How to implement a custom expvar.Var for complex data types in Go?

To implement a custom expvar.Var for complex data types in Go, you need to create a struct that implements the expvar.Var interface. Here's an example:

package main

import (
	"encoding/json"
	"expvar"
	"log"
	"net/http"
)

type MyVar struct {
	data interface{}
}

func (v *MyVar) String() string {
	// Convert complex data to a string representation
	bytes, err := json.Marshal(v.data)
	if err != nil {
		log.Println("Error marshaling data:", err)
		return ""
	}
	return string(bytes)
}

func (v *MyVar) Set(data interface{}) {
	v.data = data
}

func main() {
	// Create an instance of your custom var
	myVar := &MyVar{}

	// Register your var with expvar
	expvar.Publish("my_var", myVar)

	// Serve the expvar endpoints
	http.Handle("/debug/vars", expvar.Handler())
	log.Fatal(http.ListenAndServe(":8080", nil))
}

In the above example, MyVar is a struct that holds your complex data and implements the expvar.Var interface. The String() method converts the complex data to a string representation using JSON marshaling. The Set() method sets the complex data.

In the main() function, you create an instance of your MyVar struct and register it with expvar using the Publish() function. Then, you use the expvar.Handler() to serve the expvar endpoints on the /debug/vars route.

Now, when you run your Go program and navigate to http://localhost:8080/debug/vars, you should see your custom variable listed with its JSON representation.