Nil-assignment gotcha

nil-interface.go

package main

import (
	"fmt"
)

type Fooer interface {
	Foo()
}

type X struct{}

func (x X) Foo() { return }

func getNilX() *X {
	return nil
}

func main() {
	var fooer Fooer
	fooer = getNilX()
	if fooer == nil {
		fmt.Println("fooer DOES == nil")
	} else {
		fmt.Printf("fooer != nil: %v\n", fooer)
		// we hit this because an interface variable stores both a type and a value
		// fooer, right now, has been assigned something like:
		// struct {
		// 	typ: reflect.Type
		//	val: reflect.Value
		// } {
		//	type: reflect.TypeOf(&X{}),
		//	val:  *X(nil),
		// }
		// So while the VALUE is nil, the variable 'fooer' itself is non-nil.
	}
}

output.txt

fooer != nil: <nil>