haskell - 和内部Maybe错误处理

Question

有Haskell模式可以消除此自定义函数的编写吗？想法是将Maybe的Nothing作为错误处理(作为包装Either的一部分):

eitherMaybeHandle :: String -> Either String (Maybe a) -> Either String a
eitherMaybeHandle err = \case
  Left e ->
    Left e
  Right Nothing ->
    Left err
  Right (Just a) ->
    Right a

Answer 1

首先，您可以使用sequence将Either a (Maybe b)转换为Maybe (Either a b)。然后，您可以将fromMaybe连同您的a类型的值一起应用到结果中，以获得Either a b。

import Data.Maybe (fromMaybe)

eitherMaybeHandle :: a -> Either a (Maybe b) -> Either a b
eitherMaybeHandle err = fromMaybe (Left err) . sequence