Re: Make <|> for ZipLists lazy for infinite lists

I prefer the original definition ZipList xs <|> ZipList ys = ZipList (xs ++ drop (length xs) ys) for sake of clarity. On 05.06.2018 09:00, Chris Wong wrote:

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 mailto:ndospark320@naver.com> wrote:

Current definion of (<|>) for ZipLists:

ZipList http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicativ...xs http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicativ...<|> http://hackage.haskell.org/package/base-4.11.1.0/docs/src/GHC.Base.html#%3C%...ZipList http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicativ...ys http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicativ...=ZipList http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicativ...(xs http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicativ...++ http://hackage.haskell.org/package/base-4.11.1.0/docs/src/GHC.Base.html#%2B%...drop http://hackage.haskell.org/package/base-4.11.1.0/docs/src/GHC.List.html#drop(length http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Data.Foldable.html...xs http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicativ...)ys http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicativ...)

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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