I have some JSON that I am unmarshalling into various structs so that I can then process the data, seemingly this is turning into the hardest part of the project!!!
The format of this JSON is that if the field is missing then it is essentially nil. This is following on from Default struct values but thought it deserved it's own question on SO.
A zero therefore is a valid value and I need to be able to discern this in my Go code. Is there a way to get Go to unmarshal into this struct with pointers at all?
In the example playground you can see what I mean, it "appears" to work but when I come to print out one of the pointer values it always prints the pointer address and not the actual value.
package main
import "fmt"
import "log"
import "encoding/json"
const inputMissing = `
["AAAAAA", {"testCode" : "Sss"}, 123456]
`
const inputZero = `
["AAAAAA", {"testCode" : "Sss", "missingInt" : 0, "defaultInt" : 0,"missingString" : "", "defaultString" : ""}, 123456]
`
type RawMessage struct {
AlwaysString string
ClientData ClientData
ReceptionTime int
}
type ClientData struct {
TestCode string
MissingInt *int
DefaultInt int
MissingString *string
DefaultString string
}
func main() {
var n RawMessage
if err := json.Unmarshal([]byte(inputMissing), &n); err != nil {
log.Fatal(err)
}
fmt.Printf("%#v\n", n)
var o RawMessage
if err := json.Unmarshal([]byte(inputZero), &o); err != nil {
log.Fatal(err)
}
fmt.Printf("%#v\n", o)
fmt.Printf("Should print the value of the int, not pointer... %i", o.ClientData.MissingInt)
}
func (n *RawMessage) UnmarshalJSON(buf []byte) error {
tmp := []interface{}{&n.AlwaysString, &n.ClientData, &n.ReceptionTime}
wantLen := len(tmp)
if err := json.Unmarshal(buf, &tmp); err != nil {
return err
}
if g, e := len(tmp), wantLen; g != e {
return fmt.Errorf("wrong number of fields in RawMessage: %d != %d", g, e)
}
return nil
}
Your code is correct. Test the pointer against nil and dereference the pointer if you want the value:
fmt.Printf("Should print the value of the int, not pointer... %d", *o.ClientData.MissingInt)
Your confusion arises from the default formatting of the fmt package, which prints the hex value in case of pointer fields, and not the pointed value.
If you're using the %#v verb for printing, you may implement the fmt.GoStringer interface on your struct to override this "behavior":
func (c ClientData) GoString() string {
mi := "<missing>"
if c.MissingInt != nil {
mi = strconv.Itoa(*c.MissingInt)
}
ms := "<missing>"
if c.MissingString != nil {
ms = *c.MissingString
}
return fmt.Sprintf("{TestCode: %s, MissingInt: %s, DefaultInt: %d, MissingString: %s, DefaultString: %s}",
c.TestCode, mi, c.DefaultInt, ms, c.DefaultString)
}
And changing the last printing line to:
fmt.Printf("Should print the value of the int, not pointer... %d",
*o.ClientData.MissingInt)
Your output becomes more readable (try it on the Go Playground):
main.RawMessage{AlwaysString:"AAAAAA", ClientData:{TestCode: Sss, MissingInt: <missing>, DefaultInt: 0, MissingString: <missing>, DefaultString: }, ReceptionTime:123456}
main.RawMessage{AlwaysString:"AAAAAA", ClientData:{TestCode: Sss, MissingInt: 0, DefaultInt: 0, MissingString: , DefaultString: }, ReceptionTime:123456}
Should print the value of the int, not pointer... 0
As you can see from the output above, the MissingXX fields have the static value <missing> displayed.
Note: If you were to use the %v verb, you'd have to implement the fmt.Stringer interface, which is basically the same, just the method name is not GoString() but simply String().
Related
I have a scenario where the JSON that has dynamic set of fields that need to get unmarshalled in to a struct.
const jsonStream = `{
"name": "john",
"age": 23,
"bvu62fu6dq": {
"status": true
}
}`
type Status struct {
Status bool
}
type Person struct {
Name string `json:"name"`
Age int `json:"age"`
Status map[string]Status `json:"status"`
}
func main() {
dec := json.NewDecoder(strings.NewReader(jsonStream))
for {
var person Person
if err := dec.Decode(&person); err == io.EOF {
break
} else if err != nil {
log.Fatal(err)
}
fmt.Println(person)
fmt.Println(person.Status["bvu62fu6dq"])
}
}
The output:
{john 23 map[]}
{false}
When it gets unmarshalled, the nested status struct is not being correctly resolved to the value in the JSON (shows false even with true value in JSON), is there any issue in the code?
