Help understanding Question.ex from Designing Elixir System with OTP book

Backgrond

I am following the book Designing Elixir System with OTP book and I hit the (Fun)ctions part of the “Do Fun Things with Big Loud Wildbeests” layer.

To test my understanding I am adding dialyzer to everything while trying to make it compile.

Problem

However, I have hit a roadblock. I can’t understand the Questions.ex file. In the book they talk about lists and generators (which are functions) but I don’t understand what this lists and functions are supposed to have inside or return.

Here is the file in question (get it? The file in question? I’ll show myself out )

defmodule Mastery.Core.Question do
  @moduledoc """
  Questions consist of the text the user was asked, the used to create them and
  the substitutions used to build the template.
  """

  alias Mastery.Core.Template

  @type t() :: %__MODULE__{
    asked: String.t,
    template: Template.t,
    substitutions: %{substitution: any}
  }

  @enforce_keys [:asked, :substitutions, :template]
  defstruct ~w[asked substitutions template]a

  @spec new(Template.t) :: __MODULE__.t
  def new(%Template{} = template) do
    template.generators
    |> Enum.map(&build_substitution/1)
    |> evaluate(template)
  end

  # @spec evaluate(?????, Template.t) :: __MODULE__.t
  defp evaluate(substitutions, template), do:
    %__MODULE__{
      asked: compile(template, substitutions),
      substitutions: substitutions,
      template: template
    }

  # @spec compile(Template.t, ??????) :: any????
  defp compile(template, substitutions) do
    template.compiled
    |> Code.eval_quoted(assigns: substitutions)
    |> elem(0)
  end

  # @spec build_substitution({atom???, [any???] | function()}) :: {atom, any???}
  defp build_substitution({name, choices_or_gen}), do:
    {name, choose(choices_or_gen)}

    # ???? What is this supposed to return? any? a String.t ?
  defp choose(choices) when is_list(choices), do: Enum.random(choices)
  defp choose(generator) when is_function(generator), do: generator.()

end

Questions

As you can see, I am having trouble identifying what my functions return, namely the last function, choose/1.

I have re-read the book and I couldn’t find anything regarding the types these functions should manage. I am focusing and guessing based on the official docs of EEx.

Can anyone help me figure this out?