{-# OPTIONS -XTemplateHaskell -XMultiParamTypeClasses #-}
import UpdatableViewStub 

spec Q a = None 
         | More a (Q a) deriving (Show, Eq) where 
    isEmpty :: Q a -> Bool 
    isEmpty None = True 
    
    emptyQ :: Q a 
    emptyQ = None

    enQ :: a -> Q a -> Q a 
    enQ a None = More a None 
    enQ a (More b x) = More b (enQ a x)

    deQ :: Q a -> Q a
    deQ (More a x) = x

    first :: Q a -> a  
    first (More a x) = a 
impl Qi a = Qi [a] [a] deriving (Show, Eq) where
    enQ :: a -> Qi a -> Qi a 
    enQ a (Qi x y) = Qi x (a:y)

    deQ :: Qi a -> Qi a
    deQ (Qi (a:x) y) = Qi x y
    deQ (Qi [] y)    = deQ (Qi (reverse y) [])

    first :: Qi a -> a 
    first (Qi (a:x) y) = a 
    first (Qi [] y)    = first (Qi (reverse y) [])
abstractedBy
    view = $(foldT ''Qi) ( (idB <**> reverseB) >>>> appendB) >>>> l2l 
          where l2l = $(foldT ''[]) ( $(toC 'None) <||> $(toC 'More) )

mapQ f None       = None 
mapQ f (More a x) = More (f a) (mapQ f x)

play1 0     q = first q 
play1 (n+1) q = do hd 
                   play1 n (enQ hd tl)
   where hd = first q 
         tl = deQ   q