Meaning of a struct with embedded anonymous interface?

sort package:

type Interface interface {
	Len() int
	Less(i, j int) bool
	Swap(i, j int)
}

...

type reverse struct {
	Interface
}

What is the meaning of anonymous interface Interface in struct reverse?

In this way reverse implements the sort.Interface and we can override a specific method
without having to define all the others

type reverse struct {
        // This embedded Interface permits Reverse to use the methods of
        // another Interface implementation.
        Interface
}

Notice how here it swaps (j,i) instead of (i,j) and also this is the only method declared for the struct reverse even if reverse implement sort.Interface

// Less returns the opposite of the embedded implementation's Less method.
func (r reverse) Less(i, j int) bool {
        return r.Interface.Less(j, i)
}

Whatever struct is passed inside this method we convert it to a new reverse struct.

// Reverse returns the reverse order for data.
func Reverse(data Interface) Interface {
        return &reverse{data}
}

The real value comes if you think what would you have to do if this approach was not possible.

1. Add another Reverse method to the sort.Interface ?
2. Create another ReverseInterface ?
3. ... ?

Any of this change would require many many more lines of code across thousands of packages that want to use the standard reverse functionality.

Ok, the accepted answer helped me understand, but I decided to post an explanation which I think suits better my way of thinking.

The "Effective Go" has example of interfaces having embedded other interfaces:

// ReadWriter is the interface that combines the Reader and Writer interfaces.
type ReadWriter interface {
    Reader
    Writer
}

and a struct having embedded other structs:

// ReadWriter stores pointers to a Reader and a Writer.
// It implements io.ReadWriter.
type ReadWriter struct {
    *Reader  // *bufio.Reader
    *Writer  // *bufio.Writer
}

But there is no mention of a struct having embedded an interface. I was confused seeing this in sort package:

type Interface interface {
	Len() int
	Less(i, j int) bool
	Swap(i, j int)
}

...

type reverse struct {
	Interface
}

But the idea is simple. It's almost the same as:

type reverse struct {
    IntSlice  // IntSlice struct attaches the methods of Interface to []int, sorting in increasing order
}

methods of IntSlice being promoted to reverse.

And this:

type reverse struct {
    Interface
}

means that sort.reverse can embed any struct that implements interface sort.Interface and whatever methods that interface has, they will be promoted to reverse.

sort.Interface has method Less(i, j int) bool which now can be overridden:

// Less returns the opposite of the embedded implementation's Less method.
func (r reverse) Less(i, j int) bool {
        return r.Interface.Less(j, i)
}

My confusion in understanding

type reverse struct {
    Interface
}

was that I thought that a struct always has fixed structure, i.e. fixed number of fields of fixed types.

But the following proves me wrong:

package main

import "fmt"

// some interface
type Stringer interface {
	String() string
}

// a struct that implements Stringer interface
type Struct1 struct {
	field1 string
}

func (s Struct1) String() string {
	return s.field1
}


// another struct that implements Stringer interface, but has a different set of fields
type Struct2 struct {
	field1 []string
	dummy bool
}

func (s Struct2) String() string {
	return fmt.Sprintf("%v, %v", s.field1, s.dummy)
}


// container that can embedd any struct which implements Stringer interface
type StringerContainer struct {
	Stringer
}


func main() {
	// the following prints: This is Struct1
	fmt.Println(StringerContainer{Struct1{"This is Struct1"}})
	// the following prints: [This is Struct1], true
	fmt.Println(StringerContainer{Struct2{[]string{"This", "is", "Struct1"}, true}})
	// the following does not compile:
	// cannot use "This is a type that does not implement Stringer" (type string)
	// as type Stringer in field value:
    // string does not implement Stringer (missing String method)
    fmt.Println(StringerContainer{"This is a type that does not implement Stringer"})
}

The statement

type reverse struct {
    Interface
}

enables you to initialize reverse with everything that implements the interface Interface. Example:

&reverse{sort.Intslice([]int{1,2,3})}

This way, all methods implemented by the embedded Interface value get populated to the outside while you are still able to override some of them in reverse, for example Less to reverse the sorting.

This is what actually happens when you use sort.Reverse. You can read about embedding in the struct section of the spec.

I will give my explanation too. The sort package defines an unexported type reverse, which is a struct, that embeds Interface.

type reverse struct {
        // This embedded Interface permits Reverse to use the methods of
        // another Interface implementation.
        Interface
}

This permits Reverse to use the methods of another Interface implementation. This is the so called composition, which is a powerful feature of Go.

The Less method for reverse calls the Less method of the embedded Interface value, but with the indices flipped, reversing the order of the sort results.

// Less returns the opposite of the embedded implementation's Less method.
func (r reverse) Less(i, j int) bool {
        return r.Interface.Less(j, i)
}

Len and Swap the other two methods of reverse, are implicitly provided by the original Interface value because it is an embedded field. The exported Reverse function returns an instance of the reverse type that contains the original Interface value.

// Reverse returns the reverse order for data.
func Reverse(data Interface) Interface {
        return &reverse{data}
}

I find this feature very helpful when writing mocks in tests.

