How do I print return value of a function?

Here is my function definition which returns a string

"addClassIfActive": func(tab string, ctx *web.Context) string

I'm trying to print it like this:

<a href="/home/"{{ printf "%s" addClassIfActive "home" .Context }}>Home</a>

http response is getting terminated when I'm trying to print.

What am I doing wrong?

Returning a boolean, and then using if works, still I'm curious how to print string returned from a function

The problem you have is that "home" and .Context will be the 3:rd and 4:th argument of printf and not the arguments of addClassIfActive. The return value of addClassIfActive becomes the 2:nd argument for printf.

But the solution is simple: you don't have to use printf to print.

If your function just returns a string, you can simply print it by writing:

{{addClassIfActive "home" .Context}}

Full working example:

package main

import (
	"html/template"
	"os"
)

type Context struct {
	Active bool
}

var templateFuncs = template.FuncMap{
	"addClassIfActive": func(tab string, ctx *Context) string {
		if ctx.Active {
			return tab + " content"
		}

		// Return nothing
		return ""
	},
}

var htmlTemplate = `{{addClassIfActive "home" .Context}}`

func main() {
	data := map[string]interface{}{
		"Context": &Context{true}, // Set to false will prevent addClassIfActive to print
	}

	// We create the template and register out template function
	t := template.New("t").Funcs(templateFuncs)
	t, err := t.Parse(htmlTemplate)
	if err != nil {
		panic(err)
	}

	err = t.Execute(os.Stdout, data)
	if err != nil {
		panic(err)
	}

}

Output:

>home content

Playground

You can't call functions in templates.

What you can do is use FuncMaps:

templates.go

var t = template.New("base")
// ParseFiles or ParseGlob, etc.
templateHelpers := template.FuncMap{
		"ifactive":    AddClassIfActive,
	}
	t = t.Funcs(templateHelpers)

your_template.tmpl

...
<span class="stuff">{{ if eq .Context | ifactive }} thing {{ else }} another thing {{ end }}</span>
...

I haven't tested this exact syntax, but I am using FuncMaps elsewhere. Make sure to read the better docs at text/template on FuncMaps for more examples.