 16000000), ("Fight Club", 17000000)]
+
+movies :: [Movie]
+movies =
+  [ Movie "Pulp Fiction" "Quentin Tarantino" 9
+  , Movie "Fight Club" "David Fincher" 9
+  , Movie "Okja" "Joon-ho Bong" 8
+  ]
+
+main =
+  mapM_ (\m@Movie{..} -> do
+    case HashMap.lookup m scores of
+      Nothing -> putStrLn $ "Score for " ++ title ++ " unavailable"
+      Just score -> putStrLn $ "Score for " ++ title ++ ": " ++ show score) movies