signup_form.ex

defmodule Teller.Web.SignupForm do
  use Form,
    email: :email,
    password: :password,
    password_confirmation: :password

  validates :email, presence: true
  validates :password, length: [min: 8], confirmation: true
end

user_controller.ex

defmodule Teller.Web.UserController do
  use Teller.Web, :controller

  alias Teller.User

  plug :scrub_params, "user" when action in [:create, :update]
  
  # ... snip
  
  def create(conn, %{"user" => user_params}) do
    case Teller.Web.SignupForm.save(%User{}, user_params) do
      {:ok, _} ->
        conn
          |> put_flash(:info, "User created successfully.")
          |> redirect(to: user_path(conn, :show))
      {:error, form} ->
        render(conn, "new.html", form: form)
    end
  end
  
  # ... snip
end