使用 ReaderT Maybe 或 MaybeT Reader？

Question

我想写一个 haskell 程序读取配置并做一些事情。配置是 Data.Map，将被注入 Reader。而当我们找不到配置项时，应该中断读取。刚刚停止，不需要错误消息。所以我只想要一个 Maybe monad（不是 Either monad）。

问题是，我应该如何堆叠两个 monad，ReaderT Maybe 或 MaybeT Reader？

Answer 1

我终于明白了。两个都OK。

假设每个配置项的值只是一个 Int。

ReaderT ConfigMap Maybe Int 的值类似于：

ReaderT (\r -> Just 123)

或

ReaderT (\r -> Nothing)

MaybeT (Reader ConfigMap) Int 的值类似于：

MaybeT (Reader (\r -> Just 123))

或：

MaybeT (Reader (\r -> Nothing))

不管是哪一个，我们都可以先做一些读取，然后根据是否读取returns和Nothing来决定是否继续。这些值具有不同的外形但具有相同的功能。

我的小演示：

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

type Config = M.Map String Int

getConfig :: String -> MaybeT (Reader Config) Int
getConfig key = MaybeT $ do
  m <- ask
  return $ M.lookup key m

readAll :: Config -> Maybe (Int, Int)
readAll m =
  let r = runMaybeT $ do
      a <- getConfig "a"
      b <- getConfig "b"
      return (a, b)
  in runReader r m

main :: IO ()
main = do
  putStrLn $ show (readAll $ M.fromList [("x", 3), ("b", 4)])

我的第二个演示：

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

type Config = M.Map String Int

getConfig :: String -> ReaderT Config Maybe Int
getConfig key = ReaderT $ \r -> M.lookup key r

readAll :: Config -> Maybe (Int, Int)
readAll m =
  let r = runReaderT $ do
      a <- getConfig "a"
      b <- getConfig "b"
      return (a, b)
  in r m

main :: IO ()
main = do
  putStrLn $ show (readAll $ M.fromList [("a", 3), ("b", 4)])

使用 ReaderT Maybe 或 MaybeT Reader？

Use ReaderT Maybe or MaybeT Reader?

haskell

monad-transformers