问F#模式匹配与递归与循环&if.然后用于解析嵌套结构

EN

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
            | _ -> ()
    }