Change interface value by reference

package main

import (
	"fmt"
)

// -------- Library code. Can't change ------------
type Client struct {
	transport RoundTripper
}

type RoundTripper interface {
	Do()
}

type Transport struct{}

func (d Transport) Do() {}

var DefaultTransport RoundTripper = Transport{}

// -------- My code. Can change ------------
func changeTransport(r RoundTripper) {
	if r == nil {
		fmt.Println("transport is nil")
		r = DefaultTransport
	}
}

func main() {
	c := Client{}
	changeTransport(c.transport)
	fmt.Println(c.transport)
}

Output:

transport is nil
<nil>

Expected:

transport is nil
{}

Playground

I also tried this based on https://stackoverflow.com/a/44905592/6740589:

func changeTransport(r RoundTripper) {
	if r == nil {
		fmt.Println("transport is nil")
		d, ok := DefaultTransport.(Transport)
		if !ok {
			log.Fatal("impossible")
		}

		if t, ok := r.(*Transport); ok {
			t = &d
			fmt.Println("ignoreme", t)
		} else {
			log.Fatal("Uff")
		}

	}
}

Output:

transport is nil
2009/11/10 23:00:00 Uff

Playground

Use pointer of the RoundTripper interface as the changeTransport function argument to change pointer's value:

// -------- My code. Can change ------------
func changeTransport(r *RoundTripper) {
	if r != nil && *r == nil {
		fmt.Println("transport is nil")
		*r = DefaultTransport
	}
}

func main() {
	c := Client{}
	changeTransport(&c.transport)
	fmt.Println(c.transport)
}

transport is nil
{}

<kbd>PLAYGROUND</kbd>