Just goofing around with arrows and foldr while reading Hutton's excellent paper on folds (http://www.cs.nott.ac.uk/~gmh/fold.pdf). Wondering if this can be done automatically and more generally? module Main where import Control.Arrow import Data.List -- sum and length expressed as foldr. fsum = foldr (\n -> (+n)) 0 flen = foldr (\n -> (+1)) 0 -- compute average using arrows.. -- compute the sum of a list, compute the length, and do a divide. -- this traverses the list twice using two foldrs. avg1 = uncurry (/) . (fsum &&& flen) avg2 = uncurry (/) . (foldr (\n -> (+n)) 0 &&& foldr (\n -> (+1)) 0) -- But the two foldr's can be fused together -- here we're mixing the two foldr constants 0 and 0 to (0,0) -- and we're mixing the two functions (\n -> (+n)) and -- (\n -> (+1)) to (\n -> (+n) *** (+1)). avg3 = uncurry (/) . foldr (\n -> (+n) *** (+1)) (0, 0) main = do print $ avg1 [1,2,3,4] print $ avg2 [1,2,3,4] print $ avg3 [1,2,3,4] Tim Newsham http://www.thenewsh.com/~newsham/