Elixir and Behaviours by Ecto example

Feb 2, 2018

An Elixir behaviour is just a list of functions. A module that declares that it implements a particular behaviour must implement all of these functions.

I would say that behaviours are kind of interfaces from the OOP world.

Ecto uses behaviours for adding ability to create custom types. Let’s open ecto/lib/ecto/type.ex.
@callback directive is used to define befaviours. It expects a function name and function specification.

defmodule Ecto.Type do
  @callback type :: t
  @callback cast(term) :: {:ok, term} | :error
  @callback load(term) :: {:ok, term} | :error
  @callback dump(term) :: {:ok, term} | :error

  ...
end

So if you want to use custom types then you should implement all of these 4 functions: type, cast, load and dump. Otherwise elixir will throw a compilation warning.

arc_ecto uses ecto’s custom type generation:

defmodule Arc.Ecto.Definition do
  defmacro __using__(_options) do
    definition = __CALLER__.module

    quote do
      defmodule Module.concat(unquote(definition), “Type”) do
        @behaviour Ecto.Type
        def type, do: Arc.Ecto.Type.type
        def cast(value) do
          Arc.Ecto.Type.cast(unquote(definition), value)
        end
        def load(value), do: Arc.Ecto.Type.load(unquote(definition), value)
        def dump(value), do: Arc.Ecto.Type.dump(unquote(definition), value)
      end
    end

  ...
end

@behaviour declares that Arc.Ecto.Definition implements Ecto.Type and then defines necessary functions.

Then you can use Arc’s custom type in the your app:

defmodule Spark.ImageUploader do
  use Arc.Definition
  use Arc.Ecto.Definition
end

defmodule Spark.Gallery.Image do
  use Ecto.Schema
  use Arc.Ecto.Schema

  schema "images" do
    field :image, Spark.ImageUploader.Type
    ...
  end
end

If you don’t clearly understand what is use and why it is used here, please read my previous post about macros.

That’s all for today.
Thank you for reading this post!