Question

I’m learning Go by writing a program that generates files in Collada format, which describes geometry using XML.

You annotate your structs and almost everything works as you’d expect, except I can’t figure out how to marshal arrays into one XML element – I always end up generating N elements.

In other words, I’d like

<input>
    <p>0 1 2</p>
</input> 

instead of

<input>
    <p>0</p>
    <p>1</p>
    <p>2</p>
</input> 

The code is as follows

package main

import (
    "encoding/xml"
    "os"
)

func main() {
    type Vert struct {
        XMLName xml.Name    `xml:"input"`
        Indices     []int   `xml:"p"`
    }

    v := &Vert{Indices:[]int{0, 1, 2}}
    output, err := xml.MarshalIndent(v, "", "    ")
    if err == nil {
        os.Stdout.Write(output)
    }
}

Various comments (and code) from encoding/xml/marshal.go seem to imply that I’m out of luck:

// Marshal handles an array or slice by marshalling each of the elements.
// Slices and arrays iterate over the elements. They do not have an enclosing tag.

Strangely, if I change my array type to uint8, the array is not marshalled at all.

If I am out of luck, I’ll probably use the xml:”,innerxml” annotation to substitute the array myself.

Answer 1

As you guessed, encoding/xml will not be able to do this out-of-the-box. You can do something like this instead:

import (
    "strconv"
    "strings"
)

type Vert struct {
    P string `xml:"p"`
}

func (v *Vert) SetIndices(indices []int) {
    s := make([]string, len(indices))
    for i := range indices {
        s[i] = strconv.FormatInt(int64(indices[i]), 10)
    }
    v.P = strings.Join(s, " ")
}

EDIT: I originally wrote a getter instead of setter.