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

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

相当于MaybeT的Nothing的扩展似乎是MaybeT $ return Nothing，这有点冗长，而我不得不明确使用MaybeT构造函数对我来说是意外的。

在Nothing monad中写MaybeT有更短/更清晰的方法吗？

最佳答案

灵感来自@luqui的评论

要在纯Nothing单声道中表现为Maybe，maybeTNothing应该满足以下等式

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

这正是mzero实例中的MonadPlus对 MaybeT 的要求。这样我们就可以使用mzero

maybeTNothing = mzero

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