How to pass data struct as parameter in golang

XML to Json

package main
import (
"encoding/json"
"encoding/xml"
"fmt"
)
type Persons struct {
Person []struct {
Name string
Age  int
}
}
type Places struct {
Place []struct {
Name    string
Country string
}
}
type Parks struct {
Park struct {
Name     []string
Capacity []int
}
}
const personXml = `
<Persons>
<Person><Name>Koti</Name><Age>30</Age></Person>
<Person><Name>Kanna</Name><Age>29</Age></Person>
</Persons>
`
const placeXml = `
<Places>
<Place><Name>Chennai</Name><Country>India</Country></Place>
<Place><Name>London</Name><Country>UK</Country></Place>
</Places>
`
const parkXml = `
<Parks>
<Park><Name>National Park</Name><Capacity>10000</Capacity></Park>
<Park>Asian Park</Name><Capacity>20000</Capacity></Park>
</Parks>
`
func WhatIamUsing() {
var persons Persons
xml.Unmarshal([]byte(personXml), &persons)
per, _ := json.Marshal(persons)
fmt.Printf("%s\n", per)
var places Places
xml.Unmarshal([]byte(placeXml), &places)
pla, _ := json.Marshal(places)
fmt.Printf("%s\n", pla)
var parks Parks
xml.Unmarshal([]byte(parkXml), &parks)
par, _ := json.Marshal(parks)
fmt.Printf("%s\n", par)
}

What i want is a generic function which takes xml string and dataStruct
and returns a Json output. But below function is throwing an error
How to impliment this?

func Xml2Json(xmlString string, DataStruct interface{}) (jsobj string, err error) {
var dataStruct DataStruct
xml.Unmarshal([]byte(personXml), &dataStruct)
js, _ := json.Marshal(dataStruct)
return fmt.Sprintf("%s\n", js), nil
}
func main() {
jsonstring, _ := Xml2Json(personXml, Persons)
}

Error message:

prog.go:73: DataStruct is not a type

prog.go:80: type Persons is not an expression

goplay link: http://play.golang.org/p/vayb0bawKx

You can not store a type (like Persons) in an interface. You could pass a reflect.Type to your function. Then, your call would look like Xml2Json(personXml, reflect.TypeOf(Persons)) which is quite ugly in my opinion.

The better approach is probably:

func Xml2Json(xmlString string, value interface{}) (string, error) {
if err := xml.Unmarshal([]byte(xmlString), value); err != nil {
return "", err
}
js, err := json.Marshal(value)
if err != nil {
return "", err
}
return string(js), nil
}

You can use this function with Xml2Json(personXml, new(Persons)) if you are not interested in the value itself, and

var persons Persons
Xml2Json(personXML, &persons)

when you also want to retrieve the struct value for later processing.

The function Xml2Json declares a parameter named DataStruct. In the same scope the identifier DataStruct cannot denote a type name. If you want to use the type name DataStruct within the same scope, you must name your parameter differently.

In the main function the problem is that a function call syntax expects an expression list within the parenthesis. There you're passing a type name, which clearly cannot be an expression.

So to answer your question: No, you cannot pass a type as an argument to a function. But you can pass an instance of a type (in this case a pointer to such instance) to get the effect you're about:

package main
import (
"encoding/json"
"encoding/xml"
"fmt"
"log"
)
type Persons struct {
Person []struct {
Name string
Age  int
}
}
type Places struct {
Place []struct {
Name    string
Country string
}
}
type Parks struct {
Park struct {
Name     []string
Capacity []int
}
}
const (
personXml = `
<Persons>
<Person><Name>Koti</Name><Age>30</Age></Person>
<Person><Name>Kanna</Name><Age>29</Age></Person>
</Persons>
`
placeXml = `
<Places>
<Place><Name>Chennai</Name><Country>India</Country></Place>
<Place><Name>London</Name><Country>UK</Country></Place>
</Places>
`
parkXml = `
<Parks>
<Park><Name>National Park</Name><Capacity>10000</Capacity></Park>
<Park><Name>Asian Park</Name><Capacity>20000</Capacity></Park>
</Parks>
`
)
func Xml2Json(xmlString string, DataStruct interface{}) (jsobj string, err error) {
if err = xml.Unmarshal([]byte(xmlString), DataStruct); err != nil {
return
}
js, err := json.Marshal(DataStruct)
if err != nil {
return
}
return fmt.Sprintf("%s", js), nil
}
func main() {
var p Persons
jsonstring, err := Xml2Json(personXml, &p)
if err != nil {
log.Fatal(err)
}
fmt.Println(jsonstring)
var q Places
jsonstring, err = Xml2Json(placeXml, &q)
if err != nil {
log.Fatal(err)
}
fmt.Println(jsonstring)
var r Parks
jsonstring, err = Xml2Json(parkXml, &r)
if err != nil {
log.Fatal(err)
}
fmt.Println(jsonstring)
}

Playground

Output:

{"Person":[{"Name":"Koti","Age":30},{"Name":"Kanna","Age":29}]}
{"Place":[{"Name":"Chennai","Country":"India"},{"Name":"London","Country":"UK"}]}
{"Park":{"Name":["National Park","Asian Park"],"Capacity":[10000,20000]}}