Nil

In Go we use the constant nil instead of null. This means "nothing exists." A slice can be uninitialized, and in this state it is nil. We must be careful with nil things.

Unlike in many languages, we cannot use nil in place of a string. Nil is not allowed in string typed collections. An empty string may be used instead.

Example program

Let us consider this example Go program. When we use the "var" keyword, we declare variables. But these variables are not initialized when created.

package main

import "fmt"

func main() {
    // This is an uninitialized slice.
    var temp []int

    // The uninitialized slice is nil.
    if temp == nil {
        fmt.Println("Uninitialized slice is nil")
    }
}
Uninitialized slice is nil

String error

This program shows the error when trying to use nil in a string slice. Here we could use an empty string to mean "no value."

package main

func main() {
    temp := []string{}
    temp = append(temp, nil)
}
# command-line-arguments
C:\programs\file.go:8: cannot use nil as type string in append

Some research

It is useful to research each programming concept until you understand it. In Go we find that slices can be left uninitialized, and these equal nil.

With nil we can test for uninitialized (or nonexistent) things. A slice variable that has not been assigned yet is nil.