What Is a Nil Interface vs a Nil Concrete Value in Go

A nil interface value is an interface variable that holds no type or value, whereas a nil concrete value is a specific type (like a pointer or slice) that is nil but still stored inside a non-nil interface. You can distinguish them because a nil concrete value is not equal to a nil interface, even if the underlying data is nil. Use reflect.TypeOf() to check if the interface holds a type; if it returns nil, the interface itself is nil, but if it returns a type, the interface is non-nil even if the value inside is nil.

var i interface{}
var p *int = nil
i = p

// i is not nil because it holds the type *int, even though p is nil
if i == nil {
    fmt.Println("i is nil")
} else {
    fmt.Println("i is not nil, but holds a nil value")
}

// Check the concrete type
if reflect.TypeOf(i) == nil {
    fmt.Println("Interface is nil")
} else {
    fmt.Println("Interface holds a type:", reflect.TypeOf(i))
}

Think of an interface as a box that can hold any item. A nil interface is an empty box with nothing inside. A nil concrete value is a box that contains a specific item (like a pointer), but that item itself is empty or null. The box is not empty because it holds the item, even if the item has no value. This matters because code checking if box == nil will fail if the box contains a nil item.