Why am I seeing ZgotmplZ in my Go HTML template output?

When I'm calling a Go template function to output HTML, it displays ZgotmplZ.

Sample code:

http://play.golang.org/p/tfuJa_pFkm

package main

import (
    "html/template"
    "os"
)

func main() {
    funcMap := template.FuncMap{
        "printSelected": func(s string) string {
            if s == "test" {
                return `selected="selected"`
            }
            return ""
        },

        "safe": func(s string) template.HTML {
            return template.HTML(s)
        },
    }
    template.Must(template.New("Template").Funcs(funcMap).Parse(`
    <option {{ printSelected "test" }} {{ printSelected "test" | safe }} >test</option>
    `)).Execute(os.Stdout, nil)

}

Output:

<option ZgotmplZ ZgotmplZ >test</option>

"ZgotmplZ" is a special value that indicates that unsafe content reached a CSS or URL context at runtime. The output of the example will be:

<img src="#ZgotmplZ">

You can add a safe and attr function to the template funcMap:

package main

import (
    "html/template"
    "os"
)

func main() {
    funcMap := template.FuncMap{
        "attr": func(s string) template.HTMLAttr {
            return template.HTMLAttr(s)
        },
        "safe": func(s string) template.HTML {
            return template.HTML(s)
        },
    }

    template.Must(template.New("Template").Funcs(funcMap).Parse(`
    <option {{.attr | attr}}>test</option>
        {{.html | safe}}
     `)).Execute(os.Stdout, map[string]string{
        "attr": `selected="selected"`,
        "html": `<option selected="selected">option</option>`,
    })
}

The output will look like:

<option selected="selected">test</option>
<option selected="selected">option</option>

You may want to define some other functions which can convert string to template.CSS, template.JS, template.JSStr, template.URL etc.