accounts_example.ex

defmodule MyApp.Accounts do
  # (...)

  @spec sign_up(params :: %{String.t() => String.t()}) ::
          {:ok, User.t()} | {:error, Ecto.Changeset.t()}
  def sign_up(%{} = params) do
    # Here, a list of params and their types
    params_schema = %{
      name: :string,
      # (...)
    }

    # Here I set up a schemaless validation
    # (A bit verbose, like you said - but it's worth the LOCs)
    changeset =
      {%{}, params_schema}
      |> cast(params, Map.keys(params_schema))
      |> validate_required([:name, :email, :etc])
      # (...) Some more validation, if needed
    
    # I don't do lots of validations here because they'll probably be duplicates
    # of ones defined by some `changeset/2` function (in this example,
    # `User.changeset/2`).

    # Here's the thing: the call to `apply_action/2` is made inside the `with`,
    # turning its returned errors part of the Core function return types,
    # alongside ones from `create_user/1`, making it easier to react to errors
    # in param validation from whichever Interface you'll be using this
    # Core function.
    with {:ok, params} <- apply_action(changeset, :sign_up),
         {:ok, user} <- create_user(params) do
      {:ok, user}
    end
  end
end