What is the interface guard pattern in Go?

Question

Accepted Answer

Interface guard is a pattern for **compile-time checking** that a type implements an interface.

```go
// Interface
type Repository interface {
    FindByID(id int) (*User, error)
    Save(user *User) error
    Delete(id int) error
}

// Implementation
type PostgresRepo struct {
    db *sql.DB
}

func (r *PostgresRepo) FindByID(id int) (*User, error) { ... }
func (r *PostgresRepo) Save(user *User) error { ... }
func (r *PostgresRepo) Delete(id int) error { ... }

// Interface guard: compiler checks implementation
var _ Repository = (*PostgresRepo)(nil)

// If PostgresRepo doesn't implement Repository — compilation error!

// Additionally: check interface consistency
var (
    _ Repository = (*PostgresRepo)(nil)
    _ Repository = (*MockRepo)(nil)  // Must also implement it
)

// Standard library uses this pattern
// io.ReadWriter is checked via:
var _ io.ReadWriter = (*os.File)(nil)
```

**Advantages:**
- Compile-time errors instead of runtime.
- Intent documentation.
- Safe refactoring.

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