无法从 (MonadRandom r) 推导出 (Functor r)
下面的简单代码
import Control.Monad
import Control.Monad.Random
psum :: (MonadRandom r) => Int -> r Double -> r Double
psum n x = fmap sum $ replicateM n x
会产生错误：
Could not deduce (Functor r) arising from a use of `fmap'
from the context (MonadRandom r)
这对我来说很奇怪，因为
class (Monad m) => MonadRandom m where ...
在 Control.Monad.Random.Class 源文件中，并且由于 monads 是函子，GHC 应该推断出 r 在我的上下文中是一个函子。我还尝试导入 Control.Monad.Random.Class 但没有成功。
手动添加 r 上的 Functor 约束是可行的，但我发现这很丑陋。
我在这里缺少什么？
The simple following code
import Control.Monad
import Control.Monad.Random
psum :: (MonadRandom r) => Int -> r Double -> r Double
psum n x = fmap sum $ replicateM n x
yields the error:
Could not deduce (Functor r) arising from a use of `fmap'
from the context (MonadRandom r)
This is weird to me because of
class (Monad m) => MonadRandom m where ...
in Control.Monad.Random.Class source file, and since monads are functors, GHC should have deduced that r is a functor in my context. I also tried to import Control.Monad.Random.Class with no success.
Adding manually the Functor constraint on r works, but I find this quite ugly.
What am I missing here ?
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理论上，Monad 是函子，但遗憾的是，Functor 并不是
Monad的超类，没有什么充分的理由。您还可以使用
liftM代替fmap，而不是添加Functor r。编辑：似乎确实没有充分的理由。这些类在 Haskell 1.3 中一起引入，并且超类已经存在并用于
MonadPlus和MonadZero。Theoretically monads are functors, but sadly
Functoris not a superclass ofMonadfor no good reason.Instead of adding
Functor ryou can also useliftMinstead offmap.Edit: There really seems to be no good reason. The classes were introduced together in Haskell 1.3, and superclasses already existed and were used for
MonadPlusandMonadZero.