How to Implement Interface Methods Returning Interfaces in Golang?

Implementing Interface Method Returning an Interface in Golang

When attempting to implement an interface that returns another interface, a common error arises when the return type of the method does not match the expected type defined in the interface. Consider the following code snippet:

type IA interface {
    FB() IB
}

type IB interface {
    Bar() string
}

type A struct {
    b *B
}

func (a *A) FB() *B {
    return a.b
}

type B struct{}

func (b *B) Bar() string {
    return "Bar!"
}

Running this code will result in the following error:

cannot use a (type *A) as type IA in function argument:
    *A does not implement IA (wrong type for FB method)
        have FB() *B
        want FB() IB

Solution:

The problem lies in the return type of the FB method in the A struct. The IA interface expects FB to return an IB, but in the implementation, it is returning a *B. To rectify this, modify the return type of FB to IB as follows:

func (a *A) FB() IB {
    return a.b
}

Sharing Interfaces Across Packages:

It is possible to define interfaces in different packages and share them. In such cases, when implementing the interface in a struct, the full package path must be used for the return type of the method. For instance, if IA and IB are defined in a package called foo, and the implementation is in a package called bar, the declaration would be:

type IA interface {
    FB() foo.IB
}

And the implementation would be:

func (a *A) FB() foo.IB {
    return a.b
}

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