[Haskell-cafe] beginner's problem about lists

Hi all, I'm trying to implement a function that returns the shorter one of two given lists, something like shorter :: [a] -> [a] -> [a] such that shorter [1..10] [1..5] returns [1..5], and it's okay for shorter [1..5] [2..6] to return either. Simple, right? However, it becomes difficult when dealing with infinite lists, for example, shorter [1..5] (shorter [2..] [3..]) Could this evaluate to [1..5]? I haven't found a proper implementation. Again it's ok for shorter [2..] [3..] to return whatever that can solve the above problem correctly. An infinite list could work, I guess, but I don't know how. Thanks for any help.