haskell - ReaderT 的 Ask 未正确推导 Monad 类型

Question

我正在尝试用纯脚本中的 ReaderT 编写代码。但我的编译器没有正确推断类型

type Doc' = ReaderT Level (Writer (Array String)) String

line' :: String -> Doc' 
line' input = do 
       space <-  ask  -- error line
       lift $ tell $ [(power " " space) <> input <> "\n"] 

抛出的错误是

Could not match type
        
    Unit
        
  with type

    String

  while trying to match type t0 t1
     with type ReaderT @Type Int (WriterT (Array String) Identity) String
  while checking that expression (bind ask) (\space ->
                                             (apply lift) ((...) [ ...
                                                                 ]
                                                          )
                                          )
  has type ReaderT @Type Int (WriterT (Array String) Identity) String
  in value declaration line'

  where t0 is an unknown type
        t1 is an unknown type

Why the ask can't deduct the type automatically... I'm very new to FP,am I doing something fundamentally wrong?Why its saying cant match type of Unit with String. I'm not using Unit anywhere

Answer 1

这不起作用的原因是 tell 的类型为 tell :: (MonadTell w m, Monad m) => w -> m Unit 。您使用lift :: (MonadTrans t, Monad m) => m a -> t m a ，但这不会“返回类型”a，因此您的line'的类型是:

--                                           Unit 🖟  🖟
type Doc' = ReaderT Level (Writer (Array String)) <strong>Unit</strong>

line' :: String -> Doc' 
line' input = do 
       space <- ask
       lift $ tell $ [(power " " space) <> input <> "\n"]