Arcade and attribut rules

[leonoestergaard]

Hi Im trying to create my first attribut rule. Im using ArcGis Pro 2.5

When validating the following code

var area= FeatureSetByName($datastore,"TestArea")
var fs = Contains(area,$feature)
var res = First(fs)

..

It fails with this message

Invalid expression
Error on line 3.
General evaluation error

..

If i Change First to Count, i do not get a validation error. Why can i not use first?

[JoeBorgione · Accepted solution]

I too have just entered into the Arcade/Attribute rules arena, and claim no expertise at all, but in Chasing my tail with Arcade/Attribute Rules  Jake Skinner‌ provides a solution for me.  I just wonder of the 'optional' arguments to the function aren't so optional.  Data Functions | ArcGIS for Developers