Yes please! This bit me today. We like to use omitempty to minimize the fields we need to include in a response. But we do need to differentiate between a nil slice and an empty slice, so we had to remove omitempty for slices and always include the field, so now it's an array or null. 🙁

Partial dup of #11939, which we may get to someday. Mostly JSON is done but better handling of zeros is on the long-term wish list.

Closing in favor of #11939, which now mentions this one.

@rsc how is this even a partial dupe of that issue? Its focus is defining additional zero values to be counted as "empty" by omitempty. This issue is focused on the fact that there is no way to differentiate between nil and empty slices with omitempty. How does #11939 address that?

@rsc That proposal you referred seems to make omitempty exclude even more values from being encoded. This proposal is for doing the opposite – keep values that are today hidden from the output!

To clarify this proposal, here's an example. I want to be able to omit the Contents property for the Black Box (as it works today), but keep it for the Empty Box:

package main

import "encoding/json"
import "fmt"

type Box struct {
	Label    string
	Contents []string `json:",omitempty"`
}

func main() {
	var d []byte
	d, _ = json.Marshal(Box{Label: "Black Box"})
	fmt.Println(string(d))
	d, _ = json.Marshal(Box{Label: "Empty Box", Contents: []string{}})
	fmt.Println(string(d))
	d, _ = json.Marshal(Box{Label: "Banana Box", Contents: []string{"Banana", "Another banana"}})
	fmt.Println(string(d))
}

Here's the output I want, after changing omitempty to omitnil:

{"Label":"Black Box"}
{"Label":"Empty Box","Contents":[]}
{"Label":"Banana Box","Contents":["Banana","Another banana"]}

Actual output:

{"Label":"Black Box"}
{"Label":"Empty Box"}
{"Label":"Banana Box","Contents":["Banana","Another banana"]}

Further clarification: The patch mentioned above adds support to encoding/json to differentiate on []string(nil) vs. []string{} (through a new keyword omitnil so backwards compatibility is not broken), allowing nil slices to be excluded without excluding empty slices.

I guess I'll reopen this, but to be honest it sounds like a bad idea to me. Distinguishing between a nil slice and an empty slice is sufficiently confusing that it is generally a mistake. I don't think that the additional complexity in the already very complex encoding/json package is worth it.

@ianlancetaylor well, the encoder distinguishes. It encodes an empty slice as [] and a nil slice as null. But including a null property in the JSON is kinda pointless. Including [] is not.

In go, you shouldn't need to distinguish between a nil slice and an empty slice because they're generally treated the same. In JSON and many languages in which people parse JSON, null arrays and empty arrays are treated differently.

Ideally, the consumers of your JSON will be able to handle output that has either a null array, an empty array, or a missing key. In the real world, customers often demand that you supply output exactly how they want it. Product managers insist that engineers do the things that customers who pay boatloads of money want, so that's why I had to write the omitnil patch and run a product I built for a previous employer on a fork of go that includes this patch (and the poor souls who inherited that product have to continue maintaining that fork). It appears that my situation was not unique and others have to deal with similar demands, so it would be helpful if they didn't also have to maintain their own forks of go.

On hold for #11939.

+1 to this request. I have a struct with a a field

Metadata interface{} `json:"metadata,omitempty,omitnil"`

but if the value is nil, it still serializes it as "metadata": null

@rsc You had mentioned a JSON sweep for 1.12, was this reviewed?

To summarize:

• Today:
  • []int(nil) becomes "field": null by default
  • []int{} becomes "field": [] by default
  • omitempty will make both cases omit the field (which is unexpected for the empty slice case, basically making omitempty not an option because of course JSON should represent an empty array)
• Proposed:
  • omitnil would only omit the []int(nil) case (note: not a breaking change)

And to respond to @ianlancetaylor's comment:

Distinguishing between a nil slice and an empty slice is sufficiently confusing that it is generally a mistake

If this is something you stand behind, then please let's make []int(nil) encode into [], not null! Making this change would be backwards incompatible, unfortunately.

We need the omitnil behavior to minimize the data that is reported from an IoT device.

