在 WriterT 中包装 Maybe 以添加日志记录

Question

我完全被困住了，我觉得我现在可以使用一些帮助，只是为了保持清醒。我想要一些简单的东西，例如将日志记录功能添加到返回 Maybe 的函数中，但无论我多么努力，我都无法获得正确的类型。

这是（据我所知）我能得到的最接近的：

import Data.Maybe
import Control.Monad
import Control.Monad.Writer

q :: Integer -> Maybe Integer
q x = if x > 7
      then Just x
      else Nothing

qlog :: Integer -> WriterT [String] Maybe Integer
qlog x = do
  tell ["Querying " ++ show x]
  return $ q x

这仍然给我一个打字错误：

Couldn't match type ‘Maybe Integer’ with ‘Integer’
Expected type: WriterT [String] Maybe Integer
  Actual type: WriterT [String] Maybe (Maybe Integer)
In a stmt of a 'do' block: return $ q x
In the expression:
  do { tell ["Querying " ++ show x];
       return $ q x }
In an equation for ‘qlog’:
    qlog x
      = do { tell ["Querying " ++ show x];
             return $ q x }

我应该如何调整代码以使其编译和工作？

非常感谢您的帮助，Haskeller 同胞！

Answer 1

为了进行类型检查，内部 monad 应该是 lifted:

lift :: (Monad m, MonadTrans t) => m a -> t m a

所以，你需要写而不是return $ q x：lift $ q x;如：

qlog :: Integer -> WriterT [String] Maybe Integer
qlog x = do
  tell ["Querying " ++ show x]
  lift $ q x