Question

在阅读瓦德尔关于僧mon的论文(并 sections一些部分)之后,我决定通过该文件更加密切地工作,为他所描述的每个僧.确定ctor的遗嘱和参考案例。使用同义词

type M a = State -> (a, State)
type State = Int

瓦勒用于界定国家僧mon,我有以下(使用相关名字,以便我稍后能够用新的类型声明来界定这些名字)。

fmap  :: (a -> b) -> M a -> M b
fmap  f m = st -> let (a, s) = m st in (f a, s)

pure  :: a -> M a
pure  a = st -> (a, st)

(<@>) :: M (a -> b) -> M a -> M b
sf <@> sv = st -> let (f, st1) = sf st
                       (a, st2) = sv st1
                    in (f a, st2)

return  :: a -> M a
return  a = pure  a

bind :: M a -> (a -> M b) -> M b
m `bind` f = st -> let (a, st1) = m st
                        (b, st2) = f a st1
                     in (b, st2)

当我转向在新的类型声明中使用一种类型构造,例如

newtype S a = S (State -> (a, State))

一切都相距甚远。一切只是略有改动,例如,

instance Functor S where
 fmap f (S m) = S (st -> let (a, s) = m st in (f a, s)) 

instance Applicative S where
 pure a = S (st -> (a, st))

但是,由于lam布达的表述是隐藏在这种建筑内,因此GHC没有发生任何事情。现在,我看到的唯一解决办法是确定一项职能:

isntThisAnnoying s (S m) = m s

a. 保证金的固定价值和实际收益,例如:

fmap f m = S (st -> let (a, s) = isntThisAnnoying st m in (f a, s))

是否有另一种方式不使用这些辅助职能?

Answer 1

如果你看here,你将看到它们这样界定:

newtype State s a = State { runState :: (s -> (a,s)) }

为了给内人一个名字。

Answer 2

The usual way is to define newtype newtype S a = S {runState : State -> (a, State)}. Then instead of your isntThisAnnoying s (S m) you can write runState t s where t is the same as S m.
You have to use a newtype because type synonyms cannot be typeclass instances.

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