Why does comparing a nil slice to an interface{} result in a different outcome than directly comparing a nil slice to nil?

Passing Nil Slice to Interface: Unraveling the Enigma

In Go, the distinction between a nil slice and a nil interface{} value can lead to unexpected results. Consider the following playground excerpt:

<code class="go">package main

import "fmt"

func main() {
    var i []int = nil
    yes(i) // true
    no(i)  // false
}

func yes(thing []int) {
    fmt.Println(thing == nil)
}

func no(thing interface{}) {
    fmt.Println(thing == nil)
}</code>

Why do these functions produce different outputs?

Understanding Interface{}

An interface{} variable resembles a struct with two fields: type and data. In this case, it looks like {[]int (type), nil (data)}.

Nil Slice vs. Nil Interface

In yes, the nil slice is passed directly. The comparison checks if nil equals nil, which evaluates to true.

In no, the nil slice is wrapped in an interface{}, resulting in {[]int (type), nil (data)}. Comparing this interface with nil checks if the type is nil, which is not the case. Therefore, the output is false.

Reason for the Difference

This behavior stems from the fact that interfaces encapsulate both type and value. Comparing an interface to nil checks for a nil type, while comparing the underlying data value is different. In the case of no, the type is not nil, so the comparison returns false.

Conclusion

The seemingly paradoxical behavior is due to the subtle differences between nil slices and nil interfaces. Understanding this distinction is crucial to avoid confusion when working with interfaces in Go.

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