Strange evaluation of expression

[sirpi]

Hi,

I created this rule:

getUnion = response {
  set1 := {"1", "2"}
  set2 := {"3", "4"}
  set3 := {"5", "6"}
  response = {"union": set1 | set2 | set3 }
}

And it returns
{"union": {"1", "2"}}
which is suprising.

If I wrap the sets with brackets: {"union": (set1 | set2 | set3) }
then the result is what we would expect (all six values)

I am pretty sure the issue is the precedence of operations. But what I don't understand is that I cannot reach the same result {"union": {"1", "2"}} using any brackets. Can someone tell me why I get this result? Should not this be some sort of error?

Note: one of the developers in our team found this while implementing a production policy which I presented in a simplified form.

[anderseknert]

Hi there! Heh, yeah that's a peculiar case :) What happens here is that the parser will see {"union": set1 | and say, "oh, that's a map comprehension,!" — a map comprehension where the the key is "union" and the value is the set1 variable. set2 | set3 will then be considered the body of the comprehension, but in this case the body will do nothing to contribute to the key/value of the map.

[anderseknert]

Docs on object comprehensions found here: https://www.openpolicyagent.org/docs/latest/policy-language/#object-comprehensions

[sirpi]

oh, thanks! Totally makes sense but I could not figure it out on my own. And I generate maps this way on a daily basis :)