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

ruby -rpuppet source_exists.rb
source_exists.rb:8: syntax error, unexpected $end, expecting kEND