How do the fmt.Stringer and error interfaces work together and how do you avoid infinite recursion?

Types often implement both fmt.Stringer (String() string) and error (Error() string). A subtle trap: inside String(), calling fmt.Sprintf("%v", e) on the receiver causes infinite recursion because %v checks for Stringer and calls String() again.

type AppError struct {
    Code    int
    Message string
}

// BUGGY String() — infinite recursion
// func (e AppError) String() string {
//     return fmt.Sprintf("%v", e) // calls e.String() → infinite loop
// }

// CORRECT: format fields directly, not the receiver
func (e AppError) String() string {
    return fmt.Sprintf("AppError[%d]: %s", e.Code, e.Message)
}

// Implements both error and Stringer
func (e AppError) Error() string { return e.String() }

// Usage
err := AppError{Code: 404, Message: "not found"}
fmt.Println(err)          // uses Stringer: AppError[404]: not found
fmt.Println(err.Error())  // error: AppError[404]: not found
var e error = err         // also satisfies error interface
fmt.Println(e)            // same output via error.Error()

// Safe pattern: convert to a plain type inside String()
type Point struct{ X, Y int }
func (p Point) String() string {
    // Using struct literal — NOT the receiver — avoids recursion
    return fmt.Sprintf("(%d, %d)", p.X, p.Y)
}

Detection: the Go runtime detects some infinite-recursion panics (stack overflow), but they can be hard to trace. Always format individual fields, not the receiver itself, inside String() and Error(). Use %d, %s, %f directly on the fields.

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