Here is such an example:

package main_test

import (
	"fmt"
	"testing"
)

// Item represents the entity retrieved from the store
// It's not relevant in this example
type Item struct {
	First, Last string
}

// Store abstracts the DB store
type Store interface {
	Create(string, string) (*Item, error)
	GetByID(string) (*Item, error)
	Update(*Item) error
	HealthCheck() error
	Close() error
}

// this is a mock implementing Store interface
type storeMock struct {
	Store
	// healthy is false by default
	healthy bool
}

// HealthCheck is mocked function
func (s *storeMock) HealthCheck() error {
	if !s.healthy {
		return fmt.Errorf("mock error")
	}
	return nil
}

// IsHealthy is the tested function
func IsHealthy(s Store) bool {
	return s.HealthCheck() == nil
}

func TestIsHealthy(t *testing.T) {
	mock := &storeMock{}
	if IsHealthy(mock) {
		t.Errorf("IsHealthy should return false")
	}

	mock = &storeMock{healthy: true}
	if !IsHealthy(mock) {
		t.Errorf("IsHealthy should return true")
	}
}

By using:

type storeMock struct {
    Store
    ...
}

One doesn't need to mock all Store methods. Only HealthCheck can be mocked, since only this method is used in the TestIsHealthy test.

Below the result of the test command:

$ go test -run '^TestIsHealthy$' ./main_test.go           
ok  	command-line-arguments	0.003s

A real world example of this use case one can find when testing the AWS SDK.

To make it even more obvious, here is the ugly alternative - the minimum one needs to implement to satisfy the Store interface:

type storeMock struct {
	healthy bool
}

func (s *storeMock) Create(a, b string) (i *Item, err error) {
	return
}
func (s *storeMock) GetByID(a string) (i *Item, err error) {
	return
}
func (s *storeMock) Update(i *Item) (err error) {
	return
}

// HealthCheck is mocked function
func (s *storeMock) HealthCheck() error {
	if !s.healthy {
		return fmt.Errorf("mock error")
	}
	return nil
}

func (s *storeMock) Close() (err error) {
	return
}

Embedding interfaces in a struct allows for partially "overriding" methods from the embedded interfaces. This, in turn, allows for delegation from the embedding struct to the embedded interface implementation.

The following example is taken from this blog post.

Suppose we want to have a socket connection with some additional functionality, like counting the total number of bytes read from it. We can define the following struct:

type StatsConn struct {
  net.Conn

  BytesRead uint64
}

StatsConn now implements the net.Conn interface and can be used anywhere a net.Conn is expected. When a StatsConn is initialized with a proper value implementing net.Conn for the embedded field, it "inherits" all the methods of that value; the key insight is, though, that we can intercept any method we wish, leaving all the others intact. For our purpose in this example, we'd like to intercept the Read method and record the number of bytes read:

func (sc *StatsConn) Read(p []byte) (int, error) {
  n, err := sc.Conn.Read(p)
  sc.BytesRead += uint64(n)
  return n, err
}

To users of StatsConn, this change is transparent; we can still call Read on it and it will do what we expect (due to delegating to sc.Conn.Read), but it will also do additional bookkeeping.

It's critical to initialize a StatsConn properly, otherwise the field retains its default value nil causing a runtime error: invalid memory address or nil pointer dereference; for example:

conn, err := net.Dial("tcp", u.Host+":80")
if err != nil {
  log.Fatal(err)
}
sconn := &StatsConn{conn, 0}

Here net.Dial returns a value that implements net.Conn, so we can use that to initialize the embedded field of StatsConn.

We can now pass our sconn to any function that expects a net.Conn argument, e.g:

resp, err := ioutil.ReadAll(sconn)
if err != nil {
  log.Fatal(err)

And later we can access its BytesRead field to get the total.

This is an example of wrapping an interface. We created a new type that implements an existing interface, but reused an embedded value to implement most of the functionality. We could implement this without embedding by having an explicit conn field like this:

type StatsConn struct {
  conn net.Conn

  BytesRead uint64
}

And then writing forwarding methods for each method in the net.Conn interface, e.g.:

func (sc *StatsConn) Close() error {
  return sc.conn.Close()
}

However, the net.Conn interface has many methods. Writing forwarding methods for all of them is tedious and unnecessary. Embedding the interface gives us all these forwarding methods for free, and we can override just the ones we need.

I will try another, low level approach to this.
Given the reverse struct:

type reverse struct {
    Interface
}

This beside others means, that reverse struct has a field reverse.Interface, and as a struct fields, it can be nil or have value of type Interface.
If it is not nil, then the fields from the Interface are promoted to the "root" = reverse struct. It might be eclipsed by fields defined directly on the reverse struct, but that is not our case.

When You do something like:
foo := reverse{}, you can println it via fmt.Printf("%+v", foo) and got

{Interface:<nil>}

When you do the

foo := reverse{someInterfaceInstance}

It is equivalent of:

foo := reverse{Interface: someInterfaceInstance}

It feels to me like You declare expectation, that implementation of Interface API should by injected into your struct reverse in runtime. And this api will be then promoted to the root of struct reverse.
At the same time, this still allow inconsistency, where You have reverse struct instance with reverse.Interface = < Nil>, You compile it and get the panic on runtime.

When we look back to the specifically example of the reverse in OP, I can see it as a pattern, how you can replace/extend behaviour of some instance / implementation kind of in runtime contrary to working with types more like in compile time when You do embedding of structs instead of interfaces.

Still, it confuses me a lot. Especially the state where the Interface is Nil :(.