Why Does `fmt.Println` Print 'bad error' Instead of the Integer Value When a Type Implements the `error` Interface?

Why Does an Interface Return ""bad error"" for a Custom Type's Error Method?

In Go, an interface can be used to define methods for a type. When an object implements an interface, it provides an implementation for each of the methods defined by that interface.

Consider the following example:

type T int

func (t T) Error() string {
    return "bad error"
}

func main() {
    var v interface{} = T(5)
    fmt.Println(v) // Output: "bad error"
}

In this example, the Error method is defined for the type T. However, when the value v is printed using fmt.Println, the output is "bad error" instead of the expected integer value 5. To understand this behavior, let's refer to the fmt package documentation:

• If an operand implements the error interface, the Error method will be invoked to convert the object to a string, which will then be formatted as required by the verb (if any).
• For each Printf-like function, there is also a Print function that takes no format and is equivalent to saying %v for every operand.

In this case, the fmt.Println function is used, which is equivalent to fmt.Printf("%v", v). Here's an explanation of what happens:

• Since T implements the error interface, the Error method is invoked on the value v.
• The Error method returns the string "bad error."
• This string is then formatted by the %v verb and printed as the output.

To print the integer value of v instead, you can use fmt.Printf("%d",v).

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