extract.rb

# <modulepath>/custom_functions/lib/puppet/parser/functions/extract.rb
require 'yaml'

Puppet::Parser::Functions.newfunction(:extract, :type => :rvalue, :doc => "YAML Extract") do |args|
  raise Puppet::Error, "#extract: accepts only two arguments, you passed #{args.length}" if args.length != 2

  file  = args[0]
  field = args[1]

  begin
    file_hash = YAML.load_file(file)
  rescue Errno::ENOENT => e
    raise Puppet::Error, "#extract: file #{file} not found, please try again"
    return nil
  end

  file_hash[field]
end