proposal: encoding/xml: support marshal self-closing tag in short-form mode

[ECUST-XX]

Goals

Add an option, to support marshal a self-closing tag xml struct in short-form mode which has empty content.

In xml standard, a self-closing tag is permitted.
https://www.w3.org/TR/xml/#sec-starttags

Definitions

func ExampleMarshalShortForm() {
	type Address struct {
		City, State string
	}
	type Person struct {
		XMLName   xml.Name `xml:"person"`
		Id        int      `xml:"id,attr"`
		FirstName string   `xml:"name>first"`
		LastName  string   `xml:"name>last"`
		Age       int      `xml:"age"`
		Height    float32  `xml:"height,omitempty"`
		Married   bool
		Address
		Comment       string   `xml:",comment"`
		FavoriteMusic []string `xml:"favorite_music"`
		LuckyNumber   int      `xml:"lucky_number"`
	}

	v := &Person{Id: 13, FirstName: "John", LastName: ""}
	v.Comment = " Need more details. "
	v.Address = Address{"Hanga Roa", ""}
	v.FavoriteMusic = []string{"Made in Heaven", ""}

	output, err := xml.MarshalIndentShortForm(v, "  ", "    ")
	if err != nil {
		fmt.Printf("error: %v\n", err)
	}

	os.Stdout.Write(output)
	// Output:
	//   <person id="13">
	//       <name>
	//           <first>John</first>
	//           <last />
	//       </name>
	//       <age>0</age>
	//       <Married>false</Married>
	//       <City>Hanga Roa</City>
	//       <State />
	//       <!-- Need more details. -->
	//       <favorite_music>Made in Heaven</favorite_music>
	//       <favorite_music />
	//       <lucky_number>0</lucky_number>
	//   </person>
}

Related questions

#21399

[ianlancetaylor]

Just to be clear, I think you are proposing a new function MarshalIndentShortForm. That will be like MarshalIndent, but a tag with no contents will use a self-closing tag. Is that about right?

[ECUST-XX]

Just to be clear, I think you are proposing a new function MarshalIndentShortForm. That will be like MarshalIndent, but a tag with no contents will use a self-closing tag. Is that about right?

Yes. That's right.

[adonovan]

What is the cost of using a self-closing tag wherever it is allowed?

[tnypxl]

@adonovan Cost in terms of what? Can you elaborate?

[adonovan]

tnypxl: Cost in terms of what? Can you elaborate?

I meant all kinds of costs. Obviously any change has the potential to cause spurious failures in existing overly-specific tests, but are there more substantial reasons why not to simply enable it always? The output would be more compact, which seems like a good thing.

[tnypxl]

@adonovan Thanks for the clarification. I do agree with your last point. I am running into this issue while researching ways to build a XML config generator.

Without having looked into the current std lib implemtantion, I imagine an always-on approach would break some tests that expect a closing tag from empty elements. I'd be interested in a struct field tag option (e.g, closeempty or selfclose) that lets me decide on a field by field basis. But I imagine for large complex XML trees, that isn't ideal either.

[ydnar]

We have a small change that implements the selfclosing option to the xml struct tag here:

nbio/xml#10

Happy to turn this into a formal proposal + CL if folks are interested.

[msurmeli]

Having a field based struct tag would allow us to decide whether to use self closing tags per field. Personally I would prefer to see your approach as proposal. @ydnar

[ianlancetaylor]

See #69273 for an alternate proposal.