Excel powerquery函数可选参数

如何使用可选参数pls创建电源查询函数？ 我尝试过创建函数语法的各种排列，目前如下所示：

let
    fnDateToFileNameString=(inFileName as text, inDate as date, optional inDateFormat as nullable text) => 
    let

        nullCheckedDateFormat = Text.Replace(inDateFormat, null, ""),
        value =     if nullCheckedDateFormat = "" 
            then inFileName
            else Text.Replace(inFileName, inDateFormat, Date.ToText(inDate, inDateFormat ))
    in 
        value
in
    fnDateToFileNameString

let
    fnDateToFileNameString=(inFileName as text, inDate as date, optional inDateFormat as nullable text) => 
    if inDateFormat = null or inDateFormat = ""
    then inFileName
    else Text.Replace(inFileName, inDateFormat, Date.ToText(inDate, inDateFormat ))
in
    fnDateToFileNameString

我通过了一个测试，看起来：

 = fnDateToFileNameString("XXXXXXXXXXXXXXXXXX", #date(2015, 3, 21), null)

这意味着：

"An error occurred in the fnDateToFileNameString" query. Expression.Error: we cannot convert the value null to type Text.
    Details:
    Value=
    Type=Type

---

问题出在Text.Replace中，因为第二个参数不能为null：在文本值中替换字符

null

没有意义。如果将nullCheckedDateFormat更改为以下格式，则函数将正常工作：

nullCheckedDateFormat=如果inDateFormat=null，则“else inDateFormat，

这对于下一步来说有点多余，因此您可以像这样重写函数：

let
    fnDateToFileNameString=(inFileName as text, inDate as date, optional inDateFormat as nullable text) => 
    let

        nullCheckedDateFormat = Text.Replace(inDateFormat, null, ""),
        value =     if nullCheckedDateFormat = "" 
            then inFileName
            else Text.Replace(inFileName, inDateFormat, Date.ToText(inDate, inDateFormat ))
    in 
        value
in
    fnDateToFileNameString

let
    fnDateToFileNameString=(inFileName as text, inDate as date, optional inDateFormat as nullable text) => 
    if inDateFormat = null or inDateFormat = ""
    then inFileName
    else Text.Replace(inFileName, inDateFormat, Date.ToText(inDate, inDateFormat ))
in
    fnDateToFileNameString