calculator_test.exs

defmodule CalculatorTest do
  use ExUnit.Case
  import ExUnit.CaptureIO

  doctest Calculator

  describe "+ 1.0" do
    defmodule FakeIO do
      defdelegate puts(message), to: IO
      def gets("operation> "), do: "+ 1.0"
    end

    test "accumulator should be 1.0" do
      run = fn ->
        # Simulate two loops of our REPL since the accumulator is
        # printed at the start of the call from interpret
        Calculator.interpret(0.0, FakeIO)
        |> Calculator.interpret(FakeIO)
      end
      assert capture_io(run) =~ "1.0"
    end
  end
end