Puppet Conditional in Template

extdata:sample.yaml

---
parameters:
  server_tags: "role:all=true,role:dev=true"

modulename:lib:facter:some_custom_fact.rb

# filename ends with .rb, not .pp.
# see http://docs.puppetlabs.com/guides/custom_facts.html
# see http://www.scribd.com/doc/56291200/63/Puppet-Using-Facter-and-Templates
# see Pro Puppet, chapter 10 "Extending Facter and Puppet"
Facter.add("some_custom_fact") do
  setcode do
    'Some custom data piece. Possibly an IP address.'
  end
end

modulename:templates:filename.pp

# To call the `has_role` function, you must prefix it with `function_`,
# and send it to the `scope` object.
# So, `has_role('foo')` becomes `scope.function_has_role('foo')`.
# `has_role` is a function added by the truth module.
    <% if scope.function_has_role('dev') %>
# facts, including those set by ENCs, are available.
# some_custom_fact is a custom fact created by a custom module.
    allow <%= some_custom_fact %>;
    <% end %>