A lot of our fields are pointers to types because they are generated by another tool which uses pointers to distinguish between required and optional fields. (GraphQL)

type Battery {
    Voltage *float32 `json:"voltage"`
    Current *float32 `json:"current"`
    ... 150 additional optional fields ...
}

There is a difference between "sensor read 0V" or "sensor did not read a value at all". To limit the amount of data that is sent we want to omit all fields with nil values - not just slices.

This should be configurable on the encoder as well as the fields, e.g. with an SetOmitNil() function like you do with the indent settings.

To that extent I would disagree with @rsc that "JSON is done" since in more complex projects we don't always have full control over the types that we're using and in our case we have way too many different types to re-generate them with different tags just to satisfy the Go JSON encoder behavior.

Also, encoding behavior of the same type may change per use-case. For the IoT case we want to omit the fields to save data but for an API we may want to show all fields to make the type discoverable. jsonpb has an EmitDefaults option for that case but that only works for protobuf objects. The json tags hard-code the encoding behavior.

Additionally, it should be possible to register custom encoder/decoder functions per type for example to allow time.Time fields to render in a specific format if we have to use a type from an imported package. I can think of other examples of float formatting.

Another thing was the long discussion on whether the JSON decoder should reject/report fields for which there is no field to support config validation.

We cannot use a more efficient encoding scheme like protobuf for various reasons. AWS IoT rules are only triggered on JSON payloads is one of them but more importantly, almost all tools work with JSON data so adding different serialization schemes - even if they are more efficient - adds an extra maintenance burden.

Having a high-quality and maintained JSON encoder in the standard library is important. But so is making it flexible enough since - for better or for worse - JSON is the lingua-franca in a lot of projects. I think in that respect the Go JSON encoder falls a bit short.

What I'm currently doing is to maintain a fork of the JSON package and keep it as an internal package to support this:

var b bytes.Buffer
e := json.NewEncoder(&b)
e.SetOmitNil(true)
err := e.Encode(v)

The patch below is quite simple but I do understand that LoC is not the only measure and that every feature has to be supported forever. So there has to be a clear benefit.

diff --git a/src/encoding/json/encode.go b/src/encoding/json/encode.go
index 67412763d6..ded4ac4867 100644
--- a/src/encoding/json/encode.go
+++ b/src/encoding/json/encode.go
@@ -342,6 +342,8 @@ type encOpts struct {
 	quoted bool
 	// escapeHTML causes '<', '>', and '&' to be escaped in JSON strings.
 	escapeHTML bool
+	// omitnil causes nil fields to be skipped
+	omitnil bool
 }
 
 type encoderFunc func(e *encodeState, v reflect.Value, opts encOpts)
@@ -650,6 +652,9 @@ FieldLoop:
 			fv = fv.Field(i)
 		}
 
+		if opts.omitnil && fv.Kind() == reflect.Ptr && fv.IsNil() {
+			continue
+		}
 		if f.omitEmpty && isEmptyValue(fv) {
 			continue
 		}
diff --git a/src/encoding/json/stream.go b/src/encoding/json/stream.go
index e29127499b..8c2d08d311 100644
--- a/src/encoding/json/stream.go
+++ b/src/encoding/json/stream.go
@@ -178,6 +178,7 @@ type Encoder struct {
 	w          io.Writer
 	err        error
 	escapeHTML bool
+	omitnil    bool
 
 	indentBuf    *bytes.Buffer
 	indentPrefix string
@@ -199,7 +200,7 @@ func (enc *Encoder) Encode(v interface{}) error {
 		return enc.err
 	}
 	e := newEncodeState()
-	err := e.marshal(v, encOpts{escapeHTML: enc.escapeHTML})
+	err := e.marshal(v, encOpts{escapeHTML: enc.escapeHTML, omitnil:enc.omitnil})
 	if err != nil {
 		return err
 	}
@@ -250,6 +251,10 @@ func (enc *Encoder) SetEscapeHTML(on bool) {
 	enc.escapeHTML = on
 }
 
+func (enc *Encoder) SetOmitNil(on bool) {
+	enc.omitnil = on
+}
+
 // RawMessage is a raw encoded JSON value.
 // It implements Marshaler and Unmarshaler and can
 // be used to delay JSON decoding or precompute a JSON encoding.

