{-

Queue implementation with an underlying list.

This implementation exposes the data constructor 'Q'.

-}

module ListQueue(

ListQueue(Q),emptyQueue,queueEmpty,enqueue,dequeue,front)

where

emptyQueue :: ListQueue a

queueEmpty :: ListQueue a -> Bool

enqueue :: a -> ListQueue a -> ListQueue a

dequeue :: ListQueue a -> ListQueue a

front :: ListQueue a -> a

newtype ListQueue a = Q [a]

deriving (Show, Eq)

emptyQueue = Q []

queueEmpty (Q []) = True

queueEmpty (Q _ ) = False

enqueue x (Q q) = Q (q ++ [x])

dequeue (Q (_:xs)) = Q xs

dequeue (Q []) = error "dequeue: empty queue"

front (Q (x:_)) = x

front (Q []) = error "front: empty queue"