Your types don't really match with the JSON you have:
type Status struct {
Status bool
}
type Person struct {
Name string `json:"name"`
Age int `json:"age"`
Status map[string]Status `json:"status"`
}
Maps to JSON that looks something like this:
{
"name": "foo",
"age": 12,
"status": {
"some-string": {
"Status": true
}
}
}
The easiest way to unmarshal data with a mix of known/unknown fields in a go type is to have something like this:
type Person struct {
Name string `json:"name"`
Age int `json:"age"`
Random map[string]interface{} `json:"-"` // skip this key
}
Then, first unmarshal the known data:
var p Person
if err := json.Unmarshal([]byte(jsonStream), &p); err != nil {
panic(err)
}
// then unmarshal the rest of the data
if err := json.Unmarshal([]byte(jsonStream), &p.Random); err != nil {
panic(err)
}
Now the Random map will contain every and all data, including the name and age fields. Seeing as you've got those tagged on the struct, these keys are known, so you can easily delete them from the map:
delete(p.Random, "name")
delete(p.Random, "age")
Now p.Random will contain all the unknown keys and their respective values. These values apparently will be an object with a field status, which is expected to be a boolean. You can set about using type assertions and convert them all over to a more sensible type, or you can take a shortcut and marshal/unmarshal the values. Update your Person type like so:
type Person struct {
Name string `json:"name"`
Age int `json:"age"`
Random map[string]interface{} `json:"-"`
Statuses map[string]Status `json:"-"`
}
Now take the clean Random value, marshal it and unmarshal it back into the Statuses field:
b, err := json.Marshal(p.Random)
if err != nil {
panic(err)
}
if err := json.Unmarshal(b, &p.Statuses); err != nil {
panic(err)
}
// remove Random map
p.Random = nil
The result is Person.Statuses["bvu62fu6dq"].Status is set to true
Demo
Cleaning this all up, and marshalling the data back
Now because our Random and Statuses fields are tagged to be ignored for JSON marshalling (json:"-"), marshalling this Person type won't play nice when you want to output the original JSON from these types. It's best to wrap this logic up in a custom JSON (un)-Marshaller interface. You can either use some intermediary types in your MarshalJSON and UnmarshalJSON methods on the Person type, or just create a map and set the keys you need:
func (p Person) MarshalJSON() ([]byte, error) {
data := make(map[string]interface{}, len(p.Statuses) + 2) // 2 being the extra fields
// copy status fields
for k, v := range p.Statuses {
data[k] = v
}
// add known keys
data["name"] = p.Name
data["age"] = p.Age
return json.Marshal(data) // return the marshalled map
}
Similarly, you can do the same thing for UnmarshalJSON, but you'll need to create a version of the Person type that doesn't have the custom handling:
type intermediaryPerson struct {
Name string `json:"name"`
Age int `json:"age"`
Random map[string]interface{} `json:"-"`
}
// no need for the tags and helper fields anymore
type Person struct {
Name string
Age int
Statuses map[string]Status // Status type doesn't change
}
func (p *Person) UnmarshalJSON(data []byte) error {
i := intermediaryPerson{}
if err := json.Unmarshal(data, &i); err != nil {
return err
}
if err := json.Unmarshal(data, &i.Random); err != nil {
return err
}
delete(i.Random, "name")
delete(i.Random, "age")
stat, err := json.Marshal(i.Random)
if err != nil {
return err
}
// copy known fields
p.Name = i.Name
p.Age = i.Age
return json.Unmarshal(stat, &p.Statuses) // set status fields
}
In cases like this, it's common to create a type that handles the known fields and embed that, though:
type BasePerson struct {
Name string `json:"name"`
Age int `json:"age"`
}
and embed that in both the intermediary and the "main"/exported type:
type interPerson struct {
BasePerson
Random map[string]interface{} `json:"-"`
}
type Person struct {
BasePerson
Statuses map[string]Status
}
That way, you can just unmarshal the known fields directly into the BasePerson type, assign it, and then handle the map:
func (p *Person) UnmarshalJSON(data []byte) error {
base := BasePerson{}
if err := json.Unmarshal(data, &base); err != nil {
return err
}
p.BasePerson = base // takes care of all known fields
unknown := map[string]interface{}{}
if err := json.Unmarshal(data, unknown); err != nil {
return err
}
// handle status stuff same as before
delete(unknown, "name") // remove known fields
// marshal unknown key map, then unmarshal into p.Statuses
}
Demo 2
This is how I'd go about it. It allows for calls to json.Marshal and json.Unmarshal to look just like any other type, it centralises the handling of unknown fields in a single place (the implementation of the marshaller/unmarshaller interface), and leaves you with a single Person type where every field contains the required data, in a usable format. It's a tad inefficient in that it relies on unmarshalling/marshalling/unmarshalling the unknown keys. You could do away with that, like I said, using type assertions and iterating over the unknown map instead, faffing around with something like this:
for k, v := range unknown {
m, ok := v.(map[string]interface{})
if !ok {
continue // not {"status": bool}
}
s, ok := m["status"]
if !ok {
continue // status key did not exist, ignore
}
if sb, ok := s.(bool); ok {
// ok, we have a status bool value
p.Statuses[k] = Status{
Status: sb,
}
}
}
But truth be told, the performance difference won't be that great (it's micro optimisation IMO), and the code is a tad too verbose to my liking. Be lazy, optimise when needed, not whenever
Type doesn't meet with your json value.
