Zero values

Explanation

Zero values or zero initialization are simple to implement. Coming from languages like Java it may seem complicated that some values can be nil while others are not. In summary from Zero Value: The Go Programming Language Specification:

Pointers, functions, interfaces, slices, channels, and maps are the only types that can be nil. The rest are initializated to false, zero, or empty strings based on their respective types.

If a functions that checks some condition, problems may arise:

func isAlive() bool {
    //Not implemented yet
    return false
}

The zero value will be false even before implementation. Unit tests dependant on the return of this function could be giving false positives/negatives.

A typical workaround is to also return an error, which is idiomatic in Go:

package main

import "fmt"

func isAlive() (bool, error) {
    //Not implemented yet
    return false, fmt.Errorf("Not implemented yet")
}

func main() {
    _, err := isAlive()
    if err != nil {
        fmt.Printf("ERR: %s\n", err.Error())
    }
    
}

play it on playground

When returning both a struct and an error you need a User structure for return, which is not very elegant. There are two counter-options:

Work with interfaces: Return nil by returning an interface.
Work with pointers: A pointer can be nil

For example, the following code returns a pointer:

func(d *DB) GetUser(id uint64) (*User, error) {
    //Some error ocurred
    return nil, err
}