Unmarshaling XML with an attribute on a self-closing tag

[indraniel]

I’m exploring “encoding/xml”‘s Unmarshal function to get a better understanding of its features.

An Unmarshaling scenario I’ve been looking at is to create a proper golang struct to parse an XML snippet that looks like:

    <ASSEMBLY>
      <COMMON>Build 37</COMMON>
      <STANDARD short_name="GRCh37-lite"/>
    </ASSEMBLY>

I’ve been able to successfully the above XML using two structures, described below ( and in http://play.golang.org/p/RiI422j163 ):

package main

import (
    "encoding/xml"
    "fmt"
)

func main() {

    type Standard struct {
            ShortName string `xml:"short_name,attr"`
    }

    type Assembly struct {
        XMLName xml.Name `xml:"ASSEMBLY"`
        Common string `xml:"COMMON"`
        Std Standard `xml:"STANDARD"`
    }


    var v Assembly

    data := `
        <ASSEMBLY>
          <COMMON>Build 37</COMMON>
          <STANDARD short_name="GRCh37-lite"/>
        </ASSEMBLY>
    `
    err := xml.Unmarshal([]byte(data), &v)
    if err != nil {
        fmt.Printf("error: %v", err)
        return
    }
    fmt.Printf("%#v\n", v)
}

However, I’m only interested in parsing just value of the COMMON element and the “short_name” attribute on the STANDARD element.

Is there a way I could elegantly Unmarshal my interesting elements of the XML directly into a single struct like

type Assembly struct {
    XMLName xml.Name  `xml:"ASSEMBLY"`
    Common string
    ShortName string
}

?

​

[Egon]

AFAIK, xml package isn't capable of doing that (easily). One possibility would be to use an embedded struct.

    type Assembly struct {

        XMLName xml.Name `xml:"ASSEMBLY"`

        Common string `xml:"COMMON"`

        Std struct {

            ShortName string `xml:"short_name,attr"`

        } `xml:"STANDARD"`

    }

The other version would involve implementing Marshaler/Unmarshaler interfaces, it would allow hiding the second struct inside those methods.

+ Egon

[Matt Harden]

Yes, I would go with implementing xml.Unmarshaler. It isn't very hard: http://play.golang.org/p/StN_AQGYkS

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[indraniel]

Many Thanks for providing illustrative examples for both approaches! I was also looking at Francesc Campoy’s recent presentation: JSON, interfaces and go generate. Although it is about JSON, the unmarshaling ideas given in the presentation seem similar to what both of you have suggested for XML unmarshaling.