7.2 Interface Types

An interface type specifies a set of methods that a concrete type must possess to be considered an instance of that interface.

The io.Writer type is one of the most widely used interfaces because it provides an abstraction of all the types to which bytes can be written, which includes files, memory buffers, network connections, HTTP clients, archivers, hashers, and so on. The io package defines many other useful interfaces. A Reader represents any type from which you can read bytes, and a Closer is any value that you can close, such as a file or a network connection. (By now you’ve probably noticed the naming convention for many of Go’s single-method interfaces.)

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package io

type Reader interface {
    Read(p []byte) (n int, err error)
}

type Closer interface {
    Close() error
}

Looking farther, we find declarations of new interface types as combinations of existing ones. Here are two examples:

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type ReadWriter interface {
    Reader
    Writer
}

type ReadWriteCloser interface {
    Reader
    Writer
    Closer
}

The syntax used above, which resembles struct embedding, lets us name another interface as a shorthand for writing out all of its methods. This is called embedding an interface. We could have written io.ReadWriter without embedding, albeit less succinctly, like this:

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type ReadWriter interface {
    Read(p []byte) (n int, err error)
    Write(p []byte) (n int, err error)
}

or even using a mixture of the two styles:

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type ReadWriter interface {
    Read(p []byte) (n int, err error)
    Writer
}

All three declarations have the same effect. The order in which the methods appear is immaterial. All that matters is the set of methods.

Exercise 7.4: The strings.NewReader function returns a value that satisfies the io.Reader interface (and others) by reading from its argument, a string. Implement a simple version of NewReader yourself, and use it to make the HTML parser (§5.2) take input from a string.

Exercise 7.5: The LimitReader function in the io package accepts an io.Reader r and a number of bytes n, and returns another Reader that reads from r but reports an end-of-file condition after n bytes. Implement it.

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func LimitReader(r io.Reader, n int64) io.Reader