proposal: text/template: New template option additionalkey

[drewgonzales360]

👋 Hey folks, I'd like to add a new option when rendering templates that will error in the event that there are additional fields in the input. The original issue I'm trying to fix is in helm, so if anyone has an alternative solution to that, I'm open to it.

// Known options:
//
// missingkey: Control the behavior during execution if a map is
// indexed with a key that is not present in the map.
//
//	"missingkey=default" or "missingkey=invalid"
//		The default behavior: Do nothing and continue execution.
//		If printed, the result of the index operation is the string
//		"<no value>".
//	"missingkey=zero"
//		The operation returns the zero value for the map type's element.
//	"missingkey=error"
//		Execution stops immediately with an error.
//	"additionalkey=default"                                                      # New thing
//		Do nothing when additional keys are found.
//	"additionalkey=error"                                                        # New thing
//		Execution finishes with an error if a key is provided, but unused.

Then the following code (taken from the text/template example) would panic

package main

import (
	"html/template"
	"os"
)

type Inventory struct {
	Material   string
	Count      uint
	ExtraField string
}

func main() {
	sweaters := Inventory{"wool", 17, "unused field"}
	tmpl, err := template.New("test").Parse("{{.Count}} items are made of {{.Material}}")
	
	// Throw an error if extra fields are found. In this case ExtraField is unused.
	tmpl.Option("additionalkey=error")

	if err != nil {
		panic(err)
	}
	err = tmpl.Execute(os.Stdout, sweaters)
	if err != nil {
		panic(err)
	}
}

[seankhliao]

In helm, everything is under .Values.
As an example, if I use .Values.image.tag, does this count as a usage of .Values ? At what level is "used" counted at?
Also, {{ .Values }} can be a implemented as a function call, it doesn't always have to return the same data, even within the same template. If there's some form of recursive tracking, how will that work?

[seankhliao]

cc @robpike

[robpike]

I do not understand this proposal. How does one "provide" a key to a map yet have it be "unused"? Is the question, if I build a data structure (using a map, but maybe that's not important), pass it to a template, and don't use a field of it during execution, raise an error? If so, that is not something I would be in favor of doing in the template library. It's too peculiar a desire for a general-purpose library, and never a correctness problem in the template itself.

If you must address this, you could use one of the static analyzers if it's the static problem you're after; if it's the dynamic one, you're looking for a solution in the wrong place.

[drewgonzales360]

Is the question, if I build a data structure (using a map, but maybe that's not important), pass it to a template, and don't use a field of it during execution, raise an error?

Yes, that's exactly the question/proposal.

Thanks for the quick and thoughtful responses. I'll close this issue and find a fix elsewhere.