const jsonStream = `{
"name": "john",
"age": 23,
"bvu62fu6dq": {
"status": true
}
}`
For above json your code should look like below snnipet to work (some modifications in your existing code).
package main
import (
"encoding/json"
"fmt"
"io"
"log"
"strings"
)
const jsonStream = `{
"name": "john",
"age": 23,
"bvu62fu6dq": {
"status": true
}
}`
type bvu62fu6dq struct {
Status bool
}
type Person struct {
Name string `json:"name"`
Age int `json:"age"`
Status bvu62fu6dq `json:"bvu62fu6dq"`
}
func main() {
dec := json.NewDecoder(strings.NewReader(jsonStream))
for {
var person Person
if err := dec.Decode(&person); err == io.EOF {
break
} else if err != nil {
log.Fatal(err)
}
fmt.Println(person)
fmt.Println(person.Status)
}
}
Based on your json data you have to map with type fields.
Run code snippet
i cannot parse the json value i am sending a playground link
Any idea about that? here is the link and codes
https://play.golang.org/p/qhZpS_-618s
package main
import (
"encoding/json"
"fmt"
//mapstructure "github.com/mitchellh/mapstructure"
)
type presence struct{
id string
m_type string
deny string
}
type jsonHandler struct {
name string
dat map[string]interface{}
}
func main() {
s := `["Presence",{"id":"905356870666#c.us","type":"unavailable","deny":true}]`
data := jsonHandler{}
json.Unmarshal([]byte(s), &data)
fmt.Printf("Operation: %s", data.name)
}
Output :
Operation:
Program exited.
Try with this one: https://play.golang.com/p/UICf_uNNFdC
I've commented a lot in order to enhance code readability. Be sure to handle error properly and remove debug print.
package main
import (
"encoding/json"
"log"
"strings"
)
type Presence struct {
Presence string
ID string `json:"id"`
Type string `json:"type"`
Deny bool `json:"deny"`
}
type JsonHandler struct {
Name string `json:"name"`
Dat Presence `json:"dat"`
}
func main() {
var (
// Used for unmarshal a given json
packedData []json.RawMessage
err error
// Data that does not have a related json key
name []byte
// Used for extract the raw data that will be unmarshalled into the Presence struct
temp []byte
// Nested json
jsonPresence Presence
handler JsonHandler
)
s := `["Presence",{"id":"905356870666#c.us","type":"unavailable","deny":true}]`
log.Println("Dealing with -> " + s)
// Unmarshall into a raw json message
err = json.Unmarshal([]byte(s), &packedData)
if err != nil {
panic(err)
}
// Extract the presence
log.Println("Presence: ", string(packedData[0]))
// Extract the nested json
log.Println("Packed: ", string(packedData[1]))
// NOTE: 0 refers to the first value of the JSON
name, err = packedData[0].MarshalJSON()
if err != nil {
panic(err)
}
log.Println("Value that does not have a key: " + string(name))
handler.Name = strings.Replace(string(name), "\"", "", -1)
// NOTE: 1 refers to the second value of the JSON, the entire JSON
// Unmarshal the nested Json into byte
temp, err = packedData[1].MarshalJSON()
if err != nil {
panic(err)
}
// Unmarshal the raw byte into the struct
err = json.Unmarshal(temp, &jsonPresence)
if err != nil {
panic(err)
}
log.Println("ID:", jsonPresence.ID)
log.Println("Type:", jsonPresence.Type)
log.Println("Deny:", jsonPresence.Deny)
handler.Dat = jsonPresence
log.Println("Data unmarshalled: ", handler)
}
Go Playground Link: https://play.golang.org/p/qe0jyFVNTH1
Few Problem are present in this:
1. Json Package can't refer the Unexported Structure Elements.So please use Deny instead of deny in the following snippet.This is applicable to all variables declared inside the structure
2. The json fields tag are incorrect. eg.mapstructure:"id" should be json:"id"
3. The json to be parsed contains two distinct elements i.e string "Presence" and nested json object.It can't be parsed as a single element.It is better to declare "Presence" as a key and nested json as the value.
