interface-pollution

name	interface-pollution
description	Detect and avoid unnecessary interface abstractions

Interface Pollution Detection

Interface pollution occurs when interfaces are created before they're needed. Define interfaces at consumption point, not production.

Pollution Patterns

WRONG - Producer defines interface prematurely

// In package "storage"
type UserRepository interface {
    GetUser(id int) (*User, error)
    SaveUser(u *User) error
}

type PostgresRepo struct{}

func (p *PostgresRepo) GetUser(id int) (*User, error) { ... }
func (p *PostgresRepo) SaveUser(u *User) error { ... }

CORRECT - Consumer defines minimal interface

// In package "handler"
type UserGetter interface {
    GetUser(id int) (*User, error)
}

func HandleRequest(repo UserGetter) http.Handler {
    // Only needs GetUser, not entire repository
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        user, _ := repo.GetUser(123)
        // ...
    })
}

// Concrete type lives in storage package
// Handler depends on small interface, not full implementation

Detection Checklist

You have interface pollution if:

• Interface defined in same package as implementation
• Only one implementation exists
• Interface has >5 methods
• Name ends in "Interface" or "Impl"

Refactoring Strategy

Before:

type DataService interface {
    Read() error
    Write() error
    Validate() error
    Transform() error
}

After:

// Split by actual usage
type Reader interface { Read() error }
type Writer interface { Write() error }

// Compose where needed
type ReadWriter interface {
    Reader
    Writer
}