Golang interfaces

Similar to other languages, interfaces in go define a contract of method-signatures that implementors must have.
Unlike other languages, golang interfaces are implicit -- an object with all of the required methods automatically satisfies an interface.

Basics

// declare an interface
type Writer interface {
    Write([]byte) (int, error)
}

// implement an interface (automatic if methods match)
type PrinterWriter struct {}

func (w PrinterWriter) Write(data []byte) (int, error) {
    // ...
}

var w Writer = PrinterWriter{}  // <-- type 'Writer'
w.Write([]byte("Foobar"))

Libraries do not need to expose interfaces in go, you can create them for the subset of methods that are useful to you.