-------------------------------------------------------------------------
--  
--         Queues3.hs
--  
--         An abstract data type of queues, implemnted as two lists, with
--         new elements added at the beginning of the second list.      
--                                  
--         (c) Addison-Wesley, 1996-2011.                   
--  
-------------------------------------------------------------------------             

module Queues3 
  ( Queue , 
    emptyQ ,       --  Queue a
    isEmptyQ ,     --  Queue a -> Bool 
    addQ ,         --  a -> Queue a -> Queue a
    remQ           --  Queue a -> (  a , Queue a )
   ) where 

data Queue a = Queue [a] [a]

emptyQ :: Queue a

emptyQ :: forall a. Queue a
emptyQ = [a] -> [a] -> Queue a
forall a. [a] -> [a] -> Queue a
Queue [] []

isEmptyQ :: Queue a -> Bool

isEmptyQ :: forall a. Queue a -> Bool
isEmptyQ (Queue [] []) = Bool
True
isEmptyQ Queue a
_          = Bool
False

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

addQ :: forall a. a -> Queue a -> Queue a
addQ a
x (Queue [a]
xs [a]
ys) = [a] -> [a] -> Queue a
forall a. [a] -> [a] -> Queue a
Queue [a]
xs (a
xa -> [a] -> [a]
forall a. a -> [a] -> [a]
:[a]
ys)

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

remQ :: forall a. Queue a -> (a, Queue a)
remQ (Queue (a
x:[a]
xs) [a]
ys)    = (a
x , [a] -> [a] -> Queue a
forall a. [a] -> [a] -> Queue a
Queue [a]
xs [a]
ys)
remQ (Queue [] ys :: [a]
ys@(a
z:[a]
zs)) = Queue a -> (a, Queue a)
forall a. Queue a -> (a, Queue a)
remQ ([a] -> [a] -> Queue a
forall a. [a] -> [a] -> Queue a
Queue ([a] -> [a]
forall a. [a] -> [a]
reverse [a]
ys) [])
remQ (Queue [] [])        = [Char] -> (a, Queue a)
forall a. HasCallStack => [Char] -> a
error [Char]
"remQ"