Question

我是monad变形金刚的新手，很抱歉这个问题很简单。我有值val :: MaybeT IO String和函数fn :: String -> IO [String]。所以绑定后，我有val >>= liftM fn :: MaybeT IO (IO [String])。如何删除重复的IO monad并获得类型为MaybeT IO [String]的结果？

Answer 1

使用lift（或liftIO）代替liftM。

> :t val >>= lift . fn
val >>= lift . fn :: MaybeT IO [String]

liftM用于在monad中应用纯函数。 lift和liftIO用于将操作提升到变压器中。

liftM  :: Monad m => (a -> b) -> m a -> m b
lift   :: (Monad m, MonadTrans t) => m a -> t m a
liftIO :: MonadIO m => IO a -> m a

Monad变形金刚monad复制

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