calculator_test2.exs

describe "+ 1.0 * 5" do
  defmodule FakeIO do
    defdelegate puts(message), to: IO
    def gets("operation(1)> "), do: "+ 1.0"
    def gets("operation(2)> "), do: "* 5"
    def gets(_), do: "= 0.0"
  end

  test "accumulator should be 5.0" do
    run = fn ->
      Calculator.interpret(0.0, 1, FakeIO)
      |> Calculator.interpret(2, FakeIO)
      |> Calculator.interpret(3, FakeIO)
    end
    assert capture_io(run) =~ "5.0"
  end
end