Calling a template with several pipeline parameters

In a Go template, sometimes the way to pass the right data to the right template feels awkward to me. Calling a template with a pipeline parameter looks like calling a function with only one parameter.

Let's say I have a site for Gophers about Gophers. It has a home page main template, and a utility template to print a list of Gophers.

http://play.golang.org/p/Jivy_WPh16

Output :

*The great GopherBook*    (logged in as Dewey)

    [Most popular]  
        >> Huey
        >> Dewey
        >> Louie

    [Most active]   
        >> Huey
        >> Louie

    [Most recent]   
        >> Louie

Now I want to add a bit of context in the subtemplate : format the name "Dewey" differently inside the list because it's the name of the currently logged user. But I can't pass the name directly because there is only one possible "dot" argument pipeline! What can I do?

• Obviously I can copy-paste the subtemplate code into the main template (I don't want to because it drops all the interest of having a subtemplate).
• Or I can juggle with some kind of global variables with accessors (I don't want to either).
• Or I can create a new specific struct type for each template parameter list (not great).

Solution 1:

You could register a "dict" function in your templates that you can use to pass multiple values to a template call. The call itself would then look like that:

{{template "userlist" dict "Users" .MostPopular "Current" .CurrentUser}}

The code for the little "dict" helper, including registering it as a template func is here:

var tmpl = template.Must(template.New("").Funcs(template.FuncMap{
    "dict": func(values ...interface{}) (map[string]interface{}, error) {
        if len(values)%2 != 0 {
            return nil, errors.New("invalid dict call")
        }
        dict := make(map[string]interface{}, len(values)/2)
        for i := 0; i < len(values); i+=2 {
            key, ok := values[i].(string)
            if !ok {
                return nil, errors.New("dict keys must be strings")
            }
            dict[key] = values[i+1]
        }
        return dict, nil
    },
}).ParseGlob("templates/*.html")

Solution 2:

You can define functions in your template, and have these functions being closures defined on your data like this:

template.FuncMap{"isUser": func(g Gopher) bool { return string(g) == string(data.User);},}

Then, you can simply call this function in your template:

{{define "sub"}}

    {{range $y := .}}>> {{if isUser $y}}!!{{$y}}!!{{else}}{{$y}}{{end}}
    {{end}}
{{end}}

This updated version on the playground outputs pretty !! around the current user:

*The great GopherBook*    (logged in as Dewey)

[Most popular]  

>> Huey
>> !!Dewey!!
>> Louie



[Most active]   

>> Huey
>> Louie



[Most recent]   

>> Louie

EDIT

Since you can override functions when calling Funcs, you can actually pre-populate the template functions when compiling your template, and update them with your actual closure like this:

var defaultfuncs = map[string]interface{} {
    "isUser": func(g Gopher) bool { return false;},
}

func init() {
    // Default value returns `false` (only need the correct type)
    t = template.New("home").Funcs(defaultfuncs)
    t, _ = t.Parse(subtmpl)
    t, _ = t.Parse(hometmpl)
}

func main() {
    // When actually serving, we update the closure:
    data := &HomeData{
        User:    "Dewey",
        Popular: []Gopher{"Huey", "Dewey", "Louie"},
        Active:  []Gopher{"Huey", "Louie"},
        Recent:  []Gopher{"Louie"},
    }
    t.Funcs(template.FuncMap{"isUser": func(g Gopher) bool { return string(g) == string(data.User); },})
    t.ExecuteTemplate(os.Stdout, "home", data)
}

Although I am not sure how that plays when several goroutines try to access the same template...

The working example