What is the fmt.Stringer interface and how does it control how a type is printed?

The fmt package defines the Stringer interface: a type that implements String() string controls how it appears when printed with fmt.Println, fmt.Printf("%v"), and related functions. Implementing error works the same way for error messages.

// fmt.Stringer interface:
// type Stringer interface { String() string }

type Direction int

const (
    North Direction = iota
    South; East; West
)

func (d Direction) String() string {
    return [...]string{"North", "South", "East", "West"}[d]
}

fmt.Println(North)          // North  (not: 0)
fmt.Printf("%v\n", East)   // East
fmt.Printf("%s\n", West)   // West

// Another example: Point with custom format
type Point struct{ X, Y float64 }

func (p Point) String() string {
    return fmt.Sprintf("(%.1f, %.1f)", p.X, p.Y)
}

p := Point{3.5, 7.2}
fmt.Println(p)          // (3.5, 7.2)
fmt.Printf("%v\n", p)  // (3.5, 7.2)

// TRAP: infinite recursion inside String()
// func (p Point) String() string {
//     return fmt.Sprintf("%v", p)  // calls String() again → stack overflow!
// }
// FIX: cast to a non-Stringer type
// return fmt.Sprintf("%v", struct{ X, Y float64 }(p))

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