haskell - Haskell 中的嵌套状态

Question

我正在尝试定义具有不同类型状态的状态机系列。特别是，更“复杂”的状态机具有通过组合更简单状态机的状态形成的状态。

(这类似于面向对象的设置，其中一个对象具有多个属性，这些属性也是对象。)

这是我想要实现的简化示例。

data InnerState = MkInnerState { _innerVal :: Int }

data OuterState = MkOuterState { _outerTrigger :: Bool, _inner :: InnerState }

innerStateFoo :: Monad m => StateT InnerState m Int
innerStateFoo = do
  i <- _innerVal <$> get
  put $ MkInnerState (i + 1)
  return i

outerStateFoo :: Monad m =>  StateT OuterState m Int
outerStateFoo = do
  b <- _outerTrigger <$> get
  if b
    then
       undefined
       -- Here I want to "invoke" innerStateFoo
       -- which should work/mutate things
        -- "as expected" without
       -- having to know about the outerState it
       -- is wrapped in
    else
       return 666

更一般地说，我想要一个通用框架，其中这些嵌套更复杂。这是我想知道该怎么做的事情。

class LegalState s

data StateLess

data StateWithTrigger where
  StateWithTrigger :: LegalState s => Bool -- if this trigger is `True`, I want to use
                                   -> s    -- this state machine
                                   -> StateWithTrigger

data CombinedState where
  CombinedState :: LegalState s => [s] -- Here is a list of state machines.
                                -> CombinedState -- The combinedstate state machine runs each of them

instance LegalState StateLess
instance LegalState StateWithTrigger
instance LegalState CombinedState

liftToTrigger :: Monad m, LegalState s => StateT s m o -> StateT StateWithTrigger m o
liftToCombine :: Monad m, LegalState s => [StateT s m o] -> StateT CombinedState m o

对于上下文，这就是我想用这个机器实现的目标:

我想设计这些叫做“流转换器”的东西，它们基本上是有状态的函数:它们消耗一个 token ，改变它们的内部状态并输出一些东西。具体来说，我对输出为 bool 值的一类流转换器感兴趣；我们将这些称为“监视器”。

现在，我正在尝试为这些对象设计组合器。他们之中有一些是:

一个 pre组合器。假设 mon是显示器。然后，pre mon是一个总是产生 False 的监视器在第一个 token 被消耗然后模仿 mon 的行为之后好像现在正在插入以前的 token 。我想模拟 pre mon 的状态与 StateWithTrigger在上面的示例中，因为新状态与原始状态一起是 bool 值。

一个 and组合器。假设 m1和 m2是监视器。然后，m1 `and` m2是一个监视器，它将 token 提供给 m1，然后提供给 m2，然后生成 True如果两个答案都是真的。我想模拟 m1 `and` m2 的状态与 CombinedState在上面的示例中，因为必须保持两个监视器的状态。

Answer 1

For context, this is what I want to achieve with this machinery:

I want to design these things called "Stream Transformers", which are basically stateful functions: They consume a token, mutate their internal state and output something. Specifically, I am interested in a class of Stream Transformers where the output is a Boolean value; we will call these "monitors".

我认为你想要达到的目标并不需要太多的机器。

newtype StreamTransformer input output = StreamTransformer
  { runStreamTransformer :: input -> (output, StreamTransformer input output)
  }

type Monitor input = StreamTransformer input Bool

pre :: Monitor input -> Monitor input
pre st = StreamTransformer $ \i ->
  -- NB: the first output of the stream transformer vanishes.
  -- Is that OK? Maybe this representation doesn't fit the spec?
  let (_, st') = runStreamTransformer st i
  in  (False, st')

and :: Monitor input -> Monitor input -> Monitor input
and left right = StreamTransformer $ \i ->
  let (bleft,  mleft)  = runStreamTransformer left  i
      (bright, mright) = runStreamTransformer right i
  in  (bleft && bright, mleft `and` mright)

这个 StreamTransformer不一定是有状态的，但承认有状态的。您不需要(而且 IMO 不应该!在大多数情况下!!)为了定义这些类型类(或者确实永远!:)，但这是另一个主题)。

notStateful :: StreamTransformer input ()
notStateful = StreamTransformer $ \_ -> ((), notStateful)

stateful :: s -> (input -> s -> (output, s)) -> StreamTransformer input output
stateful s k = StreamTransformer $ \input ->
  let (output, s') = k input s
  in  (output, stateful s' k)

alternateBool :: Monitor anything
alternateBool = stateful True $ \_ s -> (s, not s)