Get XML namespace prefix in Go using Unmarshal

I would like to know if it is possible to get the XML namespace prefix using the
Unmarshal method in encoding/xml.

For example, I have:

<application xmlns="http://wadl.dev.java.net/2009/02" xmlns:xs="http://www.w3.org/2001/XMLSchema">
</application>

I would like to know how to retrieve the xs defining the prefix for XMLSchema, without having to use the Token method.

Just get it like every other attribute:

type App struct {
	XS string `xml:"xs,attr"`
}

Playground: http://play.golang.org/p/2IOmkX1Jov.

It gets trickier if you also have an actual xs attribute, sans xmlns. Even if you add the namespace URI to XS's tag, you will probably get an error.

EDIT: If you want to get all declared namespaces, you can define a custom UnmarshalXML on your element and scan it's attributes:

type App struct {
	Namespaces map[string]string
	Foo        int `xml:"foo"`
}

func (a *App) UnmarshalXML(d *xml.Decoder, start xml.StartElement) error {
	a.Namespaces = map[string]string{}
	for _, attr := range start.Attr {
		if attr.Name.Space == "xmlns" {
			a.Namespaces[attr.Name.Local] = attr.Value
		}
	}

	// Go on with unmarshalling.
	type app App
	aa := (*app)(a)
	return d.DecodeElement(aa, &start)
}

Playground: http://play.golang.org/p/u4RJBG3_jW.

Currently (Go 1.5), it does not seems to be possible.

The only solution I found, was to use rewind the element:

func NewDocument(r io.ReadSeeker) (*Document, error) {

	decoder := xml.NewDecoder(r)

	// Retrieve xml namespace first
	rootToken, err := decoder.Token()


	if err != nil {
		return nil, err
	}

	var xmlSchemaNamespace string

	switch element := rootToken.(type) {
	case xml.StartElement:

		for _, attr := range element.Attr {
			if attr.Value == xsd.XMLSchemaURI {
				xmlSchemaNamespace = attr.Name.Local
				break
			}
		}
	}

    /* Process name space */

	// Rewind
	r.Seek(0, 0)
    
    // Standart unmarshall
	decoder = xml.NewDecoder(r)
	err = decoder.Decode(&w)
   
    /* ... */
}