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


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.


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 :smiley: )

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
    |> evaluate(template)

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

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

  # @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.()



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?

The returned substitutions are few into your template, so it’s really what your template expects. The examples and tests in the book use numbers to feed math problems, but you could use strings to construct grammar problems or whatever. For this reason, I feel any() is the proper type.


Thank you for your reply. I know understand where my issues are and I can fix the code!