“纯适用的任何一种"是否有标准名称或实现? [英] Is there a standard name or implementation of the "purely applicative Either"?

问题描述

我经常找到我所谓的纯粹适用的Either"的用法，即只要我们也没有实现Monad实例，就可以使用Either和Applicative实例.

I frequently find use for what I call the "purely applicative Either", i.e. Either with the Applicative instance available so long as we don't implement a Monad instance as well.

newtype AEither e a = AEither { unAEither :: Either e a }
  deriving Functor

-- technically we only need Semigroup
instance Monoid e => Applicative (AEither e) where
  pure a = AEither (pure a)
  AEither e1 <*> AEither e2 = AEither (combine e1 e2) where
    combine (Right f) (Right a) = Right (f a)
    combine (Left m1) (Left m2) = Left (m1 <> m2)
    combine (Left m ) _         = Left m
    combine _         (Left m ) = Left m

这是一个非常有用的Applicative，因为它提供了比Either的Monad实例更强大的错误汇总"概念.为此，我发现自己要一遍又一遍地实现它.

It's a really useful Applicative as it provides a more powerful notion of "summarization of error" than Either's Monad instance can do. To that end, I find myself implementing it over-and-over again.

某处有一个标准实例吗?甚至有一个标准名称吗?

Is there a standard instance somewhere? Is there even a standard name?

推荐答案

这与validation包中的AccValidation类型非常相似:

This looks pretty similar to the AccValidation type in the validation package: http://hackage.haskell.org/package/validation-0.3.1/docs/Data-Validation.html

尤其是以下实例声明:

instance Semigroup err => Apply (AccValidation err) where
  AccFailure e1 <.> AccFailure e2 =
    AccFailure (e1 <> e2)
  AccFailure e1 <.> AccSuccess _  =
    AccFailure e1
  AccSuccess _  <.> AccFailure e2 =
    AccFailure e2
  AccSuccess f  <.> AccSuccess a  =
    AccSuccess (f a)

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