resolver.ex

defmodule RHRLive.Resolver do
  @callback resolve(map :: map, field :: atom, opts :: keyword) :: term

  defmacro __using__(_) do
    quote do
      @behaviour RHRLive.Resolver
      @before_compile RHRLive.Resolver

      def resolve(map, field), do: resolve(map, field, [])
    end
  end

  defmacro __before_compile__(_) do
    quote do
      def resolve(map, field, _opts), do: Map.fetch!(map, field)
    end
  end
end

defmodule MyApp.Data.MySchema do
  use MyApp.Resolver

  def resolve(schema, :some_computed_thing, _opts) do
    "$#{schema.something}"
  end
end

MyApp.Data.MySchema(schema, :id) # => 123
MyApp.Data.MySchema(schema, :something) # => 456
MyApp.Data.MySchema(schema, :some_computed_thing) # => "$456"