The Ellipsis/Not Implemented Error In M

Recently I was creating a parameter in Power BI Desktop and had it configured something like this:

I didn’t bother to choose anything in the Default Value dropdown box, and when I looked at the code for the parameter in the Advanced Editor I saw this:

"a"
meta
[IsParameterQuery=true,
 List={"a", "b", "c"},
 DefaultValue=...,
 Type="Text",
 IsParameterQueryRequired=true]

I was interested to know what the ellipsis symbol (three dots …) in the DefaultValue field in the record meant, and looking in the Language Reference I found that in M it can be used for two purposes: as an open record marker (maybe something for a future blog post) and, as in this case, as a quick way of returning an error. The language reference says that it is directly equivalent to the following expression, which returns a “Not Implemented” error:

error Error.Record("Expression.Error", "Not Implemented")

But from what I can see, it actually returns a “Value was not specified” error instead. Here’s another example of how it can be used in a function definition, and what it returns:

let
    MyFunction = (x,y) => if x>y then true else ...,
    Output = MyFunction(0,1)
in
    Output

It’s not something I think I’ll be using in my own code, but it’s good to know what it means!