definition module Data.Queue
/**
* This module provides a straightforward FIFO queue.
* It is implemented using two list based on Chris Okasaki's example in Purely Functional Data Structures.
*/

from Data.Maybe import :: Maybe
from StdOverloaded import class length

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

/**
* Create an empty queue
*/
newQueue :: Queue a

/**
* Test if the queue is empty
*/
//empty :: Queue a -> Bool
empty q :== case q of 
        Queue [] [] -> True
        _           -> False

instance length Queue

/**
* Add an element to the queue
*/
enqueue :: a (Queue a) -> Queue a

/**
* Take an element from the queue (if the queue is not empty)
*/
dequeue :: (Queue a) -> (!Maybe a,!Queue a)