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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Jian Fan wrote:

I somehow cannot figure this out. Tree is Foldable so I can use "find" on it. But how can I use find on TreeLoc? Am I missing something obvious?

But what exactly is a "TreeLoc"? Hoogle doesn't know about it. If you created it yourself, you haven't showed us how you defined it or what you are using it for. In any case, below is one way to simplify the code you posted. It is only a small simplification, but it is about the best I can do without seeing the details of what you are trying to do. Try posting this kind of question to the "haskell-beginners" mailing list. There are people there who are much better at answering this kind of question. One thing that made it hard to follow your code was your repeated use of "loc", one shadowing the other. Try to avoid that. searchTree :: (a -> Bool) -> TreeLoc a -> Maybe (TreeLoc a) searchTree pre rootLoc | pred (getLabel rootLoc) = Just rootLoc | otherwise = do loc <- firstChild rootLoc searchTree pred loc `mplus` (right loc >>= searchTree pred) Regards, Yitz