What is the difference between fmt.Stringer and fmt.GoStringer?

Go's fmt package supports two string-representation interfaces for types. They serve different audiences and contexts:

type Color struct{ R, G, B uint8 }

// Stringer — human readable
func (c Color) String() string {
    return fmt.Sprintf("#%02X%02X%02X", c.R, c.G, c.B)
}

// GoStringer — Go syntax
func (c Color) GoString() string {
    return fmt.Sprintf("Color{R: %d, G: %d, B: %d}", c.R, c.G, c.B)
}

c := Color{255, 128, 0}
fmt.Printf("%v\n",  c) // #FF8000         (uses String())
fmt.Printf("%s\n",  c) // #FF8000
fmt.Printf("%#v\n", c) // Color{R: 255, G: 128, B: 0} (uses GoString())

// Without GoString, %#v uses default Go syntax:
// main.Color{R:0xff, G:0x80, B:0x0}

// Practical use: panic messages, test failure output, %#v in debugging
// GoString makes the output directly copy-pasteable as Go code

GoString() is particularly valuable in test failure messages: when a test prints %#v of an unexpected value, the output can be copied directly into the test as the expected value — saving debugging time.

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