如何定义返回多态值的函数

Question

我希望下面的函数noToState有效，它会在所有状态之间传播，找到与给定状态号匹配的状态并返回状态。

class State a where
  allStates :: [a]

class (State a) => IntState a where
-- starting from zero, consecutive                                                 
  stateNo :: a -> Integer

noToState :: (IntState a) => Integer -> a
noToState n = case lookup n $ zip (map stateNo allStates) allStates of
  Just st -> st
  Nothing -> undefined  -- this should never happen

但是，它会产生错误：Could not deduce (IntState a0) arising from a use of ‘stateNo’。

所以在代码中我犯了什么错误？我应该怎么解决它们？感谢。

Answer 1

将其更改为：

noToState :: (IntState a) => Integer -> a
noToState n = case lookup n $ zip (map stateNo allSts) allSts of
  Just st -> st
  Nothing -> undefined  -- this should never happen
  where allSts = allStates

问题是你使用allStates两次，它可能是不同的东西