Should You Return Empty or Nil Slices in Go?

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The post Should You Return Empty or Nil Slices in Go? first appeared on Qvault.

In Go, we are often returning zero values. Idiomatic Go encourages the use of guard clauses, and guard clauses necessitate the need to return early. When returning early with an error, by convention all other return values should be zero values. The confusion arises with data types like maps and slices… should they be nil, or should they be empty?

The question is, should we use this syntax:

func getItems(url string) ([]string, error) {
    data, err := makeRequest(url)
    if err != nil {
        return nil, err
    }
    items, err := unpack(data)
    if err != nil {
        return nil, err
    }
    return data, nil
}

or this syntax?

func getItems(url string) ([]string, error) {
    data, err := makeRequest(url)
    if err != nil {
        return []string{}, err
    }
    items, err := unpack(data)
    if err != nil {
        return []string{}, err
    }
    return data, nil
}

The Differences

I ran a quick bit of code to show some of the differences between nil and empty slices:

package main

import (
    "encoding/json"
    "fmt"
)

func main() {
    var nilSlice []string
    fmt.Println(nilSlice) // Output: []
    fmt.Println(len(nilSlice), cap(nilSlice)) // Output: 0 0
    fmt.Println(nilSlice == nil) // Output: true
    dat, _ := json.Marshal(nilSlice)
    fmt.Println(string(dat)) // Output: null

    emptySlice := []string{}
    fmt.Println(emptySlice) // Output: []
    fmt.Println(len(emptySlice), cap(emptySlice)) // Output: 0 0
    fmt.Println(emptySlice == nil) // Output: false
    dat, _ = json.Marshal(emptySlice)
    fmt.Println(string(dat)) // Output: []
}

As you can see there are some similarities between nil and empty slices:

Both have zero length and cap
Both print []
Both can be used the same way in range loops and append functions (not shown here)

They differ in the following ways:

Only the nil slice will success a nil check
When encoded as JSON using the standard library, the nil slice becomes null and the empty slice becomes []

Which Should I Do?

Generally speaking, prefer nil.

According to the Go wiki, nil is the preferred style. When we just need to return an empty slice nil will work great in practically all cases. It’s also easier to type nil than []string{} or make([]string, 0) and typically gets syntax highlighted which makes it easier to read.