如何優化與多個重複案例的嵌套模式匹配？

Question

請考慮下面的代碼：

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，我不得不重複自己兩次：與Right並與error箱子。

我該如何優化？它有可能嗎？

Answer 1

我把錯誤的case部分以外的部分的處理：

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 

Answer 2

這對pattern guards一個良好的應用：

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