When to Use Packages vs Interfaces for Modularity in Go

Use interfaces to define behavior contracts between packages and packages to group related implementation details. Interfaces enable loose coupling by allowing different packages to implement the same contract without knowing each other's internal structure, while packages provide namespace organization and encapsulation for code that must be compiled together.

// Define contract in one package
type Reader interface {
    Read(p []byte) (n int, err error)
}

// Implement contract in another package
type File struct{}
func (f *File) Read(p []byte) (int, error) { /* ... */ }

Think of interfaces as a job description that lists required tasks, while packages are the actual teams hired to do the work. You use interfaces when you need different parts of your system to talk to each other without knowing exactly how the other side works. You use packages to keep related code organized together so it's easier to manage and reuse.