Why we are unable to evaluate comprehension if we have defined it inside a rule body in OPA?

The following is my sample code: https://play.openpolicyagent.org/p/oyY1GOsYaf

Here when I try to evaluate names array, it is showing: error occurred: 1:1: rego_unsafe_var_error: var names is unsafe

But when I define the same comprehension outside the allow rule definition : https://play.openpolicyagent.org/p/Xv0cF7FM8b, I am able to evaluate the selection

[
"smoke",
"dev"]

could someone help me to point out the difference and if I want to define the comprehention inside the rule is there any syntax I need to follow? Thanks in advance

Note: I am getting the final output as expected in both cases, only issue is with the names array evaluation.

The way the Rego Playground generates a query when evaluating a selection is much more simplistic than one might assume. A query will be generated from your selected text, without taking into account where in the document that text was selected. This means that even if you select a local variable inside a rule body, the query will simply contain that variable name (names, in your case); which will be perceived as a reference to a top-level variable in the document's body, even though a rule-local variable was selected. This is why your first sample returns an error, as there is no top-level variable names in the document; whereas the second sample does, and therefore succeeds.

You can test this quirk by selecting and evaluating the word hello on line 3 here: https://play.openpolicyagent.org/p/n5OPoFnlhx.

package play
    
# hello
    
hello {
    m := input.message
    m == "world"
}

Even though it's just part of a comment, it'll evaluate just as if you had selected the rule name on line 5.