How does interface embedding (composition) work in Go?

Go interfaces can embed other interfaces. The composed interface's method set is the union of all embedded interface method sets. This is Go's primary mechanism for building larger interface contracts from smaller, focused ones — following the Interface Segregation Principle.

// Small, focused interfaces (the Go standard library style)
type Reader interface {
    Read(p []byte) (n int, err error)
}
type Writer interface {
    Write(p []byte) (n int, err error)
}
type Closer interface {
    Close() error
}

// Composed interfaces
type ReadWriter interface {
    Reader  // embeds Reader
    Writer  // embeds Writer
}
type ReadWriteCloser interface {
    Reader
    Writer
    Closer
}

// *os.File satisfies all of the above
var f *os.File = os.Stdout
var rw  ReadWriter        = f  // *os.File has Read + Write
var rwc ReadWriteCloser   = f  // *os.File has Read + Write + Close
var r   Reader            = rw // ReadWriter is assignable to Reader

// Your composed interface
type DataStore interface {
    Reader
    Writer
    Flush() error
    Stats() map[string]int64
}

// Any type with Read, Write, Flush, Stats satisfies DataStore
// without knowing anything about DataStore

Design principle: prefer many small interfaces over one large interface. Small interfaces (io.Reader has 1 method, io.Writer has 1 method) are easy to satisfy and easy to mock in tests. Accept interfaces in your functions; return concrete types from constructors. The idiomatic Go proverb: "The bigger the interface, the weaker the abstraction."

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