Question

我正在跟随the "Go By Example" tutorial。在关于错误的课程结束时，我看到一些难以理解的语法：

if ae, ok := e.(*argError); ok {
    fmt.Println(ae.arg)
    fmt.Println(ae.prob)
}

我知道Go函数可以同时返回多个内容，“：=”语法意味着你声明并将变量指定为一行，并且（* argError）指的是一个指针（我想？）。我不明白的是“e。（* argError）”语法，特别是点符号（看起来像是对错误的方法调用，但没有以点命名的方法）。在这种情况下，我也不明白“ok”的目的是什么。对我有用的是伪代码解释在什么情况下会调用“fmt.Println”函数。

代码的完整上下文如下。

// In Go it's idiomatic to communicate errors via an
// explicit, separate return value. This contrasts with
// the exceptions used in languages like Java and Ruby and
// the overloaded single result / error value sometimes
// used in C. Go's approach makes it easy to see which
// functions return errors and to handle them using the
// same language constructs employed for any other,
// non-error tasks.

package main

import "errors"
import "fmt"

// By convention, errors are the last return value and
// have type `error`, a built-in interface.
func f1(arg int) (int, error) {
    if arg == 42 {

        // `errors.New` constructs a basic `error` value
        // with the given error message.
        return -1, errors.New("can't work with 42")

    }

    // A nil value in the error position indicates that
    // there was no error.
    return arg + 3, nil
}

// It's possible to use custom types as `error`s by
// implementing the `Error()` method on them. Here's a
// variant on the example above that uses a custom type
// to explicitly represent an argument error.
type argError struct {
    arg  int
    prob string
}

func (e *argError) Error() string {
    return fmt.Sprintf("%d - %s", e.arg, e.prob)
}

func f2(arg int) (int, error) {
    if arg == 42 {

        // In this case we use `&argError` syntax to build
        // a new struct, supplying values for the two
        // fields `arg` and `prob`.
        return -1, &argError{arg, "can't work with it"}
    }
    return arg + 3, nil
}

func main() {

    // The two loops below test out each of our
    // error-returning functions. Note that the use of an
    // inline error check on the `if` line is a common
    // idiom in Go code.
    for _, i := range []int{7, 42} {
        if r, e := f1(i); e != nil {
            fmt.Println("f1 failed:", e)
        } else {
            fmt.Println("f1 worked:", r)
        }
    }
    for _, i := range []int{7, 42} {
        if r, e := f2(i); e != nil {
            fmt.Println("f2 failed:", e)
        } else {
            fmt.Println("f2 worked:", r)
        }
    }

    // If you want to programmatically use the data in
    // a custom error, you'll need to get the error  as an
    // instance of the custom error type via type
    // assertion.
    _, e := f2(42)
    if ae, ok := e.(*argError); ok {
        fmt.Println(ae.arg)
        fmt.Println(ae.prob)
    }
}

这个Golang语法是什么意思？

0 个答案: