POC for type spec tests with StreamData.

math.ex

defmodule Math do
  @spec square(integer) :: integer
  def square(a) do
    a * a
  end
end

type_test.exs

defmodule TypeTest do
  use ExUnit.Case
  import PropertyTest

  property "types" do
    module = Math
    specs = Kernel.Typespec.beam_specs(module)

    for {{name, _}, _} = spec <- specs, name != :__info__ do
      {{function_name, _arity}, specs} = spec

      for {:type, _, :fun, types} <- specs do
        [{:type, _, :product, arguments}, result] = types

        result_type = type(result)
        [argument] = arguments
        argument_gen = argument
                       |> type
                       |> generator

        check all a <- argument_gen do
          result = :erlang.apply(module, function_name, [a])
          checker(result_type).(result)
        end

      end
    end
  end

  def generator(:integer), do: StreamData.integer()

  def checker(:integer), do: fn(result) ->
    assert is_integer(result)
  end

  def type({:type, _, type, _}), do: type
end