For those interested by this feature, Jettison v0.7.0 feature an omitnil option in struct field's tags, with a behavior similar to the original proposal of this thread.

As mentionned by rsc and and others maintainers in other discussions, the API of the encoding/json package is basically frozen, and 3rd-party packages are a good place to experiment and implement those features.

+1 to this request,

for the nature of JSON we really expect empty array/object not to be omitted if those fields are requested by the consumers, omitnil will definitely be valuable compare to a pure omitempty

I needed this today. I want to return a struct that may or may not also include relational data as a struct field. The relational data may be an empty list, or a list of relational objects. If I decide not to fetch it because the client has not requested it, it is nil. In this case, I'd like to exclude it from the output.

For example:

type User struct {
    Name string
    Books []Book
}

Books may or may not be requested, and nil vs. empty list have different meanings. I want to avoid exposing the nil value to clients.

I guess I'll reopen this, but to be honest it sounds like a bad idea to me. Distinguishing between a nil slice and an empty slice is sufficiently confusing that it is generally a mistake. I don't think that the additional complexity in the already very complex encoding/json package is worth it.

While this is true, json is an interchange format, and in other languages (that may be at the other end of the pipe) there is difference between absence of a nil structure ("doesn't have a value in the current context") or an empty one ("no results").

For those interested by this feature, Jettison v0.7.0 feature an omitnil option in struct field's tags, with a behavior similar to the original proposal of this thread.

As mentionned by rsc and and others maintainers in other discussions, the API of the encoding/json package is basically frozen, and 3rd-party packages are a good place to experiment and implement those features.

Thank you for the suggestion! Jettison works well.

For me, ideally, omitempty would omit only empty values which can be determined by an extra interface for custom types (#11939) and omitnil would omit only nil values. Sadly this would indeed break compatibility. Perhaps for go 2.0?

Just for reference, I'm implementing a wrapper to a protocol that can have different meaning for JSON NULL and a omitted field. So, I can't use omitempty because it doesn't even call the methods from the Marshaler interface if value is null and without omitempty, the field will always visible.

Turns out it's possible already to generate all 4 outputs, by using a pointer to a slice, and tagging the outer pointer omitempty: 🎉
https://go.dev/play/p/F45C77m3WnC

type S struct {
	Items *[]string `json:"items,omitempty"`
}

ptr to some       -> err <nil>, out {"items":["foo","bar"]}
ptr to emptySlice -> err <nil>, out {"items":[]}
ptr to nilSlice   -> err <nil>, out {"items":null}
nil ptr           -> err <nil>, out {}

In principle, this makes sense — it composes the "pointer tagged omitempty" pattern for optional field with a field value that just happens to be a slice. However:

• in practice creating or consuming the extra layer of pointer to a slice (when a slice is itself a pointer type!) is pretty cumbersome... 😩 It pretty much requires defining a separate structs for external JSON vs. internal model you want to operate upon.
  => Would still be great to get omitnil.

• the distinction this keeps between 0-length slice -> [] and nil slice -> null is a poor fit for Go code which mostly treats them as interchangable.
  => If one has to deal with an extra pointer, it would be nice to at least get proposed nilasempty proposal: encoding/json: nilasempty to encode nil-slices as [] #37711 allowing nilSlice to also output {"items":[]}. Though I'm not sure if it's feasible for mixed tags items,omitempty,nilasempty to apply to external pointer vs. underlying slice respectively.

Hi all, we kicked off a discussion for a possible "encoding/json/v2" package that addresses the spirit of this proposal.

See the "omitzero" struct tag option under the "Struct tag options" section, which omits a struct field if it is the zero Go value (or has a IsZero method that reports true). Under this semantic, []T(nil) would be omitted but []T{} would not be omitted since the latter is not the zero Go value.