Golang: An interface holding a nil value is not nil

• Migrated to personal blog: link
• Visit pacebits.com to check out recent projects I’m working on

Let’s start directly with an example (go playground):

package main
import "fmt"
func main() {
    var a interface{}
    fmt.Printf("a == nil is %t\n", a == nil)  
    var b interface{}
    var p *int = nil
    b = p
    fmt.Printf("b == nil is %t\n", b == nil)
}

What is your expectation for the output? It will end up like:

a == nil is true
b == nil is false

It is somehow counter-intuitive at first glance, but it makes a lot more sense if we explore a little bit about the reflection model in Golang.

Under the hood, an interface in Golang consists of two elements: type and value. When we assign a nil integer pointer to an interface in our example above, the interface becomes (*int)(nil), and it is not nil. An interface equals nil only if both the type and value are nil.

Here is another example of this (go playground), which is a bad pattern to return the error:

package main
import "fmt"
func main() {
    err :=…