Question

我经常发现我称之为“纯粹适用Either”，即只有Either实例的Applicative，只要我们不实现Monad实例也是。

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 Monad实例所能做的更强大的“错误摘要”概念。为此，我发现自己一遍又一遍地实施它。

某处有标准实例吗？是否有标准名称？

Answer 1

这看起来与validation包中的AccValidation类型非常相似：http://hackage.haskell.org/package/validation-0.3.1/docs/Data-Validation.html

编辑：

特别是以下实例声明：

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)

是否有“纯粹适用的Either”的标准名称或实施？

1 个答案: