进入编程语言界面的概念理解

Question

I am writing a sample Http Client/Server code to learn more about Go. During the process of writing I needed to read the response Body. So I looked at the Go Pkg Documentation http://golang.org/pkg/net/http/#Response and saw that Body io.ReadCloser i.e. Body is of type io.ReaderCloser. In order to know what is io.ReaderCloser, I clicked on the hyperlink and saw that http://golang.org/pkg/io/#ReadCloser is an interface that implements two methods Reader and Closer. Looking at this I thought I could do this

line, _ , err := response.Body.Reader.ReadLine()

Go compiler then gave me the error there is no method or type Reader in response.Body. I think conceptually I am having a problem of understanding why I could not do this. Maybe I am thinking too much in java terms. I would appreciate if somebody could point out my misunderstanding.

Thanks

Answer 1

TLDR

Body is a Reader. It doesn't contain one. Just call Reader methods directly on Body.

Gory Details

10 second intro to Go interfaces: A set of methods that types must implement, if it has all the methods, it implements the interface, period.

Next is what just bit you, embedding. If we have an interface Foo

type Foo interface {
    FooIt() error
}

and we want another interface also with the method FooIt + some stuff, we can "embed" the interface like this.

type FooPlusPlus interface {
    Foo // This embeds Foo in FooPlusPlus, making all of Foo's methods
        // part of FooPlusPlus's method pool.
    FooItAll() (bool, error)
}

Embedding provides a nice way to compose related interfaces, like Reader and Closer. You can kinda think of it like inheriting in Java land. You don't explicitly ask for the embedded interface any more than you'd ask for a parent class in Java with a subclass.

Now to implement FooPlusPlus

 type Demo int
 func (_ *Demo) FooIt() error {return nil}
 func (_ *Demo) FooItAll() (bool, error) {return false, nil}

And viola, now Demo is a FooPlusPlus.