Queues3.lhs

        An abstract data type of queues, implemnted as two lists, with
        new elements added at the beginning of the second 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 

>       data Queue a = Qu [a] [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 ys) = Qu xs (x:ys)

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

>       remQ (Qu (x:xs) ys)   = (x , Qu xs ys)
>       remQ (Qu [] ys)       = remQ (Qu (reverse ys) [])
>       remQ (Qu [] [])       = error "remQ"