Golang Unmarshal slice of type interface

Question

In this example, I would be trying to load 2D scenes that contain polygons. In the code, I would have many different structs such as Circle, Square, Rectangle, Pentagon, etc.. All would share common funcs, such as Area and Perimeter. The scene itself would be stored as a slice of the interface Polygon.

Here is the code I'm using to test this:

package main

import (
    "encoding/json"
    "fmt"
    "math"
)

type Polygon interface {
    Area() float32
}

type Rectangle struct {
    Base   float32 `json:"base"`
    Height float32 `json:"height"`
    X      float32 `json:"x"`
    Y      float32 `json:"y"`
}

func (r *Rectangle) Area() float32 {
    return r.Base * r.Height
}

type Circle struct {
    Radius float32 `json:"radius"`
    X      float32 `json:"x"`
    Y      float32 `json:"y"`
}

func (c *Circle) Area() float32 {
    return c.Radius * c.Radius * math.Pi
}

func main() {
    rect := Rectangle{Base: 10, Height: 10, X: 10, Y: 10}
    circ := Circle{Radius: 10, X: 0, Y: 0}

    sliceOfPolygons := make([]Polygon, 0, 2)

    sliceOfPolygons = append(sliceOfPolygons, &rect, &circ)

    jsonData, err := json.Marshal(sliceOfPolygons)
    if err != nil {
        panic(err)
    }

    fmt.Println(string(jsonData))

    newSlice := make([]Polygon, 0)

    err = json.Unmarshal(jsonData, &newSlice)
    if err != nil {
        panic(err)
    }
}

In this example, I setup a slice of 2 polygons, marshal it and then try to unmarshal it again. The marshal-ed string is:

[{"base":10,"height":10,"x":10,"y":10},{"radius":10,"x":0,"y":0}]

But when I try to Unmarshal it panics:

panic: json: cannot unmarshal object into Go value of type main.Polygon

If this worked it would be really useful and simple to use. I'd say that Unmarshall can't distinguish between a Rectangle and a Circle from the json string so it can't possibly know what struct to build.

Is there any way to tag the struct or tell Unmarshal how to distinguish this structs?

Answer 1

Thought that way to distinguish whether the json is Circle or Rectangle. In your JSON, there is no identify for the struct which can detect kind of objects. So let's make rules.

• Rectangle have base and height both greater than 0
• Circle have radius greater than 0

To unmarshal JSON, it should have commonly fields like below.

type Object struct {
    Base   float32 `json:"base,omitempty"`
    Radius float32 `json:"radius,omitempty"`
    Height float32 `json:"height,omitempty"`
    X      float32 `json:"x"`
    Y      float32 `json:"y"`
}

This struct can be stored Rectangle or Circle both. Then, add method IsCircle and IsRectangle.

func (obj *Object) IsCircle() bool {
    return obj.Radius > 0
}

func (obj *Object) IsRectangle() bool {
    return obj.Base > 0 && obj.Height > 0
}

You can make method like Kind() to return identity of struct instead. As you think best. Finally, you should add ToCircle/ToRectangle methods.

func (obj *Object) ToCircle() *Circle {
    return &Circle{
        Radius: obj.Radius,
        X:      obj.X,
        Y:      obj.Y,
    }
}

func (obj *Object) ToRectangle() *Rectangle {
    return &Rectangle{
        Base:   obj.Base,
        Height: obj.Height,
        X:      obj.X,
        Y:      obj.Y,
    }
}

If you want slice of Polygon interface, you should convert this slice of Object to the slice of Polygon like below.

var polygons []Polygon
for _, obj := range newSlice {
    if obj.IsCircle() {
        polygons = append(polygons, obj.ToCircle())
    } else if obj.IsRectangle() {
        polygons = append(polygons, obj.ToRectangle())
    }
}

https://play.golang.org/p/kO_F4GTYdA

UPDATE

One another approach. Make converters which convert from map[string]interface{}. The converter can detect the struct with looking fields exists.

var converters = []func(map[string]interface{}) Polygon{
    func(m map[string]interface{}) Polygon {
        rectangle := new(Rectangle)
        if base, ok := m["base"]; ok {
            rectangle.Base = toFloat32(base)
        } else {
            return nil
        }
        if height, ok := m["height"]; ok {
            rectangle.Height = toFloat32(height)
        } else {
            return nil
        }
        if x, ok := m["x"]; ok {
            rectangle.X = toFloat32(x)
        }
        if y, ok := m["y"]; ok {
            rectangle.Y = toFloat32(y)
        }
        return rectangle
    },
    func(m map[string]interface{}) Polygon {
        circle := new(Circle)
        if radius, ok := m["radius"]; ok {
            circle.Radius = toFloat32(radius)
        } else {
            return nil
        }
        if x, ok := m["x"]; ok {
            circle.X = toFloat32(x)
        }
        if y, ok := m["y"]; ok {
            circle.Y = toFloat32(y)
        }
        return circle
    },
}

And do convert

var polygons []Polygon
for _, obj := range newSlice {
    m, ok := obj.(map[string]interface{})
    if !ok {
        panic("invalid struct")
    }
    var p Polygon
    for _, converter := range converters {
        p = converter(m)
        if p != nil {
            break
        }
    }
    if p == nil {
        panic("unknown polygon")
    }
    polygons = append(polygons, p)
}

https://play.golang.org/p/PrxiMOa_1F