Handle default values in unmarshal methods

[droyo]

Elements and attributes that have default values should get those values set if they contain the zero value for their Go type after marshalling.

[onitake]

This may be difficult to discern. The zero value could very well be what the user wants stored, and when unmarshalling, there is a value stored in the XML anyway.

There is one case where it makes sense, though: When an optional element or attribute has a default value set, but is not stored in the XML. But can this be detected? I guess it would require a custom Unmarshal method for every type with children?

[onitake]

After a bit of experimentation, it seems this is even more difficult to solve.

Default values are attached to attributes, not types, so they cannot be handled easily by a type's Unmarshal methods. On top of that, if an attribute is missing in the XML (and hence needs to be replaced by its default value), its Unmarshal method is never called.

I see only two possible solutions:

1. Generate Unmarshal methods for every type and set all default values before handling elements and attributes.
2. Generate a default initialiser for every type that must be called instead of initialising to the zero value.

[onitake]

I just had an idea how passing defaults to the (un)marshaller could work:
In a structured type, add custom tags to the the fields. These could then be interpreted by (Un)MarshalText to handle instance specifics, and even constraints.

The downside is that this still requires custom unmarshallers for every type.

A variation of this would be to submit a PR against the core Go XML package and have this implemented on the built-in unmarshallers.

[onitake]

I created a proposal on the Go issue tracker for such a feature: golang/go#21425

[droyo]

You've probably thought about this more than I have. Can we do something like this for default values?

func (x *MyType) UnmarshalXML(d *xml.Decoder, start xml.StartElement) error {
	// avoid recursing into this method
	type T MyType
	t := (*T)(x)
	
	// Assign defaults
	t.Field1 = "defaultForField1"
	t.Field2 = "defaultForField2"
	t.Field4 = "defaultForField4"
	
	return d.DecodeElement(t, &start)
}

So if the elements/attributes are in the xml doc, but empty, they will still be put into the Go value as empty, and if they're not present at all, we'll get whatever defaults are set. I dunno if that's the correct behavior according to the XSD spec, but it seems reasonable. See it in action: https://play.golang.org/p/1AevVrEVnJ

[onitake]

I played around a bit and tried to build a generic default value handler.
But it's not easy. Go's reflection package is giving me headaches.

Simple assignments like above will work for basic types, but custom types (even if they are strings or ints) still require custom handling. Maybe it's better to just call each unmarshaller instead...

[onitake]

I tested out your approach, it works nicely. Special handling is still required for other field types than string and embedded types.

Something like this:

type ActuateType string
type RepresentationBaseType struct {
	Width                     uint              `xml:"width,attr"`
	Height                    uint              `xml:"height,attr"`
}
type AdaptationSetType struct {
	RepresentationBaseType
	Href                    string                 `xml:"href,attr"`
	Actuate                 ActuateType            `xml:"actuate,attr"`
	SubsegmentStartsWithSAP uint                   `xml:"subsegmentStartsWithSAP,attr"`
}
func (x *AdaptationSetType) UnmarshalXML(d *xml.Decoder, start xml.StartElement) error {
	type T AdaptationSetType
	t := (*T)(x)
	t.Actuate = ActuateType("onRequest")
	t.SubsegmentStartsWithSAP = 100
	t.RepresentationBaseType.Width = 200
	t.RepresentationBaseType.Height = 200
	return d.DecodeElement(t, &start)
}

[droyo]

Your idea about calling the unmarshaller is interesting. I don't think it's what you meant, but what if, for any types that have default attributes, we construct an xml document with those defaults, and unmarshal that into our type before unmarshalling the real input? This can be done at run-time, or during code generation. Given simplified xsd from your example:

<complexType name="adaptationSetType">
  <attribute name="width" type="int" default="200"/>
  <attribute name="height" type="int" default="200"/>
  <attribute name="subsegmentStartsWithSAP" type="unsignedInt" default="100"/>
</complexType>

We can gen something like this:

type AdaptationSetType struct {
	Width                     uint              `xml:"width,attr"`
	Height                    uint              `xml:"height,attr"`
	SubsegmentStartsWithSAP uint                   `xml:"subsegmentStartsWithSAP,attr"`
}

var defaultAdaptationSetType AdaptationSetType

func init() {
	mustUnmarshal(&defaultAdaptationSetType, `
		<AdaptationSetType width="200" height="200" subsegmentStartsWithSAP="100" />`)
}

func mustUnmarshal(v interface{}, xmldoc string) {
	if err := xml.Unmarshal(v, []byte(xmldoc)); err != nil {
		panic(err)
	}
}

func (x *AdaptationSetType) UnmarshalXML(d *xml.Decoder, start xml.StartElement) error {
	type T defaultAdaptationSetType
	t := (*T)(x)
	*t = T(defaultAdaptationSetType)
	return d.DecodeElement(t, &start)
}

I'd like to create the default type declaration during code-generation and avoid the run-time panic, but it seems really complicated; we'd have to generate, compile and run a helper program that does the unmarshal of the defaults into our type, and prints it out using fmt.Printf("%#v"), then use that output in the final program.

That said, I think you can only specify simpleTypes for defaults. We can reduce them to builtins and generate separate code for the various types of assignments:

• quoted scalar (string, NMTOKEN, etc)
• unquoted scalar (int, uint, float, double, bool etc)
• xml.Name (QName), could be tricky
• list types like []string, []int, []float32, etc
• time.Time (dateTime, gMonth etc)
• []byte

[onitake]

An interesting idea...

Another possible approach would be a simple constructor for each type that prefills fields with their default values. This may even avoid custom UnmarshalXML methods. Care must be taken, however, that this constructor is called when unmarshalling array elements.

I currently use the decoder like this:

func Decode() (*MPDtype, error) {
	decoder := xml.NewDecoder(reader)
	mpd := &MPDtype{}
	err := decoder.Decode(mpd)
	return mpd, err
}

This could be turned into:

func NewMPDtype() *MPDtype {
	t := &MPDtype{}
	// prefill defaults here
	return t
}
func Decode() (*MPDtype, error) {
	decoder := xml.NewDecoder(reader)
	mpd := NewMPDtype()
	err := decoder.Decode(mpd)
	return mpd, err
}