On 2 June 2011 02:06, Christoph Bussler <chbussler@aol.com> wrote:

I’d like to find an element in a set. If the element is there, I’d like to get it back (not just the fact that it exists). If it is not in the set, I’d like to get the next higher and the next lower one (according to the sort order).

import Prelude hiding (null)

import Data.Set

-- if the given element is in the set, return it

-- else, try to return the (next lower, next higher)

-- (please ask if you do not understand why I chose this type)

foo :: Ord a => a -> Set a -> Either a (Maybe a, Maybe a)

foo x s = case splitMember x s of

(_, True, _) -> Left x

(low, _, high) -> Right (findMaxMaybe low, findMinMaybe high)

-- *Main> foo 3 $ fromList [1..10]

-- Left 3

-- *Main> foo 3 $ fromList [4..10]

-- Right (Nothing,Just 4)

-- *Main> foo 3 $ fromList $ 2:[4..10]

-- Right (Just 2,Just 4)

-- *Main> foo 3 $ fromList $ []

-- Right (Nothing,Nothing)

findMinMaybe :: Set a -> Maybe a

findMinMaybe s

| null s = Nothing

| otherwise = Just (findMin s)

findMaxMaybe :: Set a -> Maybe a

findMaxMaybe s

| null s = Nothing

| otherwise = Just (findMax s)