When to Use Interfaces vs Concrete Types in Go

Use interfaces when you need polymorphism, dependency injection, or to decouple code from specific implementations; use concrete types when performance matters or the implementation is fixed. Interfaces add a small runtime cost for dynamic dispatch, while concrete types allow the compiler to inline methods and optimize aggressively.

// Use an interface for flexibility
func Process(r io.Reader) error {
    // Works with bytes.Reader, os.File, net.Conn, etc.
    _, err := io.ReadAll(r)
    return err
}

// Use a concrete type for performance
func FastProcess(b []byte) error {
    // Compiler inlines directly; no interface overhead
    _ = len(b)
    return nil
}

Think of an interface as a universal adapter that lets different plugs fit into the same socket, while a concrete type is a specific plug designed for one job. Use the adapter when you want to swap parts easily without breaking the machine, but use the specific plug when you need maximum speed and know exactly what part you are using.