An example of property-based testing in Elixir 1.6

ColorNameTest.exs

# Until StreamData ships in Elixir it needs to be added as a dependency.
# https://hexdocs.pm/stream_data/StreamData.html#content
# https://hex.pm/packages/stream_data
# Add this to your mix.exs deps...
#   {:stream_data, "~> 0.4.2"}
  
defmodule ColorNameTest do
  use ExUnit.Case, async: true
  use ExUnitProperties

  property "color_name/3" do
    check all red <- StreamData.integer(0..255), 
              green <- StreamData.integer(0..255), 
              blue  <- StreamData.integer(0..255), 
              max_runs: 100_000
      do
        assert color_name(red, green, blue) != :impossible_color
      end
  end

  def color_name(0, 0, 0), do: :black
  def color_name(255, 255, 255), do: :white
  def color_name(red, green, blue) when red > green and red > blue, do: :red
  def color_name(red, green, blue) when green > red and green > blue, do: :green
  def color_name(red, green, blue) when blue > red and blue > green, do: :blue 
  def color_name(_, _, _), do: :impossible_color
  
end


# Note: when the test above fails, these might come in handy ;)
# def color_name(red, green, blue) when red == green and red > blue, do: :yellow
# def color_name(red, green, blue) when red == blue and red > green, do: :purple
# def color_name(red, green, blue) when green == blue and green > red, do: :cyan
# Oh, and this one...
# def color_name(red = gray, green = gray, blue = gray), do: :gray