nil slice when passed as interface is not nil! Why? (golang)

go, null

Solution

Admittedly, it's somewhat of a quirk, but there's an explanation for it.

Imagine an `interface{}` variable as a struct composed of two fields: one is the type and another is the data. (`[]int` and `nil`). Actually, it looks just like that in the Go runtime.

struct Iface                                                                                                                   
{                                                                                                                              
    Itab*   tab;                                                                                                               
    void*   data;                                                                                                              
};

When you pass your nil slice to `yes`, only `nil` is passed as the value, so your comparison boils down to `nil == nil`.

Meanwhile, calling `no` automatically wraps your variable in an `interface{}` type and the call becomes something akin to `no(interface{[]int, nil})`. So the comparison in `no` could be seen as `interface{[]int, nil} == nil`, which turns out to be false in go.

The issue is actually explained in the Go FAQ.

Problem

See this playground: http://play.golang.org/p/nWHmlw1W01 ``` package main import "fmt" func main() { var i []int = nil yes(i) // output: true no(i) // output: false } func yes(thing []int) { fmt.Println(thing == nil) } func no(thing interface{}) { fmt.Println(thing == nil) } ``` Why the difference in output between the two functions?

Original source