You can’t, guards are fundamentally defined in the VM itself, which allows the VM to perform specific optimizations. defguard lets you essentially construct “new” guards by composing together other guards, but you can’t make new “from scratch” guards.

I don’t see the value of guards over this:

def do_stuff(%Opportunity{type: "Opportunity::Automatch"}), do: ...
def do_stuff(%Category{type: "Category::Automatch"}), do: ...

…and nothing else. Elixir will give you a runtime error if you pass anything else because there’s no clause to handle it.

I think I worded my question poorly, but anyways, the docs clearly explain how to do what I want…

(defguard) creates a macro that can be used both inside or outside guards.

defmodule Foo do
  
  defguard bar(m) when is_map(m) and m.field == true
  
end

iex> require Foo
iex> Foo.bar(%{field: true})
true

I just needed to read more better…