4. The deny variable should be bool rather than string
Wow,solved problem by adding only these codes
Here Go Lang Link : https://play.golang.org/p/doHNWK58Cae
func (n *JsonHandler) UnmarshalJSON(buf []byte) error {
tmp := []interface{}{&n.Name, &n.Dat}
wantLen := len(tmp)
if err := json.Unmarshal(buf, &tmp); err != nil {
return err
}
if g, e := len(tmp), wantLen; g != e {
return fmt.Errorf("wrong number of fields in Notification: %d != %d", g, e)
}
return nil
}
Is there an easy way to check if each field of myStruct was mapped by using json.Unmarshal(jsonData, &myStruct).
The only way I could image is to define each field of a struct as pointer, otherwise you will always get back an initialized struct.
So every jsonString that is an object (even an empty one {}) will return an initialized struct and you cannot tell if the json represented your struct.
The only solution I could think of is quite uncomfortable:
package main
import (
"encoding/json"
"fmt"
)
type Person struct {
Name *string `json:name`
Age *int `json:age`
Male *bool `json:male`
}
func main() {
var p *Person
err := json.Unmarshal([]byte("{}"), &p)
// handle parse error
if err != nil {
return
}
// handle json did not match error
if p.Name == nil || p.Age == nil || p.Male == nil {
return
}
// now use the fields with dereferencing and hope you did not forget a nil check
fmt.Println("Hello " + *p.Name)
}
Maybe one could use a library like govalidator and use SetFieldsRequiredByDefault. But then you still have to execute the validation and still you are left with the whole pointer dereferencing for value retrieval and the risk of nil pointer.
What I would like is a function that returns my unmarshaled json as a struct or an error if the fields did not match. The only thing the golang json library offers is an option to fail on unknown fields but not to fail on missing fields.
Any idea?
Another way would be to implement your own json.Unmarshaler which uses reflection (similar to the default json unmarshaler):
There are a few points to consider:
if speed is of great importance to you then you should write a benchmark to see how big the impact of the extra reflection is. I suspect its negligible but it can't hurt to write a small go benchmark to get some numbers.
the stdlib will unmarshal all numbers in your json input into floats. So if you use reflection to set integer fields then you need to provide the corresponding conversion yourself (see TODO in example below)
the json.Decoder.DisallowUnknownFields function will not work as expected with your type. You need to implement this yourself (see example below)
if you decide to take this approach you will make your code more complex and thus harder to understand and maintain. Are you actually sure you must know if fields are omitted? Maybe you can refactor your fields to make good usage of the zero values?
