Make <|> for ZipLists lazy for infinite lists

[Ivan Lazar Miljenovic]

Seems to work for me:

$ ghci
GHCi, version 8.2.2: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /Users/ivan/.ghci
Prelude> import Control.Applicative
Prelude Control.Applicative> let zAppend (ZipList xs) (ZipList ys) =
ZipList (xs ++ drop (length xs) ys)
Prelude Control.Applicative> take 10 (getZipList (ZipList [1..4] `zAppend`
ZipList [5..])) :: [Int]
[1,2,3,4,9,10,11,12,13,14]
Prelude Control.Applicative> take 10 (getZipList (ZipList [1..] `zAppend`
ZipList [5..])) :: [Int]
[1,2,3,4,5,6,7,8,9,10]
Prelude Control.Applicative>

(After all, the `length` call will never get evaluated.)

However, I do prefer your solution to avoid traversing the first list
twice, so +0.5 from me.

On 5 June 2018 at 13:36, 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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-- 
Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com
http://IvanMiljenovic.wordpress.com
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