Rego - how to mimic set generation using functions - open-policy-agent

Question

I have a rule that I expect to be reused by a variety of modules. I figured, let's turn that into a function, have the modules pass their input into a function and use a set comprehension like approach, but I'm running into the "functions must not produce multiple outputs for same inputs" error.
Here's a contrived example of what I want to accomplish. I'm thinking I'm going about this the wrong way and there's another approach to this type of problem in Rego.
Classic generator:
arr = [1,2,3,4]
result[entry] {
itm := arr[i]
r := itm % 2
r == 0
entry := { "type": "even", "val": itm }
}
result[entry] {
itm := arr[i]
r := itm % 2
r == 1
entry := { "type": "odd", "val": itm }
}
This works as expected.
"result": [
{
"type": "even",
"val": 2
},
{
"type": "even",
"val": 4
},
{
"type": "odd",
"val": 1
},
{
"type": "odd",
"val": 3
}
]
Here's the function approach that will trigger this error. I am passing that t_label variable to give the function some argument, but it's not really important.
f(t_label) := q {
q := [ entry | itm := arr[i]
r := itm % 2
r == 0
entry := { t_label: "even", "val": itm }
]
}
f(t_label) := q {
q := [ entry | itm := arr[i]
r := itm % 2
r == 1
entry := { t_label: "odd", "val": itm }
]
}
Is this a thing that is done? How is this problem generally approached using Rego?

Answer 1

You're right — unlike rules, functions can't be partial. If you really need something like that for a function you could either have a function to aggregate the result of two (or more) other function calls:
even(t_label) := {entry |
itm := arr[_]
itm % 2 == 0
entry := {t_label: "even", "val": itm}
}
odd(t_label) := {entry |
itm := arr[_]
itm % 2 == 1
entry := {t_label: "odd", "val": itm}
}
f(t_label) := even(t_label) | odd(t_label)
both := f("foo")
For your particlar example though, I think we'd be excused using a little "or hack" using map based branching:
f(t_label) := {entry |
itm := arr[_]
entry := {t_label: {0: "even", 1: "odd"}[itm % 2], "val": itm}
}