Here a fully executable test of this approach:
package sandbox
import (
"encoding/json"
"errors"
"reflect"
"strings"
"testing"
)
type Person struct {
Name string
City string
}
func (p *Person) UnmarshalJSON(data []byte) error {
var m map[string]interface{}
err := json.Unmarshal(data, &m)
if err != nil {
return err
}
v := reflect.ValueOf(p).Elem()
t := v.Type()
var missing []string
for i := 0; i < t.NumField(); i++ {
field := t.Field(i)
val, ok := m[field.Name]
delete(m, field.Name)
if !ok {
missing = append(missing, field.Name)
continue
}
switch field.Type.Kind() {
// TODO: if the field is an integer you need to transform the val from float
default:
v.Field(i).Set(reflect.ValueOf(val))
}
}
if len(missing) > 0 {
return errors.New("missing fields: " + strings.Join(missing, ", "))
}
if len(m) > 0 {
extra := make([]string, 0, len(m))
for field := range m {
extra = append(extra, field)
}
// TODO: consider sorting the output to get deterministic errors:
// sort.Strings(extra)
return errors.New("unknown fields: " + strings.Join(extra, ", "))
}
return nil
}
func TestJSONDecoder(t *testing.T) {
cases := map[string]struct {
in string
err string
expected Person
}{
"Empty object": {
in: `{}`,
err: "missing fields: Name, City",
expected: Person{},
},
"Name missing": {
in: `{"City": "Berlin"}`,
err: "missing fields: Name",
expected: Person{City: "Berlin"},
},
"Age missing": {
in: `{"Name": "Friedrich"}`,
err: "missing fields: City",
expected: Person{Name: "Friedrich"},
},
"Unknown field": {
in: `{"Name": "Friedrich", "City": "Berlin", "Test": true}`,
err: "unknown fields: Test",
expected: Person{Name: "Friedrich", City: "Berlin"},
},
"OK": {
in: `{"Name": "Friedrich", "City": "Berlin"}`,
expected: Person{Name: "Friedrich", City: "Berlin"},
},
}
for name, c := range cases {
t.Run(name, func(t *testing.T) {
var actual Person
r := strings.NewReader(c.in)
err := json.NewDecoder(r).Decode(&actual)
switch {
case err != nil && c.err == "":
t.Errorf("Expected no error but go %v", err)
case err == nil && c.err != "":
t.Errorf("Did not return expected error %v", c.err)
case err != nil && err.Error() != c.err:
t.Errorf("Expected error %q but got %v", c.err, err)
}
if !reflect.DeepEqual(c.expected, actual) {
t.Errorf("\nWant: %+v\nGot: %+v", c.expected, actual)
}
})
}
}
You could compare p with a empty struct, instead of comparing each field with nil.
// handle json did not match error
if p == Person{} {
return
}
Since Person{} will initialize with the 0 value of each field, this will result in each property that is pointers to be nil, strings will be "", ints will be 0, and so on.
The problem I'm currently having is after saving a struct to a json file and then opening the struct from the json file, somehow the properties of the struct have changed slightly.
In the struct N, sometimes A and B can point to the same J. However, after encoding then decoding they point to different Js of the value.
before encoding this returns true (expected). After decoding it, it returns false (not expected)
fmt.Println("is same pointer", n.A[0] == n.B[0])
Is this supposed to happen? Is there a way around this. Thanks.
type N struct {
A []*J
B []*J
C []*J
}
func (n *N) Save(name string) {
name = "radacted.json"
err := os.Remove(name)
file, err := os.Create(name)
defer file.Close()
if err != nil {
fmt.Println(err)
}
bytes, err := json.Marshal(n)
file.Write(bytes)
}
func Open(name string) *N {
bytes, err := ioutil.ReadFile("redacted.json")
if err != nil {
log.Fatal("decode error:", err)
}
var n NeuralNetwork
json.Unmarshal(bytes, &n)
return &n
}
It's expected and documented behaviour
Pointer values encode as the value pointed to.
You can assert values equality
*n.A[0] == *n.B[0] //should stay
fmt.Println("is same pointer", n.A[0] == n.B[0])
you are comparing the address value here so it will not be the same. Let me give you an example
suppose you have struct like this :
type Test struct {
ValueA *int
ValueB *int
}
and on your main function you add the same value but with different address in this case with different variable :
func main() {
hello := 12
hello2 := 12
testObject := Test{ValueA: &hello, ValueB: &hello2}
if *testObject.ValueA == *testObject.ValueB {
fmt.Println("Equal Value")
} else {
fmt.Println("Different Value")
}
}
Notice that the *testObject.ValueA and *testObject.ValueBis getting the exact value not the value address. If you are not using * the result would be different.
so as uvelichitel said you just need to use * when comparing your struct value.
Let's say I have the following Go struct on the server
type account struct {
Name string
Balance int
}
I want to call json.Decode on the incoming request to parse it into an account.
var ac account
err := json.NewDecoder(r.Body).Decode(&ac)
If the client sends the following request:
{
"name": "test#example.com",
"balance": "3"
}
Decode() will return the following error:
json: cannot unmarshal string into Go value of type int
Now it's possible to parse that back into "you sent a string for Balance, but you really should have sent an integer", but it's tricky, because you don't know the field name. It also gets a lot trickier if you have a lot of fields in the request - you don't know which one failed to parse.
