Question

我对Haskell相对较新。现在我想更全面地了解Reader Monad。它的目的和用法似乎更清晰。但是在查看:t reader函数类型的Haskell中，我看到reader :: MonadReader r m => (r -> a) -> m a。这种类型约束意味着什么？当我试图构建Reader时，例如

let myR = reader (\x -> x + 10)

我看到错误

<interactive>:21:11:
No instance for (MonadReader a0 m0) arising from a use of `reader'
The type variables `m0', `a0' are ambiguous
Possible fix: add a type signature that fixes these type variable(s)
Note: there are several potential instances:
  instance MonadReader r' m =>
           MonadReader r' (Control.Monad.Trans.Cont.ContT r m)
    -- Defined in `Control.Monad.Reader.Class'
  instance MonadReader r ((->) r)
    -- Defined in `Control.Monad.Reader.Class'
  instance (Control.Monad.Trans.Error.Error e, MonadReader r m) =>
           MonadReader r (Control.Monad.Trans.Error.ErrorT e m)
    -- Defined in `Control.Monad.Reader.Class'
  ...plus 10 others
In the expression: reader (\ x -> x + 10)
In an equation for `myR': myR = reader (\ x -> x + 10)

<interactive>:21:27:
No instance for (Num a0) arising from a use of `+'
The type variable `a0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
Note: there are several potential instances:
  instance Num Double -- Defined in `GHC.Float'
  instance Num Float -- Defined in `GHC.Float'
  instance Integral a => Num (GHC.Real.Ratio a)
    -- Defined in `GHC.Real'
  ...plus three others
In the expression: x + 10
In the first argument of `reader', namely `(\ x -> x + 10)'
In the expression: reader (\ x -> x + 10)

很明显我应该添加类型签名：

let myR = reader (\x -> x + 10) :: Reader Int Int

这有效，但我如何从读者reader :: MonadReader r m => (r -> a) -> m a的定义中暗示我应该将其定义为Reader Int Int？

（如果是Maybe，例如let m5 = return 5 :: Maybe Int，我似乎很清楚，可能是因为Maybe有一个类型参数，我不确定）

Answer 1

虽然可以默认Int Haskell不会将Monad默认为Reader。

let myR = reader (\x -> x + 10) :: Num a, MonadReader a m => m a

大致是类型检查器找到的内容，然后它将Num的默认规则应用于Int并获取。

let myR = reader (\x -> x + 10) :: MonadReader Int m => m Int

但是它没有为MonadReader定义默认值，所以此时必须返回一个模棱两可的错误。

尽管如此，如果您不想注释myR，您可以注释程序的其他部分，最终需要告知类型检查器{{1>你想要的。

请澄清Reader Monad类型的行为

1 个答案: