Quick question on set analysis formula

[kalmirall]

I'm trying to add in a piece to my set analysis formula to add a condition of having an assignment submission date in addition to an attempt.  The first formula is working fine:

Count(DISTINCT {$ <[assignmentsubmissions.attempt]={">=1"}>} [CanvasSchools.CanvasEnrollments.user_id])

But when I try to add in that second piece:

Count(DISTINCT {$ <[assignmentsubmissions.attempt]={">=1"}, len([assignmentsubmissions.submitted_at])={"<>0"}>} [CanvasSchools.CanvasEnrollments.user_id])

The formula wizard says I have an error in my set modifier expression.  I'm new to set analysis so any tweaks/help appreciated- hoping there is a quick and easy answer!

Thanks!

[swuehl]

You need to use field names, no expressions allowed, left of the equal sign in a set modifier.

Maybe try like this

Count(DISTINCT {$ <[assignmentsubmissions.attempt]={">=1"}, [assignmentsubmissions.submitted_at] ={"*"}>} [CanvasSchools.CanvasEnrollments.user_id])