http://programmingpraxis.com/2011/08/19/first-non-repeating-character/

counting.hs

firstNonRep :: String -> Maybe Char
firstNonRep cs = fmap fst $ find (\(c, n) -> n == 1) $ foldl count [] cs
	where 
		count []               c             = [(c, 1)]
		count (p@(c', n) : ps) c | c == c'   = (c', n+1) : ps
		                         | otherwise = p : count ps c

severallists.hs

import Data.List(delete, find)

firstNonRep :: String -> Maybe Char
firstNonRep cs = firstNonRep' [] [] cs
	where
		firstNonRep' [] repeating [] = Nothing
		firstNonRep' nonrepeating repeating [] = Just (last nonrepeating)
		firstNonRep' nonrepeating repeating (c:cs) = 
			if c `elem` nonrepeating
			then firstNonRep' (delete c nonrepeating) (c : repeating) cs
			else 
				if c `elem` repeating
				then firstNonRep' nonrepeating repeating cs
				else firstNonRep' (c : nonrepeating) repeating cs

straightforward.hs

import Data.Maybe(listToMaybe)

firstNonRep :: String -> Maybe Char
firstNonRep cs = listToMaybe $ filter nonrep cs
    where
        nonrep :: Char -> Bool
        nonrep c = count c cs == 1

        count :: Eq a => a -> [a] -> Int
        count c [] = 0
        count c (c':cs') | c == c'    = 1 + count c cs'
                         | otherwise  = count c cs'