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

Could always define a custom func for nil checking: https://golang.org/pkg/text/template/#example_Template_func — Adrian, Feb 07 '19 at 16:59
Please provide a [mcve]. We don't know how you execute that template, nor what you pass for `.Value`. — icza, Feb 07 '19 at 17:20
.Value is interface{} which is pulled from the db, so can be of any type — Tim Abell, Feb 07 '19 at 17:43
done, with playground. also my if was backwards, fixed. let me know if that's enough info — Tim Abell, Feb 07 '19 at 17:59

score 3 · Accepted Answer · answered Feb 07 '19 at 18:22

3

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

answered Feb 07 '19 at 18:22

dave

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Thanks, I'll do that. – Tim Abell Feb 07 '19 at 21:15
I went with `func isNil(value interface{}) bool{return value == nil}` – Tim Abell Feb 07 '19 at 21:34

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

1 Answers1