abilities.ex

defmodule Canary.Abilities do
  defmacro can(user, actions, targets, when: condition) do # This macro throws an error when used
    quote do
      defimpl Canada.Can, for: unquote(user).__struct__ do
        def can?(unquote(user), unquote(actions), unquote(targets)) when unquote(condition), do: true
      end
    end
  end

  defmacro can2(user, actions, targets, when: condition) do # This macro works
    quote do
      def can?(unquote(user), unquote(actions), unquote(targets)) when unquote(condition), do: true
    end
  end
end

iex(112)> defmodule A do; Canary.Abilities.can %User{id: id}, :index, %User{}, when: id in [1,2]; end
iex:112: warning: redefining module A
** (CompileError) iex:112: undefined function id/0
(elixir) expanding macro: Kernel.defimpl/3
iex:112: A (module)
(canary) expanding macro: Canary.Abilities.can/4
iex:112: A (module)

iex(112)> defmodule A do; Canary.Abilities.can2 %User{id: id}, :index, %User{}, when: id in [1,2]; end
iex:112: warning: redefining module A
{:module, A,
<<70, 79, 82, 49, 0, 0, 5, 40, 66, 69, 65, 77, 69, 120, 68, 99, 0, 0, 0, 150, 131, 104, 2, 100, 0, 14, 101, 108, 105, 120, 105, 114, 95, 100, 111, 99, 115, 95, 118, 49, 108, 0, 0, 0, 2, 104, 2, ...>>,
{:can?, 3}}