module Queue(Qu, empty, isEmpty, 
             enq, first, deq) where

data Qu a = Qu [a] [a]
empty = Qu [] []

empty :: Qu a
isEmpty :: Qu a-> Bool
enq   :: a-> Qu a-> Qu a
first :: Qu a-> a  
deq   :: Qu a-> Qu a

isEmpty (Qu xs _) = null xs

first (Qu [] _)  = error "Qu: first of mt Q"
first (Qu (x:xs) _) = x

enq x (Qu xs ys) = check xs (x:ys)  

deq (Qu [] _ )     = error "Qu: deq of mt Q"
deq (Qu (_:xs) ys) = check xs ys

check [] ys = Qu (reverse ys) []
check xs ys = Qu xs ys