haskell - react 香蕉 1.0 monadic API : How to define recursive behaviors now?

Question

在 reactive-banana <1.0.0 中，这有效:

-- takes a start value, minimum and maximum value, flag whether counter is
-- cyclic, an increment and decrement event stream and returns a behavior
mkCounter :: (Enum a,Ord a) => a -> Maybe a -> Maybe a -> Bool
          -> Event t b -> Event t c -> Behavior t a
mkCounter start minVal maxVal cyclic incE decE = counter
  let incF curr | isNothing maxVal = succ
                | Just maxv <- maxVal, curr<maxv = succ
                | Just maxv <- maxVal, Just minv <- minVal, cyclic = const minv
                | otherwise = id
      decF curr | isNothing minVal = pred
                | Just minv <- minVal, curr>minv = pred
                | Just minv <- minVal, Just maxv <- maxVal, cyclic = const maxv
                | otherwise = id
  counter = accumB start $ ((incF <$> counter) <@ incE) `union`((decF <$> counter) <@ decE)

现在，counter 是根据自身定义的。但是在新的 monadic API 中，accumB 是一个 monadic 函数，这是我看不到如何继续的地方 - Moment 没有 MonadFix 实例，那么它现在是如何工作的？

由于显而易见的原因，这不起作用(“不在范围内:计数器”)

mkCounter :: (MonadMoment m,Enum a,Ord a) => a -> Maybe a -> Maybe a -> Bool
          -> Event b -> Event c -> m (Behavior a)
mkCounter start minVal maxVal cyclic incE decE = do
  -- .. same as above ..
  counter <- accumB start $ unions [((incF <$> counter) <@ incE)
                                   ,((decF <$> counter) <@ decE)]
  return counter

现在正确的做法是什么？提前致谢!

Answer 1

好吧，这是一个愚蠢的错误。我只需要添加 MonadFix m 作为约束，因为我没有直接使用 Moment Monad:

mkCounter :: (MonadFix m,MonadMoment m,Enum a,Ord a) => a -> Maybe a -> Maybe a -> Bool
          -> Event b -> Event c -> m (Behavior a)

然后使用 mdo 就可以正常工作了。