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在WriterT中包装Maybe以添加日志记录

writer-monad option-type monad-transformers haskell

3

SkyWriter · 技术社区 · 10 年前

我完全被困了,我觉得我现在需要一些帮助,只是为了保持清醒。我想要一些简单的东西,比如向返回 Maybe ,但无论我怎么努力,我都不能把类型弄对。

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

这仍然会产生一个键入错误:

Couldn't match type âMaybe Integerâ with âIntegerâ
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 âqlogâ:
    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?

非常感谢你的帮助,哈斯凯勒兄弟!

1 回复 | 直到 10 年前

1

3

behzad.nouri 10 年前

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,代替 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