haskell - 将 Maybe 包装在 WriterT 中以添加日志记录

Question

我完全陷入困境，我觉得我现在可以使用一些帮助，只是为了保持理智。我想要一些简单的东西，比如向返回 Maybe 的函数添加日志记录功能，但无论我如何努力，我就是无法获得正确的类型。

这是(据我所知)我能得到的最接近的结果:

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

这仍然会导致我输入错误:

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并运行？

非常感谢您的帮助，Haskeller 同胞们!

Answer 1

为了进行类型检查，内部单子(monad)应该是 lifted :

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

因此，您需要编写: lift $ q x; 而不是 return $ q x如:

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