On Tue, Oct 10, 2006 at 08:10:44PM +0800, falseep@gmail.com wrote:
To: haskell-cafe@haskell.org From: falseep@gmail.com Date: Tue, 10 Oct 2006 20:10:44 +0800 Subject: [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.
a function that takes two lists and decides whether one of them is finite or not , without being given further information on the lists, does not exist. you could add a third argument 'inifinity :: Int' that sets the minium length of all lists that are considered infinite. this is where your function could stop looking for an end in either of the two parameters: -- untested shorter infinity = reverse . f infinity [] [] f 0 ar al _ _ = ar f _ ar al [] _ = ar f _ ar al _ [] = al f (i+1) ar al (x:xs) (y:ys) = f i (x:ar) (y:al) xs ys or you could specify your function to be callable with at most one infinite list. i guess that's all you can do? matthias