What is interface pollution and how do you avoid it in Go?

Interface pollution means creating interfaces prematurely, needlessly, or with too many methods — when a concrete type would be simpler and clearer. It adds indirection without benefit and makes code harder to navigate.

// Interface pollution — unnecessary interface
type UserGetter interface {
    GetUser(id int) User
    GetUserByEmail(email string) User
    GetUsersByGroup(group string) []User
    UpdateUser(u User) error
    DeleteUser(id int) error
    // ... 10 more methods
}
// If only ONE type will ever implement this, it adds noise

// Better: define the interface where it's consumed, with minimum methods
// In the notification package:
type UserFetcher interface {  // only what notifications need
    GetUser(id int) User
}

// Rule: define the interface in the CONSUMING package, not the producing package
// (Often called the 'interface ownership' principle)

// When interfaces are justified:
// 1. Multiple concrete implementations exist (or are expected soon)
// 2. Needed for testability (inject a mock)
// 3. Standard library patterns (io.Reader, http.Handler)
// 4. Plugin or extension points in a library

// Go proverb: 'Don't design with interfaces, discover them.'
// Start with concrete types; extract interfaces when the need becomes clear

The Go proverb "Don't design with interfaces, discover them" captures the idiomatic approach: write concrete types first. When a second concrete implementation is needed, or when testability requires a mock, extract an interface at that point. Premature abstraction costs readability without delivering flexibility.

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