英文:
fmt.Println calling Error instead of String()
问题
我有这段代码
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))
}
这段代码的输出是
Cannot Sqrt negative number: 1.4142135623730951
Cannot Sqrt negative number: -2 Cannot Sqrt negative number: -2
当我为ErrNegativeSqrt实现了Stringer接口时，为什么fmt.Println调用Error()方法而不是String()方法？
我是Go的新手，所以可能会漏掉一些非常明显的东西。
英文:
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.
答案1
得分: 9
> 4. 如果操作数实现了 error 接口，将调用 Error 方法将对象转换为字符串，然后按照所需的动词格式化。
>
> 5. 如果操作数实现了方法 String() string，将调用该方法将对象转换为字符串，然后按照所需的动词格式化。
error 接口位于 Stringer 之前。
更符合惯用写法的函数如下：
func Sqrt(x float64) (float64, error) {
if x < 0 {
return 0, ErrNegativeSqrt(x)
}
return math.Sqrt(x), nil
}
英文:
The documentation states how the value is converted to a string:
> 4. 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).
>
> 5. 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
}
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