All, I'm having substantial difficulty understanding Jeff Newbern's Example 7. Would love some help. My questions are: How does liftM2 know to lift fn into the _List_ monad? (Because it's the only possibility?) After fn is lifted, where is "bind" used in allCombinations? Basically, I'd some a detailed explanation of how allCombinations works, cause I'm not grokking it... - Alson --- import Monad -- allCombinations returns a list containing the result of -- folding the binary operator through all combinations -- of elements of the given lists -- For example, allCombinations (+) [[0,1],[1,2,3]] would be -- [0+1,0+2,0+3,1+1,1+2,1+3], or [1,2,3,2,3,4] -- and allCombinations (*) [[0,1],[1,2],[3,5]] would be -- [0*1*3,0*1*5,0*2*3,0*2*5,1*1*3,1*1*5,1*2*3,1*2*5], or [0,0,0,0,3,5,6,10] allCombinations :: (a -> a -> a) -> [[a]] -> [a] allCombinations fn [] = [] allCombinations fn (l:ls) = foldl (liftM2 fn) l ls -- print an example showExample :: (Show a) => String -> (a -> a -> a) -> [[a]] -> IO () showExample opName op ls = do putStrLn $ "opName over " ++ (show ls) ++ " = " putStrLn $ " " ++ (show (allCombinations op ls)) -- shows a few examples of using allCombinations main :: IO () main = do showExample "+" (+) [[0,1],[1,2,3]] showExample "*" (*) [[0,1],[1,2],[3,5]] showExample "/" div [[100, 45, 365], [3, 5], [2, 4], [2]]