ensure_argument

def ensure_argument list
  list.each do |value, tests|
    [tests].flatten.each do |test|
      case test.to_s
      when /^is_an?_(.*)$/
        value.is_a? $1.classify.constantize
      when /^is_(.*)$/
        value.respond_to?($1) && value.send($1)
      when /^(has|can)_(.*)/
        value.respond_to? $2
      end || raise(ArgumentError, "Expected #{value} to #{test}")
    end
  end
end

def debit(debitor, balance, descr)
  ensure_argument debitor => [:has_account, :is_active], 
                  balance => :is_a_bignum, descr => :is_a_string
  # ...
end