CODEWITHSUNDEEP
gogo-templates
Goranek
Goranek

Reputation: 896

Go template and function

In my go code I often use if like this

if user && user.Registered { }

equivalent code in go templates would be

{{ if and .User .User.Registered }} {{ end }}

Unfortunately code in the template fails, if .User is nil :/

Is it possible to achieve the same thing in go templates?

Upvotes: 5

Views: 5036

Answers (2)

Thundercat
Thundercat

Reputation: 121129

The template and function does not do short circuit evaluation like the Go && operator.

The arguments to the and function are evaluated before the function is called. The expression .User.Registered is always evaluated, even if .User is nil.

The fix is to use nested if:

 {{if .User}}{{if .User.Registered}}  {{end}}{{end}}

You can avoid the nested if or with by using a template function:

func isRegistered(u *user) bool {
  return u != nil && u.Registered
}

const tmpl = `{{if isRegistered .User}}registered{{else}}not registered{{end}}`

t := template.Must(template.New("").Funcs(template.FuncMap{"isRegistered": isRegistered}).Parse(tmpl))

playground example

Upvotes: 12

icza
icza

Reputation: 418505

Another option is to use the {{with}} action instead of the and template function.

Quoting from package doc of text/template:

{{with pipeline}} T1 {{end}}
    If the value of the pipeline is empty, no output is generated;
    otherwise, dot is set to the value of the pipeline and T1 is
    executed.

Using {{with}} often results in cleaner and shorter code, as inside the {{with}} the dot . is already set to the non-empty "wrapper", the .User in our case; moreover you don't have to worry about how and if the arguments of the and template function are evaluated.

Your template rewritten:

{{with .User -}}
    {{if .Registered}}REGISTERED{{end}}
{{- end}}

Testing it without and with a user:

t := template.Must(template.New("").Parse(tmpl))

fmt.Println("No user:")
if err := t.Execute(os.Stdout, nil); err != nil {
    panic(err)
}

u := struct{ Registered bool }{true}
fmt.Printf("User: %+v\n", u)
if err := t.Execute(os.Stdout, map[string]interface{}{"User": u}); err != nil {
    panic(err)
}

Output (try it on the Go Playground):

No user:
User: {Registered:true}
REGISTERED

Upvotes: 4

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