An assert_type function for puppet (untested example)

assert_type.rb

Puppet::Parser::Functions::newfunction(:assert_type, :arity => 2, :type => :rvalue, :doc =>
  "Asserts that the given value has the given type and returns the value") do |args|
    if Puppet::Pops::Types::TypeCalculator.instance?(args[0], args[1])
      args[1]
    else
      raise Puppet::ParseError, "The given value is not of type #{Puppet::Pops::Types::TypeCalculator.string(args[0])}"
    end
end