Question

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 }'

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"

如果有人可以帮我使用MaybeT变压器，我会很感激。我无法弄清楚如何使这个工作。

Answer 1

在这个小例子中使用MaybeT没有任何好处：你可以使用MaybeT将这三行写成：

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

没有自动将Maybe转换为MaybeT（虽然可能有如果Maybe实际上被定义为MaybeT Identity），所以你必须将函数应用于M.lookup的结果，如：

(MaybeT . return) :: Maybe a -> MaybeT m a

也许是编译错误

1 个答案: