2015-02-20 09:02:09 -05:00
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{-# LANGUAGE TupleSections #-}
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import qualified Data.Map as M
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import Data.List
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import Data.Maybe
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import Control.Applicative
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mapJoin xs fx ys fy = joined
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where yMap = foldl' f M.empty ys
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f m y = M.insertWith (++) (fy y) [y] m
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2016-12-05 22:15:40 +01:00
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joined = concat .
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mapMaybe (\x -> map (x,) <$> M.lookup (fx x) yMap) $ xs
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2015-02-20 09:02:09 -05:00
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2016-12-05 22:15:40 +01:00
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main = mapM_ print $ mapJoin
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2015-02-20 09:02:09 -05:00
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[(1, "Jonah"), (2, "Alan"), (3, "Glory"), (4, "Popeye")]
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snd
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[("Jonah", "Whales"), ("Jonah", "Spiders"),
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("Alan", "Ghosts"), ("Alan", "Zombies"), ("Glory", "Buffy")]
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fst
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