Why Does Comparing a Nil Slice to `nil` Differ Between Concrete Types and Interfaces in Go?

Interface type and nil slices

In Go, when passing a variable to a function that expects an interface type, the behavior may differ based on whether the variable is a slice or not.

Consider the following snippet:

<code class="go">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)
}</code>

In this program, the yes function expects a slice of integers, while the no function expects an interface type. When we call yes with the nil slice i, the output is true because the slice itself is nil. However, calling no with the nil slice results in false.

To understand why this happens, we need to delve into the implementation of interfaces in Go. Internally, an interface type is represented as a struct containing two fields: Itab, which refers to the type descriptor, and Data, which holds the actual value.

When passing a nil slice to yes, only the nil value is passed as Data, and the comparison effectively becomes nil == nil, which is true. However, when passing a nil slice to no, Go automatically wraps it in an interface type, resulting in something like no(interface{[]int, nil}). In this case, the comparison becomes interface{[]int, nil} == nil, which returns false because the interface type itself is not nil, even though the underlying data is nil.

This behavior can be attributed to the nature of interfaces and how they interact with nil values. As the Go FAQ explains, nil interfaces and nil concrete values are distinct concepts. When an interface type holds a nil value, it does not mean that the interface itself is nil. Therefore, comparing an interface type containing a nil value to nil will result in false.

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