为什么错误不匹配nil?Go

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英文:

why errors don't match nil ? Go

问题

为什么当你使用负比较将错误与nil进行比较时,会得到预期的输出,但是当你尝试使用'正'运算符进行比较时,它却'不起作用'?我认为代码如下所示:

package main

import "fmt"
import "errors"

func main() {
    err := makeError
    if err != nil {
        fmt.Println("as expected")

        err = noError
        if err == nil {
            fmt.Println("as expected")
        } else {
            fmt.Println("Why no error ?")
        }
    }
}

func makeError() error {
    return errors.New("error!")
}

func noError() (err error) {
    return
}

在这里运行代码!

英文:

Why when you use negatively compare an error with nil if provides the expected output but when you try to compare it using the 'positive' operator it 'doesn't work'? I think the code See below

package main

import "fmt"
import "errors"

func main() {
	err := makeError
	if err != nil {
		fmt.Println("as expected")

		err = noError
		if err == nil {
			fmt.Println("as expected")
		} else {
			fmt.Println("Why no error ?")
		}
	}
}

func makeError() error {
	return errors.New("error!")
}

func noError() (err error) {
	return
}

Play this thing!

答案1

得分: 4

问题在于你没有调用函数,而是将函数本身赋值给变量。所以 err 的值始终与 nil 不同。

示例:

func foo() error {
    return nil
}

func bar() error {
    return errors.New("error")
}

// err 将与 foo 函数相同
// 然后你可以执行 err()
err := foo

// err 将是一个错误变量,在这种情况下为 nil
// err != nil 将返回 false
err := foo()

// err 将是一个错误变量,等同于 errors.New("error")
err := bar()

如果你尝试创建变量然后再对其赋值,编译器会报错(如预期所示)。示例:

func foo() error {
    return nil
}

var err error
err = foo

将会得到以下编译器错误:

cannot use foo (type func() error) as type error in assignment:
func() error does not implement error (missing Error method)

playground 中可以尝试。

英文:

The problem is that you don't call your functions, but assign the function itself to your variables. So the err value is always different from nil.

Example:

func foo() error {
    return nil
}

func bar() error {
    return errors.New("error")
}

// err will be the same as the foo function
// you can then do err()
err := foo

// err will be an error var, nil in this case
// err != nil will return false
err := foo()

// err will be an error var, equivalent to errors.New("error")
err := bar()

If you try to create the variable then assign to it, your compiler will yell at you (as expected). Example:

func foo() error {
    return nil
}

var err error
err = foo

will give you the following compiler error

> cannot use foo (type func() error) as type error in assignment:
> func() error does not implement error (missing Error method)

In the playground

答案2

得分: 4

在你的示例代码中,err不是一个错误值。通过以下赋值语句:

err := makeError

err的类型是func() errorerr != nil成功是因为makeError函数不是nil。同样地,noError也是一个函数,并且不等于nil

如果你实际调用这些函数,你应该会得到你期望的结果。

英文:

In your example code, err is not an error value. With the following assignment:

err := makeError

err is of type func() error. The err != nil succeeds because the makeError function is not nil. Similarly, noError is also a function and not equal to nil.

If you actually call the functions, you should get the results you expected.

huangapple
  • 本文由 发表于 2014年7月25日 17:43:08
  • 转载请务必保留本文链接:https://go.coder-hub.com/24952686.html
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