Implicit interface conversion in golang

Here is an example of the idea I want to demonstrate.

package main
import "fmt"

// interface declaration
//

type A interface {
	AAA() string
}

type B interface{
	Get() A
}

// implementation
//

type CA struct {}

// implementation of A.AAA
func (ca *CA) AAA() string {
	return "it's CA"
}

type C struct {}

// implementation of B.Get, except for returning a 'struct' instead of an 'interface'
func (c *C) Get() *CA {
	return &CA{}
}

func main() {
	var c interface{} = &C{}
	d := c.(B)
	fmt.Println(d.Get().AAA())
	fmt.Println("Hello, playground")
}

In this example

• interface B has a method Get to return an interface A
• struct C has a member function Get to return a pointer to struct CA, which implements interface A

The result is Go can't deduce interface B from struct C, even their Get method is only different in returning type, which is convertible.

The reason I raise this question is when interface A, B and struct C, CA are in different packages, I can only:

• refine the Get method of C to func Get() A, which introduce some dependency between packages.
• refine both Get method of interface B and struct C to func Get() interface{}

I want to avoid dependency between packages and try not to rely on interface{}, can anyone give me some hint? What's the best practice in Go?

Your current *C type does not implement the interface B, therefore you can't assign a value of *C to a variable of type B nor can't you "type assert" a value of B from something holding a value of type *C.

Here's what you can do. Since you're already using a struct literal (&C{}), you may declare c to be of type *C of which you can call its Get() method, and you can convert the return value of C.Get() to A (because the return value does implement A):

var c *C = &C{}
var a A = c.Get() // This is ok, implicit interface value creation (of type A)
fmt.Println(a.AAA())
// Or without the intermediate "a", you can simply call:
fmt.Println(c.Get().AAA())

Output:

it's CA
it's CA

Or refactor:

The problem is that you have an interface (B) which you want to implement, which has a method which returns another interface (A). To implement this B interface, you have to have dependency to the package that defines A, you can't avoid this. And you have to declare C.Get() to return A (instead of a concrete struct type).

You may move A to a 3rd package and then the package that defines C will only have to depend on this 3rd package, but will not depend on the package that defines B (but still will implicitly implement the interface type B).