module Puppet::Parser::Functions
  newfunction(:source_exists, :type => :rvalue) do |args|
    dist = args[0]
    file = args[1]
    path = "/var/lib/puppet/dist/" + dist + file

    File.exists?(path)
  end
end


err: Could not retrieve catalog: Could not parse for environment production: Function 'source_exists' does not return a value at /etc/puppet/manifests/classes/common-pam.pp:2