is there a way to list used variables?

Question

Let's say I have a basic go text/template:

{{.var}} is another {{.var2}}

I want to get an array of the variables names used in the template, to be able to skip the execution if they are not available in the data I pass to execute, is it possible to do that somehow?

As my data is not a struct but a map, doing .var will always return something: if it doesn't exist, it will return an empty string when I would have hoped to get an error when executing the template.

So for the example from above, I would have liked to get:

[var var2]

Answer 1

Use a template func that returns an error if a value isn't set. Something like this:

template.FuncMap(map[string]interface{}{
    "require": func(val interface{}) (interface{}, error) {
        if val == nil {
            return nil, errors.New("Required value not set.")
        }
        return val, nil
    },
}))

Then in your template:

{{require .Value}}

http://play.golang.org/p/qZMBID7Y8L