module PropCheck
  ##
  # Helper functions that have no other place to live
  module Helper
    extend self
    ##
    # Creates a (potentially lazy) Enumerator
    # starting with `elem`
    # with each consecutive element obtained
    # by calling `operation` on the previous element.
    #
    #   >> Helper.scanl(0, &:next).take(10).force
    #   => [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
    #   >> Helper.scanl([0, 1]) { |curr, next_elem| [next_elem, curr + next_elem] }.map(&:first).take(10).force
    #   => [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
    def scanl(elem, &operation)
      Enumerator.new do |yielder|
        acc = elem
        loop do
          # p acc
          yielder << acc
          acc = operation.call(acc)
        end
      end.lazy
    end
  end
end