3

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的函数中Maybe，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，Haskellers 同胞！

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1 回答 1

3

为了进行类型检查，应该解除内部单子：

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

所以，而不是return $ q x 你需要写lift $ q x：如：

qlog :: Integer -> WriterT [String] Maybe Integer
qlog x = do
  tell ["Querying " ++ show x]
  lift $ q x
于 2016-03-27T11:33:54.430 回答