I'm working on a project that would require some level of abstraction on some data and I would like to keep the packages that use the data as independent as possible so I can swap things out in the future.
Here are 2 possible ways that I thought of, one is having a common data interface and have all the places import it. The other is to have each package define its own interfaces and do a type assertion.
Which is the most Go/general way of doing it?
// Importing interface
// src/model
type IData interface {
myint() int
}
// src/dataextractor
import src/model
type DataExtractor struct {
foo() IData
}
// src/service
import src/model
type ServiceDataExtractor interface {
foo() IData
}
type Service struct {
extractor ServiceDataExtractor
}
func (s Service) serve() {
v = s.extractor.foo()
// do stuff with v
}
vs
// type assertion
// src/dataextractor
type DataExtractorData struct{}
func (d DataExtractorData) myint()int {}
type DataExtractor struct {
foo() interface{} {
reutrn DataExtractorData{}
}
}
// src/service
type ServiceData interface {
myint() int
}
type ServiceDataExtractor interface {
foo() interface{}
}
type Service struct {
extractor ServiceDataExtractor
}
func (s Service) serve() {
data := s.extractor.foo()
v, ok := data.(ServiceData)
// do stuff with v
}
Here's the simple rules to keep in mind. An interface type should be declared in the package that wants to receive that interface type. To satisfy an interface, you don't have to import it. All you need to do is declare the methods as are specified by the interface.
When do you use interfaces? Typically, when you have a field, or parameter, that you want to be able to accept multiple data types, and you can do all you need to do with it by calling methods. How do you determine what goes in the interface? You simply ask yourself: "What methods would I need to call on this value to make use of it?"
The only seemingly meaningful method in your example is myint() int, and the only place it seems you intend to use it is in Service.
type Service struct {
extractor interface {
myint() int
}
}
func (s Service) serve() {
s.extractor.myint()
}
I have a need to create multiple structs with almost the same fields and methods with the same actions. Instead of doing that I thought, why not create a single struct and use interfaces to limit interactions. It worked great! Now the problem. I want the methods to be chainable. To do that, methods need to return a reference to the struct. After doing that, all the returns complain that (struct) does not implement <interface> (wrong type for <method> method). Which is expected.
The question is, is it possible to use a single struct with multiple interfaces that has chainable methods? Or is creating individual, duplicate, structs for every interface the only way?
I thought of using composition but I still need to define methods that will call the embedded struct methods, in which case there's no difference to creating new pure structs.
Example problem:
https://play.golang.org/p/JrsHATdi2dr
package main
import (
"fmt"
)
type A interface {
SetA(string) A
Done()
}
type B interface {
SetB(string) B
Done()
}
type t struct {
a string
b string
}
func (t *t) SetA(a string) *t { t.a = a; return t }
func (t *t) SetB(b string) *t { t.b = b; return t }
func (t *t) Done() { fmt.Println(t.a, t.b) }
func NewTA() A {
return &t{}
}
func NewTB() B {
return &t{}
}
func main() {
fmt.Println("Hello, playground")
ta := NewTA()
ta.SetA("a")
ta.Done()
tb := NewTB()
tb.SetB("b")
tb.Done()
}
When you use *t as return type in SetA and SetB that means t are not implement A and B interface. The signature of SetA and SetB function of *t doesn't match with interface A and B accordingly.
Accually SetA(a string) A and SetA(a string) *t are not same think. You used A as return type for SetA in interface but use *t as return type for t, go doesn't support this. Same for SetB function
If you do like this then it will work because now function signature matched
func (t *t) SetA(a string) A { t.a = a; return A(t) }
func (t *t) SetB(b string) B { t.b = b; return B(t) }
Code in go playground here
I have a question regarding dependency injection.
Please consider the example below.
For example, selector() is a function that select something and guarantee return an interface
In this example
bar.node.go
type NodeTemplate struct {
Name string
}
// satisfy interface declared in db.foo.go
//but never imports anything from db.foo.go
func (node *NodeTemplate) GetUuidName() string {
if node != nil {
return node.Name
}
return
}
db.foo.go
// interface declared in db.foo.go
type Node interface {
GetUuidName() string
}
Option A
// So selector receives a map of Some interface and populate a map
func SelectSomething(nodemap map[string]Node, selectFactor string) {
// selection from db and result populate in a map
}
Option B
Another pattern SelectSomething return a Node
and it Interface
So another package will depend on importing Node
and that will introduce a dependency.
func SelectSomething(seleconsomething) []*Node {
// do selection and return a slice of SomeInterface
n := &Node{} // here it need to be concret type T
return Node
}
So based on logic I've described I see the first approach is better but in that approach, select need do concrete type allocation in order to populate a map.
Consider another example
db.foo.go
type Node interface {
GetUuidName() string
}
func inserter(node *Node) error {
// do some work
node.GetUuidName()
}
For a case like in inserter case, inserter has no external dependency, inserter just needs to receive something that satisfies the interface. Declare interfaces locally and that brake a dependancy.
