[Haskell-cafe] List-to-outline

[Colin DeVilbiss]

On 2/14/06, Steve Schafer <steve at fenestra.com> wrote:

>   isChild :: Int -> Node -> Bool
>   isChild i (Nd j _) = (j > i)
>   isChild _ _ = False
>
>   prepend :: Int -> [Node] -> [Node]
>   prepend i [] = [Nd i []]
>   prepend i ns = (Nd i f):s
>     where (f,s) = span (isChild i) ns
>
>   unflatten :: [Int] -> [Node]
>   unflatten ns = foldr prepend [] ns

The following seemed a little more natural to me:

unflatten [] = []
unflatten (x:xs) = (Node x $ unflatten xh) : unflatten xt
  where (xh, xt) = span (>x) xs

That is, a node containing the first item and the hierarchy of all of
its children, followed by the hierarchies of its siblings.

Is [0,2,1,2] invalid input?  The behavior of both versions puts the
first 2 and the 1 as children of the root, but the second 2 is a child
of the 1.

Colin DeVilbiss