haskell - MaybeT monad 中没有任何内容 : more concise way?

Question

我正在尝试 MaybeT monad，特别是 MaybeT Identity String

import Control.Monad.Trans.Maybe
import Control.Monad.Identity
import Data.Maybe

main :: IO ()
main = putStrLn . show . runIdentity . runMaybeT $ maybeGetString

maybeGetString :: MaybeT Identity String
maybeGetString = return "first" >>
                 maybeTNothing >> 
                 return "second"

maybeTNothing :: MaybeT Identity String
maybeTNothing = MaybeT $ return Nothing

MaybeT 的表达式相当于 Nothing好像是 MaybeT $ return Nothing ，感觉有点冗长，而且我觉得必须明确使用 MaybeT构造函数。

有没有更短/更好/更清晰的写作方式 Nothing在 MaybeT单子(monad)？

Answer 1

受到@luqui 评论的启发

表现得像 Nothing在平原Maybe单子(monad)，maybeTNothing应该满足方程

maybeTNothing >>= f  =  maybeTNothing
v >> maybeTNothing   =  maybeTNothing

这正是 mzero 上的要求来自 MonadPlus , 其中 MaybeT是的一个实例。所以我们可以使用 mzero

maybeTNothing = mzero

https://hackage.haskell.org/package/base-4.9.1.0/docs/Control-Monad.html#t:MonadPlus