Can't return nil, but zero value of slice

I am having the case in which a function with the following code:

func halfMatch(text1, text2 string) []string {
    ...
    if (condition) {
         return nil // That's the final code path)
    }
    ...
}

is returning []string(nil) instead of nil. At first, I thought that perhaps returning nil in a function with a particular return type would just return an instance of a zero-value for that type. But then I tried a simple test and that is not the case.

Does anybody know why would nil return an empty string slice?

Nil is not a type. It is a description of the zero value for maps, chans, pointers, functions, slices, and interfaces.

When you put "nil" in your program, go gives it a type depending on the context. For the most part, you never need to explicitly type your nil. This is no exception, the compiler knows it must be a []string(nil) because the type returned is []string.

A nil string slice is a slice with no backing array and a length/capacity of zero. You may compare it to the literal "nil" and can get its length and capacity. It is a []string, just empty. If you wish to have an empty []string that is not nil, return []string{}. This creates a backing array (of length zero) and makes it no longer equivalent to nil.

I believe I know what's going on. The assert library I am using (github.com/bmizerany/assert) is using internally a reflect.DeepEqual.

The return value of func halfMatch(text1, text2 string) []string is always of type []string, but if it returns nil and is compared to a nil value via the == operator, it will return true. However, if reflect.DeepEqual is used, the type will matter and it won't consider both values the same.

playgound link with the test

I think maybe you are being confused by the output of print. (playground link)

package main

import "fmt"

func f() []string {
	return nil // That's the final code path)
}
func main() {
	result := f()
	fmt.Printf("result = %v\n", result)
	fmt.Printf("result = %v\n", result == nil)
}

Which produces

result = []
result = true

So I think the output really is nil

Go will return an enpty slice if condition is true.

There is a problem with your "test" because if you try to compare [] with nil, you get true.
I have modified your test to show you what I mean

package main
import "fmt"

func main() {
    //fmt.Println(test1("") == nil)  
	fmt.Println(test1("")) // prints [] 
}

func test1(text1 string) []string {
	if len(text1) > 0 {
		return []string{text1}
	}
	return nil
}