定义monad时的Haskell类型错误

Question

i具有Effect类型，该类型表示对堆栈的影响。Effect可以是成功修改堆栈并选择生成输出的成功效果，也可以是一个错误，可以记住堆栈失败时的外观。

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)

绑定应串联结果(以相反的顺序，我知道(。但是，目前GHC给我以下错误消息：

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，没有使用GHC的错误消息。我应该如何纠正此错误？

Answer 1

从GHC 7.10开始，不幸的是，您需要由于函数插入式 - 月球建议而实现Monad，Applicative和Functor。

您获得类型错误的原因是因为>>=的类型为：

(>>=) :: Monad m => m a -> (a -> m b) -> m b

这意味着您通过传递的功能给出 type m b。但是，由于您还给Failure debugS，这是m a类型的类型不匹配的类型，因为它本质上迫使>>=符合：

(>>=) :: Monad m => m a -> (a -> m a) -> m a  -- Wrong!

您返回的debugS必须不同。