What is the fmt.Stringer interface and how do you implement it?

The fmt.Stringer interface is the standard Go convention for providing a human-readable string representation of a type. It is used automatically by fmt.Print, fmt.Println, and %v / %s format verbs.

// Interface definition (in the fmt package)
type Stringer interface {
    String() string
}

// Custom type implementing Stringer
type Color int

const (
    Red Color = iota
    Green
    Blue
)

func (c Color) String() string {
    switch c {
    case Red:   return "Red"
    case Green: return "Green"
    case Blue:  return "Blue"
    default:    return fmt.Sprintf("Color(%d)", int(c))
    }
}

fmt.Println(Red)          // Red   (not '0')
fmt.Printf("%v\n", Green) // Green
fmt.Printf("%s\n", Blue)  // Blue

// Another example: Point type
type Point struct{ X, Y float64 }
func (p Point) String() string {
    return fmt.Sprintf("(%.2f, %.2f)", p.X, p.Y)
}
p := Point{3.14159, 2.71828}
fmt.Println(p)  // (3.14, 2.72)

// GoStringer (fmt.GoStringer) — %#v verb, Go syntax representation
type Foo struct{ X int }
func (f Foo) GoString() string { return fmt.Sprintf("Foo{X: %d}", f.X) }
fmt.Printf("%#v\n", Foo{42}) // Foo{X: 42}

Implementing Stringer makes debugging significantly easier: log output, error messages, and test failure messages show meaningful names rather than raw integer or struct values. It is a lightweight interface with large practical benefit — implementing it should be a habit for any named type that appears in logs or user-facing output.

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