I've just seen Go has incorporated generics in its latest release, and I'm trying to create a small project to understand how it works. I don't seem to figure out how it works apart from very simple functions being now generic. I'd like to be able to do things like this:
type Dao[RT any] interface {
FindOne(id string) *RT
}
type MyDao struct {
}
type ReturnType struct {
id int
}
func (m *MyDao) FindOne(id string) *ReturnType {
panic("implement me")
}
// how should this look like?
func NewMyDao() *Dao[ReturnType] {
return &MyDao[ReturnType]{}
}
Is that even possible? I don't seem to be implementing the interface that way, and I've tried many combinations of the same.
Is there a way to implement a generic interface? If not, is the alternative only to return the interface{} type?
Types don't actually implement generic interfaces, they implement instantiations of generic interfaces. You can't use a generic type (including interfaces) without instantiation. From there, it is just like pre-generics Go, including the difference between methods with pointer receiver.
Therefore it is helpful to think what the methods that use type parameters would look like if you rewrote them with concrete types.
Let's consider a generic interface and some type:
type Getter[T any] interface {
Get() T
}
type MyStruct struct {
Val string
}
There's a few possible cases
Interface with concrete type argument
Instantiate as Getter[string], implemented by types with method Get() string
// implements Getter[string]
func (m MyStruct) Get() string {
return m.Val
}
// ok
func foo() Getter[string] {
return MyStruct{}
}
Interface with type parameter as type argument
Functions that have type parameters may use those to instantiate generic types, e.g. Getter[T]. Implementors must have exactly the Get() T method. For that to be valid, they are also generic and instantiated with the same type parameter:
So this doesn't compile even if T is string
// Getter[T] literally needs implementors with `Get() T` method
func bar[T any]() Getter[T] {
return MyStruct{} // doesn't compile, even if T is string
}
Making MyStruct also parametrized works:
type MyStruct[T any] struct {
Val T
}
func (m MyStruct[T]) Get() T {
return m.Val
}
func bar[T any]() Getter[T] {
return MyStruct[T]{} // ok
}
Concrete interface with generic implementor
Let's reverse the previous cases. We keep the parametrized MyStruct[T any] but now the interface is not parametrized:
type Getter interface {
Get() string
}
In this case, MyStruct implements Getter only when it is instantiated with the necessary concrete type:
// Getter requires method `Get() string`
func baz() Getter {
return MyStruct[string]{} // instantiate with string, ok
// return MyStruct[int]{} // instantiate with something else, doesn't compile
}
Pointer receivers
This follows the same rules as above, but requires instantiating pointer types, as usual:
// pointer receiver, implements Getter[string]
func (m *MyStruct) Get() string {
return m.Val
}
func foo() Getter[string] {
return &MyStruct{} // ok
// return MyStruct{} // doesn't implement
}
and it is the same if MyStruct is generic.
// parametrized pointer receiver
func (m *MyStruct[T]) Get() T {
return m.Val
}
func foo() Getter[string] {
return &MyStruct[string]{} // ok
}
So in your case, the mental exercise of replacing the type params with concrete types gives that Dao[ReturnType] has method FindOne(id string) *ReturnType. The type that implements this method is *MyDao (pointer receiver), therefore:
func NewMyDao() Dao[ReturnType] {
return &MyDao{}
}
The type *MyDao implements the interface Dao[ReturnType]. Thus, the function should look like:
func NewMyDao() Dao[ReturnType] {
return &MyDao{}
}
Note that the return type is an instance of the generic interface, and the returned value is simply an instance of the *MyDao type.
i.e.
package main
import "fmt"
type rabbit struct {
food string
}
func (r *rabbit) test() {
fmt.Println(r.food)
}
func main() {
roger := &rabbit{"carrot"} //rabbit{"carrot"} would have done the same thing
roger.food = "salad"
roger.test()
}
whether you define roger as &rabbit{"carrot"} or rabbit{"carrot"}, you can in any case change its attributes and call pointer receivers. But if you had to implement an interface like in this example
package main
import "fmt"
type interfaccia interface {
test()
}
type rabbit struct {
food string
}
func (r *rabbit) test() {
fmt.Println(r.food)
}
func someFunc(elemento interfaccia) {
elemento.test()
}
func main() {
roger := &rabbit{"carrot"} //without pointer receiver you would bump into an error
someFunc(roger)
}
then you are forced to set roger as pointer receiver due to the interface. and if I had a recursive struct like this:
type rabbit struct {
food string
girlfriend *rabbit
}
then I'm always forced to use the pointer.
My concern is: since &rabbit{"carrot"} includes all things of rabbit{"carrot"}, why should I use the last one?
A method of a type can either have a pointer receiver or a value receiver. There is a caveat while pointer vs value receiver for methods of a type when that type implements an interface
If a type implements all methods of an interface using value receiver, then both value and pointer of that type can be used while assigning to that interface variable or while passing to a function which accept an argument as that interface.
If a type implements all methods of an interface using pointer receiver, then the only pointer of that type can be used while assigning to that interface variable or while passing to a function that accepts an argument as that interface.
see the example in go tour here
I want to compose a type of another type, but replace one of the fields (which is an interface value) with a fake. The problem I am getting is the underlying field is being used, so I can't seem to override the field.
