State 和其他 MTL monad 的应用实例？
查看 Control 的文档。应用性，我注意到它们有某些单子的实例声明（例如IO、Maybe，尤其是ST） ，但是有没有 MTL monad 的实例，例如 State 和 RWS。相反，它看起来像是一个通用的 WrappedMonad 类型定义，我猜它是为了涵盖所有其他情况。
所以我的问题是：
为什么 MTL monad 没有
Applicative实例？到目前为止，我自己找到的最好答案是一个三岁的孩子帖子，其中有人实现了这些实例，但被立即忽略。WrappedMonad 是怎么回事？我在 Reddit 上发现了一篇 Reddit 上的帖子，简要解释了这一点，但是我想我对如何使用它感到困惑。
最终我希望能够以应用风格使用 State （就像 向我推荐），但如果我必须在代码中使用 WrappedMonad 数据构造函数，那么这似乎并不是一个胜利。我还可以完全忽略 WrappedMonad，并以与 IO、ST 相同的方式自己定义 Applicative 实例> 等等：就 return 和 ap 而言...但这似乎也很愚蠢。
Looking at the docs for Control.Applicative, I notice that they have instance declarations for certain monads (e.g. IO, Maybe and notably ST), but there are no instances for MTL monads such as State and RWS. Instead it looks like there's a general-purpose WrappedMonad type defined, which I'm guessing is to cover all other cases.
So here are my questions:
Why aren't there
Applicativeinstances for MTL monads? The best answer I've been able to find on my own so far is a three year old post, where somebody implemented these instances and was summarily ignored.What's the deal with
WrappedMonad? I found a post on Reddit that explains it briefly, but I think I'm confused about how to use it.
Ultimately I'd like to be able to use State in an applicative style (as was recommended to me), but if I have to litter my code with WrappedMonad data constructors then it doesn't seem like a win. I could also ignore WrappedMonad entirely, and define the Applicative instance myself the same way that was done for IO, ST and so on: in terms of return and ap... But that seems goofy as well.
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该文档仅根据
base包的内容生成，该包不包含State- 因此它不包含Applicative实例。如果您查看 MTL 文档，您将看到列出的Applicative实例。或者，您可以打开 GHCi，导入 Control.Monad.State 和 Control.Applicative，然后使用
:i Applicative列出Applicative或:i 的实例StateT列出StateT具有实例的类（State是StateT的类型别名，其参数之一是固定的） 。任何一个都将表明确实存在这样的例子。That documentation is generated based solely on the contents of the
basepackage, which does not containState- so it does not containApplicativeinstances for it. If you look in the MTL documentation, you'll see anApplicativeinstance listed.Alternately, you can open up GHCi, import Control.Monad.State and Control.Applicative, and use
:i Applicativeto list the instances ofApplicativeor:i StateTto list the classes for whichStateThas instances (Stateis a type alias forStateTwith one of its parameters fixed). Either will show that there is indeed such an instance.mtl monad 的 Applicative 实例无法在基础文档中列出，因为此时没有可用的 mtl。好消息是 mtl 有 Applicative 实例 StateT 例如，从 mtl-2.0.0.0 开始。
The Applicative instances for the mtl monads can't be listed in the base docs, because there's no mtl available at that point. The good news is that mtl has Applicative instances StateT for example, since mtl-2.0.0.0.