Поиск в ширину на бинарном дереве

--Иерархия должностей в некоторой организации
data Positions a =  Worker a
                | Boss String (Positions a) (Positions a)
 
--Возвращает список всех работников по заданному уровню.
 
getListWLev :: Positions String -> Int -> Int -> [(String, Int)]
getListWLev (Worker x) level count = if count<=level then [(x,count)] else []
getListWLev (Boss name l r) level count = if count<=level then  getListWLev l level (count+1) ++ getListWLev r level (count+1) ++ [(name,count)]   else []
                                 
getL :: [(String, Int)] -> Int -> [String]
getL [] _ = []
getL ((name,count):xs) level =  if level == count then name : getL xs level     else getL xs level
 
getWorkersLevel :: Positions String -> Int -> [String]
getWorkersLevel t level  = getL (getListWLev t level 0 ) level
 
 
 
position = (Boss "Director" (Boss "Deputy director" (Boss "Foreman" (Boss "Master" (Worker "Worker") (Worker "Subsidiary")) (Worker "machine operator")) 
    (Boss "Senior laboratory assistant" (Worker "Chemist controler") (Worker "laboratory assistant")))    (Boss "Finance. managing director" (Worker "Accountant") 
    (Boss "Managing director of the personnel" (Worker "Personnel officer") (Worker "The spe******t in interaction with clients"))))
 
main = do print $ getWorkersLevel position 4

getWorkersLevel :: Positions String -> Int -> [String]
getWorkersLevel (Boss name _ _) 0 = [name]
getWorkersLevel (Worker name)   0 = [name]
getWorkersLevel (Boss _ l r) n = getWorkersLevel l (n-1) ++ getWorkersLevel r (n-1)
getWorkersLevel _  _ = []