Description
Currently even if one implements MarshalJSON
on a struct there is no way to prevent its inclusion into the parent struct. E.g.:
type Foo struct {
Field Field `json:"field"`
}
type Field struct {
DataPublic bool
Value interface{}
}
func (f Field) MarshalJSON() ([]byte, error) {
if f.DataPublic {
return json.Marshal(f.Value)
} else {
return []byte("null"), nil
}
}
(Full example: https://go.dev/play/p/A0wKfuZgURw)
The best one can do is return null
. But what if one wants to fully omit the field? This is not possible. Or one has to implement MarshalJSON
on the parent struct, but that is not nice from composability perspective (one would have to implement this for every struct which includes Field
).
I propose that it should be allowed to return nil, nil
from MarshalJSON
to signal that the field should be omitted in the parent struct. Currently returning nil, nil
does fails with error unexpected end of JSON input
because the resulting JSON is malformed. That means that nobody can rely on this behavior really for correct JSON output (maybe only to catch errors?), so I do not think we would be breaking any real backwards compatibility if we introduce this, or at least I believe benefits are bigger here. In any case, if you want previous behavior, you can return []byte{}
and then it will continue to error in the same way. Doing JSON marshal of a struct itself which returns nil
is compatible enough in my view: if caller just assumes they are getting []byte
slice, nil
will mostly behave for them as an empty slice. If they are able to handle it anyway.
Implementing this would also provide a solution for the issue that omitempty
does not work with structs: one could implement MarshalJSON
which checks if the value is zero and return nil
if it is so. This would be useful only when implementor of the type is the user of it at the same time, but one can always make a new type as a user and then implement MarshalJSON
there. I think it is an useful step in addressing that.
Returning nil
to omit the field seems to be also something others expected to work and has been also proposed in the past, but inside a bigger discussion, so this can serve as a proposal for just this feature.
Activity
[-]proposal: encoding/json: Allow returning nil from MarshalJSON to omit the field[/-][+]proposal: encoding/json: allow returning nil from MarshalJSON to omit the field[/+]earthboundkid commentedon Jan 18, 2022
I worry this could result in an accidental omission. How about
return nil, json.OmitFieldErr
instead?LittleFox94 commentedon Apr 28, 2022
What is the state for this, would a PR for this be accepted?
I really like the
return nil, json.OmitFieldErr
idea, though it should be eitherErrOmitField
to match conventions or strip theErr
completely - maybejson.OmitFieldHint
or something?LittleFox94 commentedon Apr 28, 2022
I just saw this comment is created from one of the ideas #11939, explicitly for
return nil, nil
andreturn nil, json.$someErr
is one of the other proposals in that original issue.icholy commentedon Apr 30, 2022
@mitar what happens if
MarshalJSON
returnsnil
withoutomitempty
.icholy commentedon Apr 30, 2022
@mitar I implemented your idea here https://github.com/icholy/json if you want to play with it https://go.dev/play/p/7Cf6BgrNGpP
edit: I've also implemented the
err_empty
branch which requires the error return. Ex:This type will marshal to
null
unless it's a struct field withomitempty
set.smikulcik commentedon May 9, 2022
Regarding #50480 (comment)
Wouldn't
nil, nil
yield the invalid JSON of empty string?rsc commentedon Jun 1, 2022
This proposal is a duplicate of a previously discussed proposal, as noted above,
and there is no significant new information to justify reopening the discussion.
The issue has therefore been declined as a duplicate.
— rsc for the proposal review group
mitar commentedon Jun 1, 2022
Sorry, where exactly has this already been discussed? So I found a mention in one long issue (on
omitempty
and structs), a much broader issue, where this was proposed but not acted upon, while this issue itself tries to be focused on exactly one proposal. So which discussion is this reopening? This is just trying to focus the discussion which have not happened. (And this issue has also 14 upvotes.)joeshaw commentedon Jun 1, 2022
@mitar #45669 is the one that's been put on the docket for this.
There are lots of different proposals to address the same underlying problem, but it's not clear yet which is the best approach to take. Hopefully discussion there will address it.
mitar commentedon Jun 1, 2022
Hm, I saw that one, but I disagree that this one would be addressed with
omitzero
. But the opposite holds true: implementing this proposal would also provide a way to solve the issue whichomitzero
attempts to address. Butomitzero
(noromitempty
) do not influence JSON marshaling when struct implementsMarshalJSON
. And this issue is about that. If a struct hasMarshalJSON
, then you cannot omit it anymore.omitzero
will not fix this.MarshalJSON() ([]byte, error)
#57554