I've demoed the problem here: https://play.golang.org/p/lHGnyjzIS-Y
package main
import (
"fmt"
)
type Printer interface {
Print()
}
type PrinterService struct {}
func (ps PrinterService) Print() { fmt.Println("PrinterService") }
type Service struct {
Client PrinterService
}
func (s Service) PrintViaMethod() { s.Client.Print() }
type FakeService struct {
Service
Client Printer
}
type SomeOtherService struct {}
func (sos SomeOtherService) Print() { fmt.Println("SomeOtherService") }
func main() {
s := FakeService{Client: SomeOtherService{}}
s.PrintViaMethod()
}
Why does it print "PrinterService"? I want it to print "SomeOtherService".
Thanks.
By s.PrintViaMethod(), you are calling the promoted method FakeService.Service.PrintViaMethod(), and the method receiver will be FakeService.Service which is of type Service, and Service.PrintViaMethod() calls Service.Client.Print(), where Service.Client is of type which PrinterService, that's why it prints "PrinterService".
In Go there is embedding, but there is no polymorphism. When you embed a type in a struct, methods of the embedded type get promoted and will be part of the method set of the embedder type. But when such a promoted method is called, it will get the embedded value as the receiver, not the embedder.
To achieve what you want, you would have to "override" the PrintViaMethod() method by providing your implementation of it for the FakeService type (with FakeService receiver type), and inside it call FakeService.Client.Print().
By doing so s.PrintViaMethod() will denote the FakeService.PrintViaMethod() method as that will be at the shallowest depth where the PrintViaMethod() exists (and not FakeService.Service.PrintViaMethod()). This is detailed in Spec: Selectors.
For example:
func (fs FakeService) PrintViaMethod() {
fs.Client.Print()
}
Then the output will be (try it on the Go Playground):
SomeOtherService
See related questions and answers with more details:
Go embedded struct call child method instead parent method
Does fragile base class issue exist in Go?
Why does it print 'PrinterService'? I want it to print 'SomeOtherService'.
Because that's what your code says to do. PrintViaMethod calls s.Client.Print(), and s.Client is a (zero value) instance of PrinterService, which outputs PrinterService.
What you probably want is to call s.Print() in main(). I don't see any reason for your PrintByMethod function at all.
As per Flimzy you are calling print s.Client.Print() which is of type PrinterService implemented as receiver to Print() function printing PrinterService. You can also change type of Client PrinterService in Service struct to Someother service
package embedded
import (
"fmt"
)
type Printer interface {
Print()
}
type PrinterService struct{}
func (ps PrinterService) Print() { fmt.Println("PrinterService") }
type Service struct {
Client SomeOtherService
}
func (s Service) PrintViaMethod() { s.Client.Print() }
type FakeService struct {
Service
Client Printer
}
type SomeOtherService struct{}
func (sos SomeOtherService) Print() { fmt.Println("SomeOtherService") }
func Call() {
s := FakeService{Client: SomeOtherService{}}
s.PrintViaMethod()
}
I have the following code:
package main
type MyInterface interface {
Test()
}
type MyType struct {
}
func (m MyType) Test(){}
func AcceptInterface(i *MyInterface){
}
func main() {
object := &MyType{}
AcceptInterface(object)
}
I was expecting this to work, because MyType implements MyInterface, but I get:
cannot use object (type *MyType) as type *MyInterface in argument to
AcceptInterface: *MyInterface is pointer to interface, not interface
I tried doing type assertion: object.(MyInterface), but that doesn't work either.
How can I accomplish this?
As the error says,
cannot use object (type *MyType) as type *MyInterface in argument to AcceptInterface: *MyInterface is pointer to interface, not interface
This means that it is expecting an interface value, not a pointer.
If you change the pointers to values in your code (by removing the & and *), the program will run with no errors:
package main
type MyInterface interface {
Test()
}
type MyType struct {
}
func (m MyType) Test(){}
func AcceptInterface(i MyInterface){
}
func main() {
object := MyType{}
AcceptInterface(object)
}
Play it
Edit 1
If you still want to use a pointer as an argument, there are two important parts of the Go language to note
From the Go Spec on what exacly is a variable that fits an instance:
A variable of interface type can store a value of any type with a method set that is any superset of the interface.
From the Go Spec on what pointers being automatically dereferenced:
As with selectors, a reference to a non-interface method with a value receiver using a pointer will automatically dereference that pointer: pt.Mv is equivalent to (*pt).Mv [and] as with method calls, a reference to a non-interface method with a pointer receiver using an addressable value will automatically take the address of that value: t.Mp is equivalent to (&t).Mp.
Those two points are important, because when combined they explain that pointers to variables can still fit instances. This is because the pointer's method set is automatically dereferenced by the Go compiler (and since the variable it is referencing can fit an instance, the pointer can, too)!
In action, this means that in order to see if a pointer fits an instance, you have to declare the instance as a value and the pointer as a pointer.
If you run this code:
package main
type MyInterface interface {
Test()
}
type MyType struct {
}
func (m MyType) Test() {}
func AcceptInterface(i MyInterface) {
}
func main() {
object := &MyType{}
AcceptInterface(object)
}
Play it
you will see that there are no errors! Notice how there is an & in the object declaration, but no * in the i declaration?
type MyInterface interface {
Test()
}
type MyType struct {
}
func (m MyType) Test(){}
Note: it means that MyType implemts the interface MyInterface, not MyType*.
You can use:
func (m *MyType) Test(){} //Notice here
func AcceptInterface(i *MyInterface){
}
func main() {
object := &MyType{}
AcceptInterface(object)
}
Use explicit typing if you want to pass a pointer to interface:
func main() {
var object MyInterface = MyType{}
AcceptInterface(&object)
}
I would not recommend using pointer interfaces as you would need to write code like (*i).Test() to call the interface pointer methods. The compiler do auto dereferencing for struct pointers, not so for interface pointers.
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 :(.