Marshal int slice to string line - go

Question

I have a struct as this:
type Int64Slice []int64
type DataWrapper struct {
ListId Int64Slice `json:"listid" required`
Domain string `json:"domain" required`
Name string `json:"name,omitempty"`
}
And I need it become:
{
"listid": "1 2 3 4 5",
"domain": "mydomain"
}
I have wrote custom MarshalJSON:
func (u Int64Slice) MarshalJSON() ([]byte, error) {
var result string
if u == nil {
result = "null"
} else {
result = strings.Trim(strings.Join(strings.Fields(fmt.Sprint(u)), " "), "[]")
Logger.Debugln(result)
}
return []byte(result), nil
}
func (d *DataWrapper) ToJSON() []byte {
result, err := json.Marshal(d)
if err != nil {
log.Fatalln(err)
panic(err)
}
return result
}
At the line Logger.Debugln(result), it prints this result:
20170830090317506 20170830090026319 20170830111023194 201708301043081 ...
json: error calling MarshalJSON for type models.Int64Slice: invalid
character '2' after top-level value

Answer 1

I think you have it backwards.
Use the bytes.Buffer type to incrementally build up the string representation of your data.
The program
package main
import (
"bytes"
"encoding/json"
"os"
"strconv"
)
type Int64Slice []int64
func (s Int64Slice) MarshalJSON() ([]byte, error) {
if s == nil {
return []byte("null"), nil
}
var b bytes.Buffer
b.WriteByte('"')
for i, v := range s {
if i > 0 {
b.WriteByte('\x20')
}
b.WriteString(strconv.FormatInt(v, 10))
}
b.WriteByte('"')
return b.Bytes(), nil
}
func main() {
var (
a Int64Slice = nil
b = Int64Slice{
42,
12,
0,
}
)
enc := json.NewEncoder(os.Stdout)
enc.Encode(a)
enc.Encode(b)
}
Prints:
null
"42 12 0"
Playground link.

Answer 2

20170830090317506 20170830090026319 20170830111023194 201708301043081 is not a valid JSON value. It is interpreted as a valid number (20170830090317506) followed by a valid space, followed by invalid data, beginning with the 2 character; thus the error you observed.
It needs quotes around it:
Try something like:
result = `"` + strings.Trim(strings.Join(strings.Fields(fmt.Sprint(u)), " "), "[]") + `"`