haskell - MaybeT 的直觉

Question

我试图通过在 ghci 中运行一些示例来理解 MaybeT:

λ: import Control.Monad.Trans.Maybe
λ: let x = return $ 42 :: MaybeT (Either String) Int
λ: :t x
x :: MaybeT (Either String) Int

然后，我运行了它:

λ: runMaybeT x
Right (Just 42)

请给我值，y，这样

runMaybeT y === Left (Just "...")
runMaybeT y === Left Nothing
runMaybeT y === Right Nothing

Answer 1

你永远不会得到 Left Nothing 或 Left (Just ..)，因为这里的 monad 是 Either String ...，所以左边总是有一个String。

以下是您可以获得的内容:

> let y = fail "Failed" :: MaybeT (Either String) Int
> runMaybeT y
Right Nothing
> let y = lift (Left "Failed") :: MaybeT (Either String) Int
> runMaybeT y
Left "Failed"