put dequeue/enqueue into a type class.

hs/DataStructures.hs

    isEmpty Nil = True
    isEmpty _ = False
class Queue q where
    enqueue :: a -> q a -> q a
    dequeue :: q a -> Maybe (a, q a)
-- fifo, remove from front, insert into rear
data BankersQueue a = BankersQueue {
                          frontSize :: Integer,
                          rear :: List a
                      } deriving (Show)
enqueue x (BankersQueue fs f rs r) = check $ BankersQueue fs f (rs + 1) (cons x r)
instance Queue BankersQueue where
    enqueue x (BankersQueue fs f rs r) = check $ BankersQueue fs f (rs + 1) (cons x r)
dequeue (BankersQueue fs Nil         rs Nil) = Nothing
--dequeue (BankersQueue fs Nil         rs r) = Just (x, check $ BankersQueue fs Nil (rs - 1) r')
dequeue (BankersQueue fs (Cons x fr) rs r) =
    Just (x, check $ BankersQueue (fs - 1) fr rs r)
    dequeue (BankersQueue fs Nil         rs Nil) = Nothing
    -- not needed because of `check` invariant?
    --dequeue (BankersQueue fs Nil         rs r) = Just (x, check $ BankersQueue fs Nil (rs - 1) r')
    --    where (Just x) = last r
    --          r' = butLast r
    dequeue (BankersQueue fs (Cons x fr) rs r) =
        Just (x, check $ BankersQueue (fs - 1) fr rs r)
dequeueN :: Queue q => Integer -> q a -> Maybe (a, q a)
dequeueN 0 q = Nothing
dequeueN 1 q = dequeue q >>= \(x, _) -> return x
dequeueN 1 q = dequeue q >>= \(x, q) -> return (x, q)
dequeueN n q = dequeue q >>= \(_, q') -> dequeueN (n-1) q'
check q@(BankersQueue fs f rs r) =