execute.log

$ puppet apply --modulepath=. -e 'include test'
Warning: ModuleLoader: module 'test' has unresolved dependencies - it will only see those that are resolved. Use 'puppet module list --tree' to see information about modules
   (file & line not available)
Error: Evaluation Error: Error while evaluating a Function Call, can't modify frozen String at /home/jan/tmp/test/manifests/init.pp:4:16 on node minimoose.home.vstone.be

init.pp

class test {
  notify {'test':
    message => test_function('foobar'),
  }
}

test_function.rb

Puppet::Functions.create_function(:test_function) do
  def test_function(value)
    value.reverse!
  end
end