module Queue_Sol (Queue, empty, isEmpty, enqueue, dequeue, toList) where

data Queue a = Queue [a] [a]
    deriving Show

empty :: Queue a
empty = Queue [] []

isEmpty :: Queue a -> Bool
isEmpty (Queue [] []) = True
isEmpty _ = False

enqueue :: a -> Queue a -> Queue a
enqueue a (Queue hs ts) = Queue hs (a : ts)

dequeue :: Queue a -> Maybe (a, Queue a)
dequeue (Queue [] []) = Nothing
dequeue (Queue [] ts) = dequeue $ Queue (reverse ts) []
dequeue (Queue (h:hs) ts) =  Just (h, Queue hs ts)

toList :: Queue a -> [a]
toList (Queue hs ts) = hs ++ reverse ts