Go gotcha: Why is nil not equal to nil?

Why is nil not equal to nil in this example?

func Foo() error {
	var err *os.PathError = nil
	// …
	return err
}

func main() {
	err := Foo()
	fmt.Println(err)        // <nil>
	fmt.Println(err == nil) // false
}
Answer

An interface value is equal to nil only if both its value and dynamic type are nil. In the example above, Foo() returns [nil, *os.PathError] and we compare it with [nil, nil].

You can think of the interface value nil as typed, and nil without type doesn’t equal nil with type. We can illustrate this by converting the nil to the correct type in the comparison:

…
fmt.Println(err == (*os.PathError)(nil)) // true

A better approach

To avoid this problem use a variable of type error instead, for example a named return value:

func Foo() (err error) {
	// …
	return // err is unassigned and has zero value [nil, nil]
}

func main() {
	err := Foo()
	fmt.Println(err)        // <nil>
	fmt.Println(err == nil) // true
}
Best practice: Use the built-in error interface type, rather than a concrete type, to store and return error values.

See Interfaces explained for an extensive guide to interfaces in Go.

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