小编nic*_*oan的帖子

我需要为特定数据类型声明一个Monad实例:

data M m a = Mk (m (Maybe a))

instance (Monad m) => Monad (M m) where
  return x = Mk (m (Just x))
  Mk (m (Nothing)) >>= f = Mk (m (Nothing))
  Mk (m (Just x)) >>= f = f x

但我得到:

test.hs:6:7: Parse error in pattern: m
Failed, modules loaded: none.

这可能很简单,但我无法弄清楚!