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Task/Hash-join/Haskell/hash-join-2.hs

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+{-# LANGUAGE TupleSections #-}
+import qualified Data.Map as M
+import Data.List
+import Data.Maybe
+import Control.Applicative
+
+mapJoin xs fx ys fy = joined
+  where yMap = foldl' f M.empty ys
+        f m y = M.insertWith (++) (fy y) [y] m
+        joined = concat . catMaybes .
+                 map (\x -> map (x,) <$> M.lookup (fx x) yMap) $ xs
+
+test = mapM_ print $ mapJoin
+    [(1, "Jonah"), (2, "Alan"), (3, "Glory"), (4, "Popeye")]
+        snd
+    [("Jonah", "Whales"), ("Jonah", "Spiders"),
+     ("Alan", "Ghosts"), ("Alan", "Zombies"), ("Glory", "Buffy")]
+        fst