Trying to answer to https://www.ruby-forum.com/topic/4416285

psych_handler_with_mark.rb

require 'psych'

class ScalarHandler < Psych::Handler

  def parser=(parser)
    @parser=parser
  end

  def mark
    @parser.mark
  end

  def scalar value, anchor, tag, plain, quoted, style
    p [value, anchor, tag, plain, quoted, style]
    p mark
  end
 
end

scalar_handler = ScalarHandler.new
parser = Psych::Parser.new(scalar_handler)
scalar_handler.parser = parser

yaml_document = <<END
foo:
  bar:
    baz: 3
  blubb:
    baz: 4
END

parser.parse(yaml_document)

=begin
["foo", nil, nil, true, false, 1]
#<struct Psych::Parser::Mark index=4, line=0, column=4>
["bar", nil, nil, true, false, 1]
#<struct Psych::Parser::Mark index=11, line=1, column=6>
["baz", nil, nil, true, false, 1]
#<struct Psych::Parser::Mark index=20, line=2, column=8>
["3", nil, nil, true, false, 1]
#<struct Psych::Parser::Mark index=25, line=3, column=2>
["blubb", nil, nil, true, false, 1]
#<struct Psych::Parser::Mark index=31, line=3, column=8>
["baz", nil, nil, true, false, 1]
#<struct Psych::Parser::Mark index=40, line=4, column=8>
["4", nil, nil, true, false, 1]
#<struct Psych::Parser::Mark index=43, line=5, column=0>
=end