Working lazy version of permutations using yield statements

permutations_lazy1.rb

def permutations_lazy(list)
  # recursion's base case: return, not yield (outside of enumerator)
  return [list] if list.size == 1
  en = Enumerator.new do |y|
    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} # in case you're not familiar with ruby, the p keyword prints the output of an object's #inspect method
end

~output.txt

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