Passing multiple arguments to golang sub-templates

Have you ever wanted to pass multiple arguments to a go sub-template? If you google it you'll be convinced it's not possible. But bear with me.

In go templates you can pass a single "argument" (pipeline in go parlance) to a "sub-template" defined block. But by creating a simple helper function you can pass as many arguments as you want. Simply add this function to your FuncMap:

func(els ...any) []any {
    return els
}

And you'll be able to create constructs such as:

{{ template "MyTemplate" (arr "first" 123 .Some.Value) }}
{{ template "MyTemplate" (arr "second" 456 .Other.Value) }}

{{ define "MyTeplate" }}
  {{ $strArg := index . 0 }}
  {{ $intArg := index . 1 }}
  {{ $valArg := index . 2 }}

  This is my str {{ $strArg }} parameter.
  ...
{{ end }}

I named arr my helper func, but you can call it whatever you want.

Enjoy!

Comments

[Andrew Pillar]

With Go templates it's much more preferable to define a struct containing the data you wish to expose to the template, for example,

data := TemplateData{
    Post: Post{Title: "Example post", Author: "Andrew"},
    User: User{Username: "andrew"},
}


tmpl.Execute(w, data)

and in your template you would have something like,

{{define "body"}}
    <h1>{{.Post.Title}}</h1>
    by {{.Post.Author}}
{{end}}

this provides a more stricter mechanism by which to control what data is in a template, and more clarity too since you would be referring to the data via the field names. Furthermore, it provides some type checking beforehand via the struct's fields.

[MoniqueLive]

Agreed. But eventually in the rare case when you need a more dynamic approach now you know how.

[Andrew Pillar]

In the case of having a more dynamic approach I would then prefer map[string]any.

[MoniqueLive]

Actually we're talking about different things.
The example you showed is related to data sent from Go to the template.
My post is about a template calling a sub-template.
Imagine an html form builder, where each field is constructed by calling a sub template:

<form...>
  {{ template "field" (arr "title" .MP3.Title) }}
  {{ template "field" (arr "artist" .MP3.Artist) }}
...
</form>

{{ define "field" }}
  {{ $fieldName := index . 0 }}
  {{ $fieldValue := index . 1 }}
  <label for="{{ $fieldName }}" ... >
  </label>
  <input id="{{ $fieldName }}" name="{{ $fieldName }}" type="text" value="{{ $fieldValue }}"/>
{{ end }}

The data sent from Go to the template is a struct (MP3 info), field names have nothing to do with the MP3 data, they're a frontend issue.

[Andrew Pillar]

In that case I would simply change your implementation to not use an arr function for grabbing arbitrary values and still use a map. See the playground link for an example as to what I mean: go.dev/play/p/5Hiajt4H2Z5

A map function is defined which takes the pairs of values and puts them into a map[string]any, these can then be accessed from the sub-template. This provides more clarity of the data being accessed since the values can be referred to by the index name.

[Simon Storl-Schulke]

This is very helpful thanks for that!

[Klaus Breyer]

Nice! This was really helping me. However, it needs to be:

{{ template "MyTemplate" (els "first" 123 .Some.Value) }}
{{ template "MyTemplate" (els "second" 456 .Other.Value) }}

[MoniqueLive]

In the end I wrote: “I named arr my helper func, but you can call it whatever you want.”, els is the name of the arguments. The name of the function you define on your FuncMap:

template.New("hello.gohtml").Funcs(template.FuncMap{
    "arr": func(els …any) []any { return els }
})