Interface methods return itself or another interface

[drumsta]

How to deal with the case below?

// Bad.
type Doer interface { Do() Doer }
type IDoer struct{}
func New() Doer { return new(IDoer)}
func (d *IDoer) Do() Doer {/*...*/ return d }

// Good.
type Doer interface { Do() Doer }
type IDoer struct{}
func New() *IDoer { return new(IDoer)}
func (d *IDoer) Do() *IDoer {/*...*/ return d}

// Very Good (Verify Interface Compliance in compile time)
var _ Doer = (*IDoer)(nil)

The above won't compile because of the error:

cannot use (*IDoer)(nil) (value of type *IDoer) as Doer value in variable declaration: *IDoer does not implement Doer (wrong type for method Do)
		have Do() *IDoer
		want Do() Doer

This use case is quite common designing libraries. As an example, standard slog package has this interface, that's Handler methods return the Handler interface.

type Handler interface {
	Enabled(context.Context, Level) bool
	WithAttrs(attrs []Attr) Handler
	WithGroup(name string) Handler
}

[butuzov]

closed as duplicate of #21