Dear All, I am trying to work out how to handle a function that might return different types of output. I assume I need to use either Maybe or Either, but I can't quite get it to work. At the moment, I have some function: checkNum3 :: Int -> Int -> (Int,Int) checkNum3 a b = if check a b then (a,b) else (a,-1) checkLists :: [Int] -> Int -> [(Int,Int)] checkLists a b = map (checkNum3 b) a checkAll3 :: [Int] -> [Int] -> [(Int,Int)] checkAll3 a b = concat (map (checkLists a) b) However, I know that checkNum3 isn't a good function - it uses setting the second element of the tuple to -1 to signal failure, which is obviously a recipe for problems later on. However, I want to return either a pair of integers, or a single integer. Any advice would be appreciated. Thanks, Matt