haskell - 如何优化具有多个重复案例的嵌套模式匹配？

Question

请考虑以下代码：

case action1 of
  Right a -> a
  Left (Failure1 a) -> a
  Left (Failure2 a) -> 
    case action2 a of
      Right a -> a
      _ -> error "Unexpected failure"
  _ -> error "Unexpected failure"

你可以看到我必须重复两次：with theRight和 with the errorcase。

我该如何优化呢？有可能吗？

Answer 1

这是模式保护的一个很好的应用：

case action1 of
  Right a -> a
  Left f
    | Failure1 a <- f       -> a
    | Failure2 a <- f
    , Right b <- action2 a  -> b
  _ -> error "Unexpected failure"

Answer 2

我将错误处理部分放在该部分之外case：

fromMaybe (error "Unexpected failure") $
    let eitherToMaybe = either (const Nothing) Just
    in case action1 of
          Right a           -> Just a
          Left (Failure1 a) -> Just a
          Left (Failure2 a) -> eitherToMaybe (action2 a)
          _                 -> Nothing