Church-encoded Lists are efficient?

ChurchList.hs

http://stackoverflow.com/questions/15589556/why-are-difference-lists-not-an-instance-of-foldable
       /15593349?noredirect=1#comment22277557_15593349

    a = singleton 1     singleton x = ChurchList $ \k z -> k x z               
    b = snoc a 2        snoc  xs  x = ChurchList $ \k z -> runList xs k (k x z)  
    c = snoc b 3  
    d = append c c      append u  v = ChurchList $ \k z -> runList u k (runList v k z)

runList a k z = k 1 z                                     a := ChurchList $ \k z -> k 1 z                     
runList b k z = runList a k (k 2 z) = k 1 (k 2 z)         b := ChurchList $ \k z -> runList a k (k 2 z)       
runList c k z = runList b k (k 3 z) = k 1 (k 2 (k 3 z))   c := ChurchList $ \k z -> runList b k (k 3 z)        
runList d k z = runList c k (runList c k z) =             d := ChurchList $ \k z -> runList c k (runList c k z)
              = k 1 (k 2 (k 3 (runList c k z)))          FORCED to WHNF

-- OR:

runList d k z =                                d := ChurchList $ \k z -> runList c k (runList c k z)
  runList c k (runList c k z) =                c := ChurchList $ \k z -> runList b k (k 3 z) 
  runList b k (k 3 (runList c k z)) =          b := ChurchList $ \k z -> runList a k (k 2 z)
  runList a k (k 2 (k 3 (runList c k z))) =    a := ChurchList $ \k z -> k 1 z 
  = k 1 (k 2 (k 3 (runList c k z)))

----------------------------------------------------------------------------------------
g = (((a `append` b) `append` c) `append` d)
  = append (append (append a b) c) d
  = ChurchList $ \k z -> runList (append (append a b) c) k (runList d k z)                -- ******
                                         ------------ g2
                                 ----------------------- g1

runList g k z                                                  -- g  gets forced to WHNF: -- ******
 = runList g1 k (runList d k z)                                -- g1 gets forced to WHNF
 = runList g2 k (runList c k (runList d k z))                  -- g2 gets forced
 = runList a  k (runList b k (runList c k (runList d k z)))    -- a  gets forced
 = k 1 (runList b k (runList c k (runList d k z)))
----------------------------------------------------------------------------------------

    e = append d c            -- left-associative                -- NB!

    runList e k z = runList d k (runList c k z)
                  = k 1 (k 2 (k 3 (runList c k (runList c k z))))    -- right-associative  -- NB!