定义Monad时出现Haskell类型错误
我有一个类型定义Monad时出现Haskell类型错误,haskell,monads,Haskell,Monads,我有一个类型Effect，它表示堆栈上的效果。效果可以是修改堆栈并可选地生成输出的成功效果，也可以是在堆栈失败时记住堆栈外观的错误 data Effect a = Failure a | Effect a [Int] instance Monad Effect where return s = Effect s [] (Failure debugS) >>= _ = Failure debugS (Effect oldS oldO) >>= f =
Effect效果data Effect a = Failure a | Effect a [Int]
instance Monad Effect where
return s = Effect s []
(Failure debugS) >>= _ = Failure debugS
(Effect oldS oldO) >>= f =
case f oldS of
(Failure debugS) -> Failure debugS
(Effect newS newO) -> Effect newS (newO ++ oldO)
example.hs:2:10:
No instance for (Applicative Effect)
arising from the superclasses of an instance declaration
In the instance declaration for ‘Monad Effect’
example.hs:4:38:
Couldn't match expected type ‘b’ with actual type ‘a’
‘a’ is a rigid type variable bound by
the type signature for
(>>=) :: Effect a -> (a -> Effect b) -> Effect b
at example.hs:4:22
‘b’ is a rigid type variable bound by
the type signature for
(>>=) :: Effect a -> (a -> Effect b) -> Effect b
at example.hs:4:22
Relevant bindings include
debugS :: a (bound at example.hs:4:14)
(>>=) :: Effect a -> (a -> Effect b) -> Effect b
(bound at example.hs:4:5)
In the first argument of ‘Failure’, namely ‘debugS’
In the expression: Failure debugS
我是Haskell的新手，不习惯GHC的错误消息。我应该如何更正此错误？从GHC 7.10开始，不幸的是，由于以下原因，您需要实现
MonadApplicativeFunctor>>=(>>=) :: Monad m => m a -> (a -> m b) -> m b
mb故障调试MA>=(>>=) :: Monad m => m a -> (a -> m a) -> m a -- Wrong!
您返回的
调试（>>=）（>>=）：：Monad m=>ma->（a->ma）->maFailuredata Effect a=Failure FailMsg |…>=FailMsgFailMsga>=data Effect sa=Failure s | Effect sa[Int]实例Monad（Effect s），其中…Effect