permutations_lazy3.rb

def permutations_lazy(list)
  # return [list] if list.size == 1
  en = Enumerator.new do |y|
    if list.size == 1
      # yield a new array with the list as its only element (instead of returning)
      y.yield [list]
    end

    list.each do |locked|
      puts "locking #{locked}"
      list_copy = list.dup
      list_copy.delete locked
      permutations_lazy(list_copy).each do |permutation|
        puts "permutation #{permutation}, locked #{locked}"
        y.yield permutation.insert(0, locked)
      end
    end
  end
  en
end

if __FILE__ == $0
  pl = permutations_lazy %w[a b c d]
  pl.first(5).each {|i| p i}
end

~output.txt

Output:
["a", "b", "c", ["d"]]
["a", "b", "d", ["c"]]
["a", "c", "b", ["d"]]
["a", "c", "d", ["b"]]
["a", "d", "b", ["c"]]