Shouldn't List().ContainsOnly(List()) return true?

Seems related to:

I’m trying to check if two lists are identical.


  • List(1,2).ContainsOnly(List(1,2)) returns true
  • List(1,2).ContainsOnly(List(1,2,3)) returns false

but now the weird one

  • List().ContainsOnly(List()) returns false, I expected true

The way around this is to use

  • List() = List()returns true
    and it works for the rest as expected
  • List(1,2) = List(1,2) returns true
  • List(1,2) = List(1,2,3) returns false

Community, have any thoughts on this? Does this seem expected to you? @jeo @Paul_Danyliuk @Federico.Stefanato @Johg_Ananda?

I think the answer is in the description of what ContainsOnly() does:

It’s searching for something … and you can’t search an empty list, so its false from the start. It doesn’t matter if you do:
List().ContainsOnly(List("foo")) - this isn’t true because an empty list doesn’t have it
List().ContainsOnly(List()) - this isn’t true either because an empty list can’t have anything in it

Kind of like how zero can be true:


I see that perspective.

Another way to say this could be List() clearly only contains nothing, and therefore any other list whose only contents are nothing must also only contain nothing.

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Interesting question, I had to research this.

It seems that mathematics agree with you, the empty set is a part of all sets…

I agree with you that this is unexpected.

I would expect the following equivalency to always hold true:


is just a shorthand for


and also this expression:

Sort(Unique(ListA)) = Sort(Unique(ListB))

And this holds in all cases except if both ListA and ListB are List(), in which case all three expressions should return true, but only the latter two do. That strikes me as inconsistent.

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