How does Go's implicit interface satisfaction enable duck typing and loose coupling?

Go's implicit interface satisfaction means: if a type has the required methods, it satisfies the interface — regardless of whether the type's author ever heard of that interface. This is sometimes called structural typing or duck typing with compile-time verification.

// Third-party library defines:
type Logger interface {
    Log(msg string)
}

// Your legacy type (written before the Logger interface existed):
type AppLogger struct{ prefix string }
func (a AppLogger) Log(msg string) { fmt.Println(a.prefix+":", msg) }

// AppLogger satisfies Logger with ZERO changes to either package
func useLogger(l Logger) { l.Log("hello") }
useLogger(AppLogger{prefix: "APP"}) // compiles fine

// Compile-time interface check — verify without running the code
// Idiom: blank identifier assignment causes a compile error if the interface isn't satisfied
var _ Logger = AppLogger{}          // value receiver version
var _ Logger = (*AppLogger)(nil)    // pointer receiver version

// Interface segregation — small interfaces are more reusable
type Reader interface { Read(p []byte) (int, error) }
type Writer interface { Write(p []byte) (int, error) }
type ReadWriter interface {
    Reader  // interface embedding
    Writer
}

// Any type satisfying ReadWriter also satisfies Reader and Writer individually
var rw ReadWriter = os.Stdout // *os.File has both Read and Write
var r  Reader     = rw        // ReadWriter subsumes Reader
var w  Writer     = rw        // ReadWriter subsumes Writer

The compile-time check idiom (var _ Logger = AppLogger{}) is a Go best practice: it causes a compile error if AppLogger ever stops satisfying Logger, catching the mismatch at compile time rather than at runtime when a type assertion or assignment fails.

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