Forum Discussion
Understanding Expression.Evaluate - isn't working
When you use Expression.Evaulate you need to include every variable and library function you want to make avialable in a record. In your case:
Expression.Evaulte(#"Need Help", [Long Step Name = #"Long Step Name"])
Thanks artemus
So in the Final step below, Expression.Evalute doesn't know what to evaluate, and I have to tell it where Long Step Name is defined?
Why am I saying:
Long Step Name = #"Long Step Name"
Is "Long Step Name" how Evaluate.Expression sees it, and #"Long Step Name" is the reference to the step?
let
Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WUtJR8krMUzAyMDJQitUB88G0Z0lqLpAdnJiTWgwWSATyDMGsJCDLCMxKBrKMlWJjAQ==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t, Column2 = _t]),
#"Long Step Name" = Source,
#"Need Help" = Expression.Identifier("Long Step Name") & "[" & Table.ColumnNames(#"Long Step Name"){0} & "]",
Final = Expression.Evaluate(#"Need Help", [Long Step Name = #"Long Step Name"])
in
Final
Hi edhans ,
My friend LarsSchreiber and I have written an article about the evaluation context in M that might explain a bit of this here (hopefully 😉 ) https://ssbi-blog.de/blog/technical-topics-english/the-environment-concept-in-m-for-power-query-and-power-bi-desktop-part-3/
You can find the official language specification here now: https://docs.microsoft.com/en-us/powerquery-m/power-query-m-language-specification
BTW: The "equivalent" of Table.Column is Record.Field (if you want to do some dynamic field referencing on row-level)
Thanks ImkeF for the reminder on Record.Field. I've used it before but not enough I could just code with it, so need to go back and refresh my memory with it.
So many ways to do the same or similar things in Power BI, but performance considerations, or just code readability, can be very different. 👍