How Can I Return a Concrete Type from an Interface Method in Go?

Interface Method Returning Interface and Concrete Types

In Go, an interface method returning an interface offers flexibility for type implementations. However, attempting to implement such a method with a concrete type can lead to errors.

Consider the example:

<code class="go">func GetStringer() fmt.Stringer</code>

Where GetStringer is an interface method returning the fmt.Stringer interface.

If a concrete type, such as Foo, is implemented to return a value of itself (*Foo), an error is encountered. This is because Foo does not implement fmt.Stringer directly.

To resolve this issue, several approaches can be taken:

Using an Adapter Type

Create an adapter type that wraps the concrete type and conforms to the required interface:

<code class="go">type MyBar struct{ Bar }

func (b *MyBar) GetStringer() fmt.Stringer {
    return b.Bar.GetStringer()
}</code>

This approach allows for extension without modifying the original type Bar.

Wrapping with the Interface

Alternatively, the concrete type can be directly wrapped with the required interface:

<code class="go">func (b *Bar) GetStringer() fmt.Stringer {
    return &Foo{"foo"}
}</code>

Here, the concrete type Bar provides an implementation of the GetStringer method that returns an instance of the Foo type, which implements fmt.Stringer.

Both methods provide options for accommodating interface method returns with concrete types in Go. The appropriate approach depends on the specific requirements and architecture of the code.

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