- Hello
-       Hello
-                                                    hello
-      hello
-    Hello
-                                                                                                   Hello

```
type function simple_keyof(ty)
    -- Ignoring unions or intersections of tables for simplicity.
    if not ty:is("table") then
        error("Can only call keyof on tables.")
    end

    local union = nil

    for property in ty:properties() do
        union = if union then types.unionof(union, property) else property
    end

    return if union then union else types.singleton(nil)
end

type person = {
    name: string,
    age: number,
}
--- keys = "age" | "name"
type keys = simple_keyof<person>
```

```
function ispositive(x)
    return x > 0
end

print(ispositive(1))
print(ispositive("2"))

function isfoo(a)
    return a == "foo"
end

print(isfoo("bar"))
print(isfoo(1))
```