Make <|> for ZipLists lazy for infinite lists

[Andreas Abel]

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 <ndospark320 at naver.com 
> <mailto: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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-- 
Andreas Abel  <><      Du bist der geliebte Mensch.

Department of Computer Science and Engineering
Chalmers and Gothenburg University, Sweden

andreas.abel at gu.se
http://www.cse.chalmers.se/~abela/