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错误：

maybet.hs:8:14:
    Couldn't match expected type `MaybeT m0 t0'
                with actual type `Maybe a0'
    In the return type of a call of `M.lookup'
    In a stmt of a 'do' expression: m <- M.lookup "a" a
    In the second argument of `($)', namely
      `do { m <- M.lookup "a" a;
              lift $ putStrLn m }'

和代码：

import qualified Data.Map as M
import Control.Monad.Trans.Maybe
import Control.Monad.Trans.Class

main = do
    let a = M.fromList [("a", "b"), ("c", "d")]
    runMaybeT $ do
        m <- M.lookup "a" a
        lift $ putStrLn m
    putStrLn "done"

如果有人可以帮助我使用 MaybeT 变压器，我将不胜感激。我不知道如何让这个工作。

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1 回答 1

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在这个小例子中使用 MaybeT 没有任何好处：您可以使用 MaybeT 编写三行代码：

Data.Foldable.traverse_  putStrLn $ M.lookup "a" a

没有从 Maybe 到 MaybeT 的自动转换（尽管如果 Maybe 实际上被定义为 MaybeT Identity 可能会有），所以你必须对 M.lookup 的结果应用一个函数，例如：

(MaybeT . return) :: Maybe a -> MaybeT m a
于 2013-01-05T06:11:19.323 回答