How to grouping by categories

support
2

I tried other way.
I set categories name to .Site.Params.

[params]
  categories = ["golang", "html"]

I try iterate it’s list and I get post with a specific categories by where method.
So I write this code, but it don’t work because this error message.

<div id="params">
  {{ $site := . }}
  {{ range .Site.Params.categories }}
    <div class="category_name">{{ . }}</div>

    <ul class="posts">
        {{ range where $site.Data.Pages .Params.tags "in" "golang" }}

        <li>{{ .Title }}</li>
        {{ end }}
    </ul>
  {{ end }}
</div>

ERROR: 2015/03/02 Error while rendering homepage: template: theme/index.html:28:
59: executing "theme/index.html" at <.Params.tags>: can't evaluate field Params
in type interface {}

But this code work well and no post displayed

<div id="params">
{{ range where .Data.Pages .Params.tags "in" "golang" }}
<div>where tags {{ .Title }} {{ .Params.tags }} </div>
{{ end }}
</div>

I think $site.Data.Pages in upper case is equal to .Data.Pages in lowre case.
But it isn’t same result, and lower case doesn’t work.

It’s double Dutch to me.