encoding/xml: inconsistent omitempty compared to encoding/json

[gopherbot]

by krolaw:

Both packages define empty as being:
"The empty values are false, 0, any nil pointer or interface value, and any array,
slice, map, or string of length zero."

However, json does not look at the value behind pointers, while xml does.  This
difference is made obvious when wanting to omit nil pointers, but retain empty values.

Without omitempty:
xml will omit tags with nil pointers (not documented)
json will display null for nil pointers.

With omitempty:
xml will omit pointer fields pointing to empty values.
json will DISPLAY pointer fields pointing to empty values.

My current solution is: have omitempty on for json and off for xml.

http://play.golang.org/p/hANosIIkM_

This is probably a documentation issue.  If it's a bug, please retain ability to omit
nil pointers, but not look at values beyond the pointer. 

Thanks.

[rogpeppe]

Comment 2:

A related issue: I believe this program should print {}, not {"X":null}.
It may be strictly speaking within the letter of the documentation
(the interface is not nil, but holds a nil value) but
I think it would be more in the spirit of the existing marshalling
(and consistent with encoding/xml) for the value inside the interface
to be considered when deciding about omitempty.
package main
import (
    "encoding/json"
    "fmt"
)
type Foo struct {
    X interface{} `json:",omitempty"`
}
func main() {
    data, err := json.Marshal(Foo{[]int(nil)})
    if err != nil {
        fmt.Println("err: %v", err)
    } else {
        fmt.Printf("%s\n", data)
    }
}

[gopherbot]

Comment 3 by krolaw:

I like that json does not look inside the value of the pointer.  Otherwise, I wouldn't
be able to set the difference between a zero (zero availability) and null value (no
update), which is important in the motel management solution I am writing.
Maybe omitempty is too general.  Maybe we need an omitnil as well.

[rsc]

Comment 4:

Labels changed: added priority-later, go1.2maybe, removed priority-triage.

Status changed to Accepted.

[rsc]

Comment 5:

Labels changed: added feature.

[robpike]

Comment 6:

Not for 1.2.

Labels changed: removed go1.2maybe.

[rsc]

Comment 7:

Labels changed: added go1.3maybe.

[rsc]

Comment 8:

Labels changed: removed feature.

[rsc]

Comment 9:

Labels changed: added release-none, removed go1.3maybe.

[rsc]

Comment 10:

Labels changed: added repo-main.