Generator module provides the make/4 function and make/2 macro which allow to atomatically derive a data generator to be used with property-based frameworks.

For example, to write a unit test for a pure function with a given spec to use with StreamData framework one could write something along the lines:

defmodule MyTest do
  use ExUnit.Case
  use ExUnitProperties

  require RTypes
  require RTypes.Generator, as: Generator

  # @spec f(arg_type) :: result_type
  arg_type_gen = Generator.make(arg_type, Generator.StreamData)
  result_type? = RTypes.make_predicate(result_type)

  property "for any parameter `f/1` returs value of `result_type`" do
    check all value <- arg_type_gen do
      assert result_type?.(f(value))
    end
  end
end

The above would generate 100 (by default) values that belong to arg_type and verify that the result belongs to result_type.