How Can I Extend JSON Output with Unknown Structs in Go?

Extending JSON Output with Unknown Structs

Problem Overview

Adding arbitrary fields to JSON output can be achieved by extending a known struct anonymously. However, this approach is limited when dealing with unknown structs or interfaces. This article explores solutions to this challenge.

Solution 1: Dynamic Type Generation using Reflection

One solution involves generating a dynamic type at runtime using the reflect package. This new type is a struct with an anonymous field of the wrapped interface type and an additional field for the extra value. By reflecting on the value and setting the fields accordingly, we can obtain the desired JSON output.

func printInterface(val interface{}) {
    // Create a new struct type with anonymous field for the interface
    t2 := reflect.StructOf([]reflect.StructField{
        {Name: "X", Anonymous: true, Type: reflect.TypeOf(val)},
        {Name: "Extra", Type: reflect.TypeOf("")},
    })

    // Create a new value of the dynamic type
    v2 := reflect.New(t2).Elem()
    // Set the value of the anonymous field to the input interface
    v2.Field(0).Set(reflect.ValueOf(val))
    // Set the extra field to the desired value
    v2.FieldByName("Extra").SetString("text")

    json.NewEncoder(os.Stdout).Encode(v2.Interface())
}

Solution 2: Marshaling and Unmarshaling

Alternatively, we can marshal the value to JSON, unmarshal it into a map, add the extra field, and marshal the result again.

func printInterface(val interface{}) error {
    // Marshal the value to JSON
    data, err := json.Marshal(val)
    if err != nil {
        return err
    }

    // Unmarshal the JSON into a map
    v2 := map[string]interface{}{}
    if err := json.Unmarshal(data, &v2); err != nil {
        return err
    }

    // Add the extra field
    v2["Extra"] = "text"

    // Marshal the map to JSON
    return json.NewEncoder(os.Stdout).Encode(v2)
}

Comparison of Solutions

The reflection-based solution generates a new type specifically for the given interface, resulting in a more tailored and potentially faster approach. The marshaling and unmarshaling solution is simpler but slower due to the additional marshaling step. The order of fields in the JSON output may also differ in the latter approach.

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