fmt.Println调用Error而不是String（）

Question

I have this code

package main

import (
    "fmt"
    "math"
)

type ErrNegativeSqrt float64

func (s ErrNegativeSqrt) String() string {
    return fmt.Sprintf("%f", float64(s))
}

func (e ErrNegativeSqrt) Error() string {
    return fmt.Sprintf("Cannot Sqrt negative number: %v", float64(e))
}

func Sqrt(x float64) (ErrNegativeSqrt, error) {
    if x < 0 {
        e := ErrNegativeSqrt(x)
        return e, e
    } else {
        return ErrNegativeSqrt(math.Sqrt(x)), nil
    }
}

func main() {
    fmt.Println(Sqrt(2))
    fmt.Println(Sqrt(-2))
}

And the output of this code is

Cannot Sqrt negative number: 1.4142135623730951 <nil> Cannot Sqrt negative number: -2 Cannot Sqrt negative number: -2

When I have implemented the Stringer interface for the ErrNegativeSqrt, why is the fmt.Println invoking the Error() method instead of the String() method?

I am new to go, so I might be missing something very obvious.

Answer 1

The documentation states how the value is converted to a string:

1. 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).

2. If an operand implements method String() string, that method will be invoked to convert the object to a string, which will then be formatted as required by the verb (if any).

The error interface comes before Stringer.

A more idiomatic way to write the function is:

func Sqrt(x float64) (float64, error) {
  if x < 0 {
    return 0, ErrNegativeSqrt(x)
  }
  return math.Sqrt(x), nil
}