[Haskell-cafe] Re: search Data.Tree.Zipper

Thanks. `mplus` is the glue I was looking for. The original algorithm has a bug. Following is what I have for now: searchTree pred rootLoc | pred (getLabel rootLoc) = Just rootLoc | otherwise = (right rootLoc >>= searchTree pred) `mplus` (firstChild rootLoc >>= searchTree pred) Jian MightyByte wrote:

I haven't tested it, but I think you're looking for something like this:

searchTree2 :: (a -> Bool) -> TreeLoc a -> Maybe (TreeLoc a) searchTree2 pred rootLoc = if pred (getLabel rootLoc) then Just rootLoc else firstChild rootLoc >>= siblings where siblings loc = searchTree2 pred loc `mplus` (searchTree2 pred =<< right loc)

On Mon, Mar 8, 2010 at 1:11 PM, Jian Fan wrote:

...

Hi,

There doesn't seem to be a function to search the tree so I come up with following function:

searchTree :: (a -> Bool) -> TreeLoc a -> Maybe (TreeLoc a) searchTree pred rootLoc =  if pred (getLabel rootLoc) then    Just rootLoc    else case firstChild rootLoc of      Just loc -> case searchTree pred loc of        Just loc -> Just loc        Nothing -> case right loc of          Just rLoc -> searchTree pred rLoc          Nothing -> Nothing      Nothing -> Nothing

Which feels quite ugly. Any suggestions? Thanks.

Jian

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