The proposed definition has one drawback: it is strict in its second argument.

It should be possible to make it lazy in its second argument while keeping the single-pass behavior, though. Something like this?

ZipList xs <|> ZipList ys = ZipList $ go xs ys 0
  where
    go [] ys n = drop n ys
    go (x:xs) ys n = x : (go xs ys $! n + 1)

On Tue, Jun 5, 2018, 15:36 박신환 <ndospark320@naver.com> wrote:

Current definion of (<|>) for ZipLists:

ZipList xs <|> ZipList ys = ZipList (xs ++ drop (length xs) ys)

doesn't work if the left argument is infinite. It should be:

ZipList []     <|> ys             = ys
xs             <|> ZipList []     = xs
ZipList (x:xs) <|> ZipList (_:ys) = ZipList (x : (ZipList xs <|> ZipList ys))
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