{{ $currentPage := . }}
<div id="nav">
  <ul class="nav navbar-nav">
    {{ range .Site.Menus.main }}
    <li>
      <a class="{{- if eq $currentPage.RelPermalink .URL -}}open{{- end -}}"
         href="{{ .URL }}">
        <span>{{ .Name }}</span>
      </a>
    </li>
    {{ end }}
  </ul>
</div>