Escape HTML in array field in struct

How do I escape HTML when I have an array field in a struct?

For a single show page, this code works:

show.go:

err := ShowTmpl.ExecuteTemplate(w, "show.html", struct {
		Title    string
		SafeBody template.HTML
}{
	t.Title,
	template.HTML(t.BodyHTML),
})

For an index page:

index.go

type as struct {
		Articles []*Article
	}
var a as

// some code to give a.Articles its values

err := IndexTmpl.ExecuteTemplate(w, "index.html", a)
if err != nil {
	http.Error(w, err.Error(), http.StatusInternalServerError)
}

index.html:

{{with .Articles}}
  {{range .}}
    <a href="/">{{.Title}}</a>
    {{.BodyHTML | html}} // Doesn't work
  {{end}}
{{end}}

How do I escape HTML when I'm ranging over a struct field?

You can achieve this in several ways:

1. With a custom function

You can use a custom function. The Template.Funcs() method allows you to register any custom functions which can be invoked from templates.

Create a simple function which converts a string to template.HTML like this:

func ToHtml(s string) template.HTML {
	return template.HTML(s)
}

You can register it like this:

t := template.Must(template.New("index.html").
	Funcs(map[string]interface{}{"ToHtml": ToHtml}).Parse(indexHtml))

Where just for demonstration purposes indexHtml is a string of your template:

const indexHtml = `{{with .}}
  {{range .}}
    <a href="/">{{.Title}}</a>
    {{ToHtml .BodyHTML}}
  {{end}}
{{end}}
`

And you can refer to it and call it from the template like this:

{{ToHtml .BodyHTML}}

Calling this template with a parameter:

a := []struct {
	Title    string
	BodyHTML string
}{{"I'm the title", "I'm some <b>HTML</b> code!"}}

err := t.ExecuteTemplate(os.Stdout, "index.html", a)

Here's the complete, working example on the Go Playground.

2. Modifying Article

If you can, it would be easier to just change the type of Article.BodyHTML to template.HTML and then it would be rendered unescaped without further ado. This would also make the intent clear (that it should/does contain safe HTML which will be rendered unescaped).

3. Adding a method to Article

You can also add a method to the Article type which would return its BodyHTML field as a template.HTML (pretty much what the custom function does in proposition #1):

func (a *Article) SafeBody() template.HTML {
    return template.HTML(a.BodyHTML)
}

Having this method you can simply call it from the template:

  {{range .}}
    <a href="/">{{.Title}}</a>
    {{.SafeBody}}
  {{end}}

Try this variant on the Go Playground.