Handling Errors in Golang

Introduction

The purpose of this lab is to understand how to handle errors in Golang.

Skills Graph

Errors

The lab provides two functions that return an error if the input argument is 42. The first function returns a basic error value, while the second function uses a custom type to represent the error.

• The errors package must be imported.
• The f1 function must return an error if the input argument is 42.
• The f2 function must return an error of type argError if the input argument is 42.
• The argError type must have two fields: arg and prob.
• The argError type must implement the Error() method.
• The main function must call both f1 and f2 with input arguments of 7 and 42.
• The main function must print the result of each function call, along with any error that was returned.
• The main function must demonstrate how to programmatically use the data in a custom error.

$ go run errors.go
f1 worked: 10
f1 failed: can't work with 42
f2 worked: 10
f2 failed: 42 - can't work with it
42
can't work with it

## See this [great post](https://go.dev/blog/error-handling-and-go)
## on the Go blog for more on error handling.

There is the full code below:

// 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"
	"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)
	}
}

Summary

This lab demonstrates how to handle errors in Golang using the error interface and custom error types. It also shows how to programmatically use the data in a custom error.