When do you recommend not using composition?

[Nitin Muppalaneni]

Is there a reason why `http.ResponseWriter` does not include io.Writer directly and instead has the `Write([]byte) (int, error)` method? Why did you choose to do it this way? I can see that it adds a dependency on the io package. Was that the concern?

And more generally, when would you recommend not using composition of interfaces?

Thanks

Nitin

[Aaron Rubesh]

I would recommend reading up on how to implement Go interfaces.

The only requirement required to satisfy the io.Writer interface is a method Write([]byte)(int,error). If this method is present, Go can then interpret your struct as a io.Writer.

By itself, io.Writer provides no functionality only a definition. That is left to the struct to fulfill.

[Nitin Muppalaneni]

I understand that interface does not provide the implementation. Perhaps my question was not clear. Let me try with the code snippets.

This is how http.ResponseWriter is written:

type http.ResponseWriter interface {

    ...

    Write([]byte) (int, error)

    ...

}

But it could have also been written this way:

type http.ResponseWriter interface {

    ...

    io.Writer

    ...

}

The second one uses embedding (https://go.dev/doc/effective_go#embedding), which I thought is preferable. I was curious why embedding was not used here and more broadly, when not to use embedding.

Thanks

Nitin

[Ian Lance Taylor]

On Fri, Apr 28, 2023 at 3:09 PM Nitin Muppalaneni <muppa...@gmail.com> wrote:
>
> I understand that interface does not provide the implementation. Perhaps my question was not clear. Let me try with the code snippets.
>
> This is how http.ResponseWriter is written:
> type http.ResponseWriter interface {
> ...
> Write([]byte) (int, error)
> ...
> }
>
> But it could have also been written this way:
> type http.ResponseWriter interface {
> ...
> io.Writer
> ...
> }
>
> The second one uses embedding (https://go.dev/doc/effective_go#embedding), which I thought is preferable. I was curious why embedding was not used here and more broadly, when not to use embedding.

I don't know if this was the actual reason, but personally I think
listing the Write method explicitly makes the documentation easier to
read.

https://pkg.go.dev/net/http#ResponseWriter

Ian

[Nitin Muppalaneni]

Thank you, Ian. It indeed makes documentation easier to read in this case. Unless the interface to be embedded is sufficiently self-documenting, as is the case with `heap.Interface` embedding `sort.Interface`, I will stick to listing the method[s] directly.

https://cs.opensource.google/go/go/+/refs/tags/go1.20.3:src/container/heap/heap.go;l=31-35#:~:text=type-,Interface,-interface%20%7B

Thank you Aaron for the earlier reply.

Nitin