Make <|> for ZipLists lazy for infinite lists

[Edward Kmett]

I rather like this version. It avoids needlessly holding onto the entire
list from the start, unlike the original version, which is leakier, and
avoids the unnecessary strictness problem of the second version.

We should definitely keep the original in a comment as a description of the
meaning of the operator, with an explanation of the change, though, as the
road here is rather convoluted, and to Andreas' point it is far easier to
understand even if it has undesirable garbage collection properties.

-Edward

On Tue, Jun 5, 2018 at 3:00 AM, Chris Wong <lambda.fairy at gmail.com> 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> 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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