I can calculate non-nested list comprehensions without a problem, but am unable to calculate nested comprehensions involving, for example, the generation of a list of pairs where the first and separate elements are drawn from two separate lists, as in: [(a, b) | a <- [1..3], b <- [1..2]] How does one calculate the expansion of this list? The two rules for expanding list comprehensions are: 1. Generator rule: [e | x <- xs, Q] = concat (map f xs) where f x = [e | Q] 2. Guard rule: [e | p, Q] = if p then [e | Q] else [] There is a third rule that I've seen on the Internet, not in an authoritative text: [e | Q1 , Q2] = concat [ [e | Q 2] | Q1 ] I don't understand what this third rule means, or whether it's relevant. Concat and map are defined as: concat :: [[a]] -> [a] concat [] = [] concat (xs:xss) = xs ++ concat xss map :: (a -> b) -> [a] -> [b] map f [] = [] map f (x:xs) = f x : (map f xs) Any help is appreciated. _________________________________________________________________ Windows Live™: Life without walls. http://windowslive.com/explore?ocid=TXT_TAGLM_WL_allup_1a_explore_032009