Using json.Marshaler in GeoJSON

geojson.go

package main

import (
	"encoding/json"
	"fmt"
	"log"
	"os"
)

//type Point2D [2]float64

type Point2D struct {
	X float64
	Y float64
}

type Properties struct {
	// ...
}

type Feature struct {
	Type       string                  `json:"type"`
	Geometry   *map[string]interface{} `json:"geometry"`
	Properties *Properties             `json:"properties,omitempty"`
}

type Geometry struct {
	// the geometry type
	Type string `json:"type"`
}

type Point struct {
	// the geometry type
	Type        string   `json:"type"`
	Coordinates *Point2D `json:"coordinates,omitempty"`
}

func (p *Point2D) MarshalJSON() ([]byte, error) {
	return []byte(fmt.Sprintf("[%.6f,%.6f]", p.X, p.Y)), nil
}

func main() {
	feature := Feature{
		Type: "Feature",
		Geometry: &map[string]interface{}{
			"type":     "Point",
			"geometry": &Point2D{12.3567, 89.0123},
		},
	}

	enc := json.NewEncoder(os.Stdout)
	enc.SetIndent("", "    ")
	if err := enc.Encode(&feature); err != nil {
		log.Fatalf("error: %v\n", err)
	}
}