Why deffact use so strange format of deffact("foo", [a, b]) instead of more “natural” deffact foo(a, b)?

You’re right, that’s a much better syntax, and its a change I’d like to make. I just haven’t figured out that part of building macros yet (I think I have an issue for that in gitlab, if not I’ll add one).

Hint, in IEx you can use quote to see AST, for example:

iex> quote do
...>   deffact foo(a, b)
...> end
{:deffact, [],
 [
   {:foo, [],
    [
      {:a, [], Elixir},
      {:b, [context: Elixir, imports: [{1, IEx.Helpers}]], Elixir}
    ]}
 ]}

And now you see that foo(a, b) will be passed as just regular function call (that is the reason why you can do stuff like def a + b in Elixir).

It also looks like I can use Macro — Elixir v1.16.3 too handle those expressions pretty easily. I’ll just need to change the type for Fact some.