Question

我在F＃中使用第三方供应商的API。在初始化时，API返回一个嵌套的msg容器的C＃对象。它填充了状态消息，可能包含错误消息。供应商提供了一个C＃样本解析例程，我已将其移植到F＃。

代码示例循环遍历嵌套的msg容器，提取致命和非致命错误，然后返回类型为BBResponseType * string的元组列表

回应枚举：

type BBResponseType =
    | Status = 0
    | Data = 1
    | Error = 2
    | FatalError = -1

我到F＃的端口看起来像这样：

 member private this.ProcessStatusMsg(eventObj: Blpapi.Event) = 
     let responseLst = List<(BBResponseType * string)>()
     for msg in eventObj do
         if msg.MessageType.Equals(SUBSTARTED) then
             if msg.GetElement(EXCEPTIONS).NumValues > 0 then // <- check for errors/exceptions                        
                 let e = msg.GetElement(EXCEPTIONS)
                 for i in 0..e.NumValues-1 do                                
                     let error = e.GetValueAsElement(i)
                     let field = error.GetElementAsString(FieldID)
                     let reason = error.GetElement(REASON)
                     let message = sprintf "Subscription Started w/errors( Field:   %s \n   Reason:   %s)" field (reason.GetElementAsString(DESCRIPTION))                                
                     responseLst.Add(BBResponseType.Error, message)
             else                                
                 let message = sprintf "Subscription Started"         
                 responseLst.Add(BBResponseType.Status, message)

         if msg.MessageType.Equals(SUBSCFAILURE) then // <- check for subscriptions failure
             if msg.HasElement(REASON) then
                 let reason = msg.GetElement(REASON)
                 let desc = reason.GetElementAsString(DESCRIPTION)
                 let message = sprintf "Real-time Subscription Failure:    %s" desc                            
                 responseLst.Add(BBResponseType.FatalError, message)
             else
                 let message = sprintf "Subscription Failure:  (reason unknown) "                                                
                 responseLst.Add(BBResponseType.FatalError, message)
     responseLst

在我完成它之后，我看着它，并想：“哇，这就像你可以得到的那样无功能，仍然在F＃中编码。”

它似乎比C＃版本更清晰简洁，但我认为必须有更好的方法来完成所有这些，而不使用这么多循环和if / then's。

如何使用模式匹配和递归来更好地解析这些嵌套结构？

Answer 1

几个指针：

而不是返回元组列表返回一个元组序列 - 使用seq { }计算表达式来创建序列。
将if / else部分提取为Message -> (BBResponseType * string)类型的函数，并在seq表达式
在这个新函数（将Message转换为元组）中使用模式匹配来确定要返回的是什么类型的（BBResponseType *字符串）。

Answer 2

补充@ Ankur的答案：

member private this.ProcessStatusMsg(eventObj: Blpapi.Event) =
    // 0. Define a parameterized active pattern to turn if/else into pattern matching
    let (|Element|_|) e msg =
        if msg.HasElement(e) then
            Some <| msg.GetElement(e)
        else None

    // 1. Wrapping the whole method body in a sequence expression 
    seq {
        for msg in eventObj do
            // 2. Extracting if/else part and using it in sequence expression
            match msg.MessageType with
            // 3. Using pattern matching to figure out what kind (BBResponseType * string) to return
            | SUBSTARTED ->
                match msg with
                // 4. Use active pattern to pattern match on message directly
                | Element EXCEPTIONS e when e.NumValues > 0 ->
                    for i in 0..e.NumValues-1 do                                
                        let error = e.GetValueAsElement(i)
                        let field = error.GetElementAsString(FieldID)
                        let reason = error.GetElement(REASON)
                        let message = sprintf "Subscription Started w/errors( Field:   %s \n   Reason:   %s)" field (reason.GetElementAsString(DESCRIPTION))                                
                        yield (BBResponseType.Error, message)
                | _ ->                                
                    let message = sprintf "Subscription Started"         
                    yield (BBResponseType.Status, message)

            | SUBSCFAILURE ->
                match msg with
                | Element REASON reason ->
                    let desc = reason.GetElementAsString(DESCRIPTION)
                    let message = sprintf "Real-time Subscription Failure:    %s" desc                            
                    yield (BBResponseType.FatalError, message)   
                | _ ->
                    let message = sprintf "Subscription Failure:  (reason unknown) "                                                
                    yield (BBResponseType.FatalError, message)

            // There are probably more cases, processing them here
            | _ -> ()
    }

评论中的第1,2和3点来自另一个答案。我添加了第0点和第4点来使用活动模式以便于模式匹配。

F＃模式匹配＆amp;递归vs循环＆amp; if..then用于解析嵌套结构

2 个答案: