Hi guys, I tried for days now to figure out a solution that Luke Palmer has presented me with, by myself, I'm getting nowhere. He has kindly provided me with this code: import Data.Monoid newtype IntTrie a = IntTrie [a] deriving Show singleton :: (Monoid a) => Int -> a -> IntTrie a singleton ch x = IntTrie $ replicate ch mempty ++ [x] ++ repeat mempty lookupTrie :: IntTrie a -> Int -> a lookupTrie (IntTrie xs) n = xs !! n instance (Monoid a) => Monoid (IntTrie a) where mempty = IntTrie (repeat mempty) mappend (IntTrie xs) (IntTrie ys) = IntTrie (infZipWith mappend xs ys) infZipWith f ~(x:xs) ~(y:ys) = f x y : infZipWith f xs ys test = mconcat [singleton (n `mod` 42) [n] | n <- [0..]] `lookupTrie` 10 It's supposed to eventually help me group a list of key value pairs and then further process them in a linear (streaming like) way. The original list being something like [('a', 23), ('b', 18), ('a', 34) ...]. There are couple of techniques employed in this solution, but I'm just guessing here. The keywords I've been looking up so far: Memmoization, Deforestation, Single Pass, Linear Map and some others. Can someone please fill me in? Günther