some x in xs: semantics with shadowing, error unclear

[srenatus]

$ opa run
OPA 0.36.0-dev (commit b20b7e6e5-dirty, built at 2022-01-04T11:30:15Z)

Run 'help' to see a list of commands and check for updates.

> import future.keywords
> y := "a"
Rule 'y' defined in package repl. Type 'show' to see rules.
> some [a, 10] in [[y, 10]]
+-----+
|  a  |
+-----+
| "a" |
+-----+

That much is clear: a is a declared variable, and it is bound to whatever [[y, 10]] resolved to ([["a", 10]]).

However, shadowing y yields:

> some [y, 10] in [[y, 10]]
1 error occurred: 1:1: rego_unsafe_var_error: var y is unsafe

The problem is not that y shadows the declared (top-level) var y, at least not alone:

> x := "?"
Rule 'x' defined in package repl. Type 'show' to see rules.
> some [x, 10] in [[y, 10]]
+-----+
|  x  |
+-----+
| "a" |
+-----+
>

x shadows in the same way, but it does yield a result.

[srenatus]

Moved. Kind of forgot I was looking into this 😅

[tsandall]

Aha, I guess we're rewriting the domain when we aren't expecting to?

some [y, 10] in [[y, 10]]

Becomes

some [$1, 10] in [[$1, 10]]

Becomes

[_1, 10] = [[_1, 10]][_]

Which is unsafe because _1 is never assigned to anything but itself... the error reporting does the expected thing by reporting the user declared variable.