Kleisli, ReaderT, Reader 在Scalaz中是一样的难道只是巧合吗 [英] Is it just a coincidence that Kleisli, ReaderT, and Reader are the same in Scalaz

问题描述

在 Scalaz 中

• Kleisli[F, A, B] 是 A => 的包装器.F[B].
• ReaderT[F, A, B] -- 阅读器 monad 转换器 -- 只是 Kleisli[F, A, B] 的别名.
• Reader[A, B] monad 是 ReaderT 的特化，具有标识 monad Id:
  type Reader[A, B] = ReaderT[Id, A, B].

• Kleisli[F, A, B] is a wrapper for A => F[B].
• ReaderT[F, A, B] -- reader monad transformer -- is just an alias of Kleisli[F, A, B].
• Reader[A, B] monad is a specialization of ReaderT with identity monad Id:
  type Reader[A, B] = ReaderT[Id, A, B].

Kleisli、ReaderT 和 Reader 是巧合还是有一些更深层的原因Scalaz 中的同构?

Is it just a coincidence or there are some deeper reasons that Kleisli, ReaderT, and Reader are isomorphic in Scalaz ?

推荐答案

你可以把它想象成通过两条不同的路线到达同一个地方.一方面，您从 reader monad 开始，它只是一种函数的包装器.然后您意识到您想将此阅读器功能集成到具有其他效果"的更大 monad 中，因此您创建了一个 ReaderT monad 转换器.在这一点上，将您的原始 Reader[E, ?] 实现为 ReaderT[Id, E, ?] 是有意义的.

You can think of it as arriving at the same place by two different routes. On one side you start with the reader monad, which is simply a kind of wrapper for functions. Then you realize that you want to integrate this reader functionality into a larger monad with other "effects", so you create a ReaderT monad transformer. At that point it makes sense to implement your original Reader[E, ?] as ReaderT[Id, E, ?].

另一方面，您需要一个类型来表示 Kleisli 箭头(即具有一元返回类型的函数).事实证明，这与 ReaderT 是一回事，所以您只需将其设为别名即可.

From the other side, you want a type to represent Kleisli arrows (i.e. functions with a monadic return type). It turns out that this is the same thing as ReaderT, so you just make that an alias.

结果证明"部分并没有什么特别神秘的地方.这有点像如果你开始使用 Addable 类型类来处理类似数字的事情，然后决定让它更通用，最终得到一个只提供关联加法"的类型类像"操作.你彻底改造了Semigroup！不过，出于历史或教学原因，或者只是为了方便，您可能仍希望保留 Addable 名称.

There's nothing terribly mysterious about the "it turns out" part. It's a little like if you started out with an Addable type class for number-like things, then decide to make it more generic, and eventually end up with a type class that just provides an associative "addition-like" operation. You've reinvented Semigroup! You may still want to keep the Addable name around, though, for historical or pedagogical reasons, or just for convenience.

这就是 Reader 和 ReaderT 发生的所有事情——您不需要需要这些别名，但它们很方便，并且可能会有所帮助提高代码的清晰度.

That's all that's happening with Reader and ReaderT—you don't need these aliases, but they can be convenient, and may help improve the clarity of your code.

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