Potential space leak in transpose

In Data.List, we define transpose :: [[a]] -> [[a]] transpose [] = [] transpose ([] : xss) = transpose xss transpose ((x : xs) : xss) = (x : [h | (h : _) <- xss]) : transpose (xs : [t | (_ : t) <- xss]) The potential difficulty is that we essentially mapMaybe over the xss list twice in the third case. So we hang on to the heads where we need the tails and the tails where we need the heads. We could fix that with something like this: transpose ((x : xs) : xss) = (x : fronts) : transpose (xs : rears) where (fronts, rears) = unzip [(h,t) | (h : t) <- xss] I don't know how much that'll cost in practice.