Queues2.lhs

        An abstract data type of queues, implemnted as a list, with
        new elements added at the beginning of the list.						
                                                                      
        (c) Simon Thompson, 1998.					
                                                                      

>       module Queue 
>         ( Queue , 
>           emptyQ ,       --  Queue a
>           isEmptyQ ,     --  Queue a -> Bool 
>           addQ ,         --  a -> Queue a -> Queue a
>           remQ           --  Queue a -> (  a , Queue a )
>          ) where 

>       newtype Queue a = Qu [a]
 
>       emptyQ :: Queue a

>       emptyQ = Qu []

>       isEmptyQ :: Queue a -> Bool

>       isEmptyQ (Qu []) = True
>       isEmptyQ _       = False

>       addQ   :: a -> Queue a -> Queue a

>       addQ x (Qu xs) = Qu (x:xs)

>       remQ   :: Queue a -> (  a , Queue a )

>       remQ q@(Qu xs)
>         | not (isEmptyQ q)   = (last xs , Qu (init xs))
>         | otherwise          = error "remQ"