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Task/Hash-join/Haskell/hash-join-1.hs

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+{-# LANGUAGE LambdaCase, TupleSections #-}
+import qualified Data.HashTable.ST.Basic as H
+import Data.Hashable
+import Control.Monad.ST
+import Control.Monad
+import Data.STRef
+
+hashJoin :: (Eq k, Hashable k) =>
+            [t] -> (t -> k) -> [a] -> (a -> k) -> [(t, a)]
+hashJoin xs fx ys fy = runST $ do
+  l <- newSTRef []
+  ht <- H.new
+  forM_ ys $ \y -> H.insert ht (fy y) =<<
+    (H.lookup ht (fy y) >>= \case
+      Nothing -> return [y]
+      Just v -> return (y:v))
+  forM_ xs $ \x -> do
+    H.lookup ht (fx x) >>= \case
+      Nothing -> return ()
+      Just v -> modifySTRef' l ((map (x,)  v) ++)
+  readSTRef l
+
+test = mapM_ print $ hashJoin
+    [(1, "Jonah"), (2, "Alan"), (3, "Glory"), (4, "Popeye")]
+        snd
+    [("Jonah", "Whales"), ("Jonah", "Spiders"),
+      ("Alan", "Ghosts"), ("Alan", "Zombies"), ("Glory", "Buffy")]
+        fst

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