What's the best way to take that incoming request, in Go, and return the error message, "Balance must be a string", for any arbitrary number of integer fields on a request?
You can use a custom type with custom unmarshaling algorythm for your "Balance" field.
Now there are two possibilities:
Handle both types:
package main
import (
"encoding/json"
"fmt"
"strconv"
)
type Int int
type account struct {
Name string
Balance Int
}
func (i *Int) UnmarshalJSON(b []byte) (err error) {
var s string
err = json.Unmarshal(b, &s)
if err == nil {
var n int
n, err = strconv.Atoi(s)
if err != nil {
return
}
*i = Int(n)
return
}
var n int
err = json.Unmarshal(b, &n)
if err == nil {
*i = Int(n)
}
return
}
func main() {
for _, in := range [...]string{
`{"Name": "foo", "Balance": 42}`,
`{"Name": "foo", "Balance": "111"}`,
} {
var a account
err := json.Unmarshal([]byte(in), &a)
if err != nil {
fmt.Printf("Error decoding JSON: %v\n", err)
} else {
fmt.Printf("Decoded OK: %v\n", a)
}
}
}
Playground link.
Handle only a numeric type, and fail anything else with a sensible error:
package main
import (
"encoding/json"
"fmt"
)
type Int int
type account struct {
Name string
Balance Int
}
type FormatError struct {
Want string
Got string
Offset int64
}
func (fe *FormatError) Error() string {
return fmt.Sprintf("Invalid data format at %d: want: %s, got: %s",
fe.Offset, fe.Want, fe.Got)
}
func (i *Int) UnmarshalJSON(b []byte) (err error) {
var n int
err = json.Unmarshal(b, &n)
if err == nil {
*i = Int(n)
return
}
if ute, ok := err.(*json.UnmarshalTypeError); ok {
err = &FormatError{
Want: "number",
Got: ute.Value,
Offset: ute.Offset,
}
}
return
}
func main() {
for _, in := range [...]string{
`{"Name": "foo", "Balance": 42}`,
`{"Name": "foo", "Balance": "111"}`,
} {
var a account
err := json.Unmarshal([]byte(in), &a)
if err != nil {
fmt.Printf("Error decoding JSON: %#v\n", err)
fmt.Printf("Error decoding JSON: %v\n", err)
} else {
fmt.Printf("Decoded OK: %v\n", a)
}
}
}
Playground link.
There is a third possibility: write custom unmarshaler for the whole account type, but it requires more involved code because you'd need to actually iterate over the input JSON data using the methods of the
encoding/json.Decoder type.
After reading your
What's the best way to take that incoming request, in Go, and return the error message, "Balance must be a string", for any arbitrary number of integer fields on a request?
more carefully, I admit having a custom parser for the whole type is the only sensible possibility unless you are OK with a 3rd-party package implementing a parser supporting validation via JSON schema (I think I'd look at this first as juju is a quite established product).
A solution for this could be to use a type assertion by using a map to unmarshal the JSON data into:
type account struct {
Name string
Balance int
}
var str = `{
"name": "test#example.com",
"balance": "3"
}`
func main() {
var testing = map[string]interface{}{}
err := json.Unmarshal([]byte(str), &testing)
if err != nil {
fmt.Println(err)
}
val, ok := testing["balance"]
if !ok {
fmt.Println("missing field balance")
return
}
nv, ok := val.(float64)
if !ok {
fmt.Println("balance should be a number")
return
}
fmt.Printf("%+v\n", nv)
}
See http://play.golang.org/p/iV7Qa1RrQZ
The type assertion here is done using float64 because it is the default number type supported by Go's JSON decoder.
It should be noted that this use of interface{} is probably not worth the trouble.
The UnmarshalTypeError (https://golang.org/pkg/encoding/json/#UnmarshalFieldError) contains an Offset field that could allow retrieving the contents of the JSON data that triggered the error.
You could for example return a message of the sort:
cannot unmarshal string into Go value of type int near `"balance": "3"`
It would seem that here provides an implementation to work around this issue in Go only.
type account struct {
Name string
Balance int `json:",string"`
}
In my estimation the more correct and sustainable approach is for you to create a client library in something like JavaScript and publish it into the NPM registry for others to use (private repository would work the same way). By providing this library you can tailor the API for the consumers in a meaningful way and prevent errors creeping into your main program.