{- - ``Data/Queue/Classes'' - (c) 2008 Cook, J. MR SSD, Inc. -} {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-} module Data.Queue.Classes where -- |Construct a new FIFO queue. I don't know whether or not I really want -- the 'a' parameter. It might go away sometime, so beware ;-). class Monad m => NewFifo q m where newFifo :: m q -- |A type class carrying an altered set of functional dependencies used to -- constrain queues when the type of the queue never escapes far enough for -- a more deliberate choice to be made. class Monad m => DefaultFifo q m a | q -> a, m a -> q -- |Construct a new FIFO of a type suitable for carrying the supplied thing. -- Does not actually do anything with the thing supplied. newDefaultFifoFor :: (DefaultFifo q m a, NewFifo q m) => a -> m q newDefaultFifoFor _thing = newFifo class Monad m => Enqueue q m a | q -> a where -- |Put an item into a queue. May block while trying to do so. -- No constraint is placed on the behavior of the queue except that -- every item put in "really ought to" come out sometime before -- 'dequeue' returns a 'Nothing'. enqueue :: q -> a -> m () class Monad m => Dequeue q m a | q -> a where -- |Pull an item out of a queue. Should not block. No ordering -- constraints are implied other than that any item that went into -- the queue "really ought to" come out before 'dequeue' returns -- 'Nothing'. dequeue :: q -> m (Maybe a) class Monad m => DequeueWhere q m a | q -> a where -- |Pull an item matching the given predicate out of a queue. dequeueWhere :: q -> (a -> Bool) -> m (Maybe a) class Monad m => PeekQueue q m a | q -> a where -- |return the whole contents of the queue (if possible) without -- altering the queue's contents. Obviously in cases where this -- can't be done lazily this can be a very expensive operation. peekQueue :: q -> m [a] -- |peek a specified number of items off the queue. The default -- implementation is hideously wasteful in cases where peekQueue is -- not able to get the contents lazily. peekQueueTaking :: Int -> q -> m [a] peekQueueTaking n q = do xs <- peekQueue q return (take n xs) class Monad m => QueueSize q m where -- |return the number of elements in the queue queueSize :: q -> m Int