小编roh*_*ant的帖子

import Control.Monad.State.Lazy

type Queue a = [a]

push :: a -> State (Queue a) ()
push x = state (\xs -> ((),xs++[x]))

pop :: State (Queue a) a
pop = state (\(x:xs) -> (x,xs))

queueManip :: State (Queue Int) Int
queueManip =
  do
    mapM_ push [1..]
    a <- pop
    return a

main :: IO()
main = do
  let (a,_) = runState queueManip []
  print a

不mapM_应该懒惰吗？除了实现队列不应该复杂O(1)吗？

因为追加(++)本身就是懒惰......