halve.hs

{-

> halve []
([],[])
> halve [1]
([],[1])
> halve [1,2]
([1],[2])
> halve [1,2,3]
([1],[2,3])
> halve [1,2,3,4]
([1,2],[3,4])

-}
halve :: [a] -> ([a], [a])
halve xs = go xs xs
  where
    go (x:ys) (_:_:zs) = (x:us, vs)
      where
        (us, vs) = go ys zs
    go ys _ = ([], ys)