But in the case of selector example, it has to do memory allocation in order to return or populate a map or return something that has concrete type T. So in both case, it has to have internal re-presentation.
So here is my question can selector somehow at run time figure out a type it receives based on the interface and instantiate an object of that type and insert to a map as an interface or return a slice of the interface. ?
By doing so selector function will have no dependancy on what it receives it just guarantee it will instantiate the same object type T
and return interface.
or can selector return interface but I guess I have to have a bi-directional interface between db package and package X or dynamic dispatcher need to do some magic ?
You want a type to behave in a certain way. That is achieved via an interface. This case is no different. Simply add the desired behavior to your interface, as demonstrated below with the Foo interface.
package main
import (
"fmt"
"reflect"
)
type Foo interface {
Bar()
TypeOf() reflect.Type
}
type Baz struct{}
func (b Baz) Bar() {
fmt.Println("I am a Fooer!")
}
func (b Baz) TypeOf() reflect.Type {
return reflect.TypeOf(b)
}
func DoSomeThing(f Foo) {
f.Bar()
fmt.Println(f.TypeOf())
}
func main() {
fmt.Println("Hello, playground")
b := Baz{}
DoSomeThing(b)
}
Run on playground
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.
Add another Reverse method to the sort.Interface ?
Create another ReverseInterface ?
... ?
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 :(.
I am trying to make something real simple on Go: to have an interface with getter and setter methods. And it seems setter methods are not allowed.
Given this code:
package main
import "fmt"
type MyInterfacer interface {
Get() int
Set(i int)
}
type MyStruct struct {
data int
}
func (this MyStruct) Get() int {
return this.data
}
func (this MyStruct) Set(i int) {
this.data = i
}
func main() {
s := MyStruct{123}
fmt.Println(s.Get())
s.Set(456)
fmt.Println(s.Get())
var mi MyInterfacer = s
mi.Set(789)
fmt.Println(mi.Get())
}
Set method does not work, because in func (this MyStruct) Set(i int), this MyStruct is not a pointer, and the changes are lost as soon at the function exits. But making it this *MyStruct would not compile. Is there any workaround?
Here is a corrected version of your code (playground). This isn't exactly Polymorphism, but the use of an interface is good Go style.
package main
import "fmt"
type MyInterfacer interface {
Get() int
Set(i int)
}
type MyStruct struct {
data int
}
func (this *MyStruct) Get() int {
return this.data
}
func (this *MyStruct) Set(i int) {
this.data = i
}
func main() {
s := &MyStruct{123}
fmt.Println(s.Get())
s.Set(456)
fmt.Println(s.Get())
var mi MyInterfacer = s
mi.Set(789)
fmt.Println(mi.Get())
}
I once found this example of how to do polymorphism in Go:
http://play.golang.org/p/6Ip9scm4c3
package main
import "fmt"
type Talker interface {
Talk(words string)
}
type Cat struct {
name string
}
type Dog struct {
name string
}
func (c *Cat) Talk(words string) {
fmt.Printf("Cat " + c.name + " here: " + words + "\n")
}
func (d *Dog) Talk(words string) {
fmt.Printf("Dog " + d.name + " here: " + words + "\n")
}
func main() {
var t1, t2 Talker
t1 = &Cat{"Kit"}
t2 = &Dog{"Doug"}
t1.Talk("meow")
t2.Talk("woof")
}
To answer the question the in the title to post:
Go does not use classes, but provides many of the same features:
* message passing with methods
* automatic message delegation via embedding
* polymorphism via interfaces
* namespacing via exports
From: http://nathany.com/good/
Solving the code you supplied, I will leave to some more learned Gopher
###AD HOC polymophism
Ad hoc polymorphism is a general way of polymorphism implementation for statically typed languages. Polymorphism in Go is ad hoc polymorphism which is very close to Bjarne's Stroustrup definition:
Polymorphism – providing a single interface to entities of different types.
Interfaces
Go interface is really powerful tool designed specially for polymorphism implementation. Interface is a type abstraction (sets of methods) which provides a way to specify the behavior of an object: if something can do this, then it can be used here. Back to Straustrup's polymorphism definition: it is possible to use objects of different types as a type of a common interface if they implement the interface.
Playground with an example.
Parametric polymorphism
Wiki:
A function or a data type can be written generically so that it can handle values identically without depending on their type.
This kind of polymorphism is more regular for dynamically typed languages like Python or Ruby but Go implements it too! Go uses type empty interface interface{} for this purpose.
Type interface{}
From Tour Of Go:
The interface type that specifies zero methods is known as the empty interface:
interface{}
An empty interface may hold values of any type. Every type implements at least zero methods.
Empty interfaces are used by code that handles values of unknown type. For example, fmt.Print takes any number of arguments of type interface{}.
And it is possible to get particular type of an object with type assertion.
And again Tour Of Go:
A type assertion provides access to an interface value's underlying concrete value.
t := i.(T)
This statement asserts that the interface value i holds the concrete type T and assigns the underlying T value to the variable t.
There we have parametric polymorphism with static duck typing.