Make <|> for ZipLists lazy for infinite lists

[Chris Wong]

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 at naver.com> wrote:

> Current definion of (<|>) for ZipLists:
>
> ZipList <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicative.html#ZipList> xs <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicative.html#local-6989586621679319881> <|> <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/GHC.Base.html#%3C%7C%3E> ZipList <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicative.html#ZipList> ys <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicative.html#local-6989586621679319882> = ZipList <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicative.html#ZipList> (xs <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicative.html#local-6989586621679319881> ++ <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/GHC.Base.html#%2B%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#length> xs <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicative.html#local-6989586621679319881>) ys <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Applicative.html#local-6989586621679319882>)
>
> 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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