Learn Functional Programming with Elixir

Controlling the Flow of Impure Functions

The first strategy for handling unexpected events is to control the flow like we covered in Chapter 3, ​Using Pattern Matching to Control the Program Flow​. You can use conditional structures, like case, if, or function clauses, to handle impure function results. They are flexible and good for handling simple cases, but not so much for complex ones. Let’s see how they do the job well:

handle_the_uncertain/case/0/shop.ex

defmodule Shop do
  def checkout(price) do
    case ask_number("Quantity?") do
      :error -> IO.puts("It's not a number")
      {quantity, _} -> quantity * price
    end
  end

  def ask_number(message) do
    message <> "\n"
      |> IO.gets
      |> Integer.parse