我对Haskell相对较新。现在我试图更全面地了解Reader Monad。它的目的和用法似乎更清楚。但在Haskell查看:t reader
函数的类型时,我看到了reader :: MonadReader r m => (r -> a) -> m a
。这种类型约束是什么意思? 当我试图建构阅读器,如请澄清Reader Monad类型的行为
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)
很显然,我要补充的类型签名:
let myR = reader (\x -> x + 10) :: Reader Int Int
这工作,但我怎么会从读者reader :: MonadReader r m => (r -> a) -> m a
的这个定义暗示我应该将其定义为Reader Int Int
?
(在可能的情况下,例如,let m5 = return 5 :: Maybe Int
它似乎是明确的对我来说,可能是因为也许有一种类型的参数,我不知道)
什么是上下文?我可以在GHCi提示符处写'let myR =(\ x - > x + 10)',其中myR ::(数字a,MonadReader a m)=> m a','myR 5'的计算结果为15。 – chepner