How can I prevent non-nil values triggering a golang template if nil block

Question

The following is incorrectly showing "null" for values of 0 , but I only want it to do that for exactly nil .

package main

import (
    "os"
    "text/template"
)

type thing struct {
    Value interface{}
}

func main() {
    tmpl, _ := template.New("test").Parse("{{if .Value }} {{.Value}} {{else}} [null] {{end}}\n")
    tmpl.Execute(os.Stdout, thing{Value: "hi"}) // outputs hi
    tmpl.Execute(os.Stdout, thing{Value: nil})  // outputs [null]
    tmpl.Execute(os.Stdout, thing{Value: 0})    // outputs [null] - should output 0
    tmpl.Execute(os.Stdout, thing{Value: 2})    // outputs 2
}

Playground link: https://play.golang.org/p/Gg8uBCOb2vE

How do I make it show the value of 0 instead?

.Value is an interface{} which in the problem case contains an int , but could contain anything.

Show default content in a template if an object is nil otherwise show based on the set property is close but not quite the same thing

Answer 1

I would just create a function that you pass to the template using template.Funcs :

https://play.golang.org/p/anxW5ooGE7N

funcs := make(map[string]interface{})
funcs["isNotNull"] = func(t interface{}) bool {
    return t != nil
}
tmpl, _ := template.New("test").Funcs(funcs).Parse("{{if isNotNull .Value }} {{.Value}} {{else}}[null] {{end}}\n")
tmpl.Execute(os.Stdout, thing{Value: "hi"}) // outputs hi
tmpl.Execute(os.Stdout, thing{Value: nil})  // outputs [null]
tmpl.Execute(os.Stdout, thing{Value: 0})    // outputs 0
tmpl.Execute(os.Stdout, thing{Value: 2})    // outputs 2