undef parameters in puppet

default.pp

class foo (
  $param1 = 'Hello World!'
) {
  notice("${param1}")
}

class bar (
  $param1 = 'Hello World!'
) {
  notice("${param1}")
}

class {'foo':
  param1 => 'Goodbye world'
}

class {'bar':
  param1 => undef,
}

runit.sh

$ puppet apply default.pp
Notice: Scope(Class[Foo]): Goodbye world
Notice: Scope(Class[Bar]): Hello World!
Notice: Finished catalog run in 0.07 seconds