Creating Your Own Error Types in Go

Overview
Prerequisites
Setup
Creating Custom Error Types
Handling Custom Errors
Conclusion

Overview

In Go, error handling is a crucial aspect of writing robust and reliable code. While Go provides the error type for representing errors, you can also create your own custom error types to encapsulate specific error scenarios. This tutorial will guide you through the process of creating and using custom error types in Go. By the end of this tutorial, you will understand how to define and handle custom errors effectively.

Prerequisites

To follow along with this tutorial, it is assumed that you have a basic understanding of Go programming language syntax and concepts. Familiarity with error handling in Go would also be beneficial.

Setup

Before we dive into creating custom error types, let’s set up a basic Go project to work with throughout the tutorial. Perform the following steps to get started:

Ensure Go is installed on your system. You can download it from the official Go website: https://golang.org/dl/
Create a new directory for your project.
Open a terminal or command prompt and navigate to the project directory.
Initialize a new Go module by running the following command:
```
 go mod init your-module-name
```
Now that our project is set up, we can proceed with creating custom error types.

Creating Custom Error Types

Creating custom error types in Go involves defining a new struct that satisfies the error interface. Let’s walk through an example to see how this can be done:

Inside your project directory, create a new Go file called errors.go.
Open the errors.go file in your preferred text editor.
Define a new struct type for your custom error:
```
 package main
    
 import "fmt"
    
 type CustomError struct {
     Reason string
 }
```
In this example, we created a CustomError struct with a single field Reason of type string. You can add any additional fields that are relevant to your custom error.
Implement the Error() method for your custom error:
```
 func (e *CustomError) Error() string {
     return fmt.Sprintf("Custom Error: %s", e.Reason)
 }
```
The Error() method is part of the error interface and should return a string representation of the error. In our example, we format the error message as "Custom Error: <reason>".
Save the errors.go file.

Congratulations! You have successfully created a custom error type in Go. Now let’s look at how to handle these custom errors.

Handling Custom Errors

Handling custom errors in Go is similar to handling standard errors using conditional statements. Let’s see how you can handle the CustomError type we created in the previous section:

Create a new Go file called main.go.
Open the main.go file in your text editor.
Import the necessary packages:
```
 package main
    
 import "fmt"
```
Define a function that returns a CustomError:
```
 func doSomething() error {
     return &CustomError{Reason: "Something went wrong"}
 }
```
Here, we created a function doSomething() that returns a CustomError instance.
In the main() function, handle the custom error:
```
 func main() {
     if err := doSomething(); err != nil {
         fmt.Println(err.Error())
     } else {
         fmt.Println("No error occurred")
     }
 }
```
In this example, we called the doSomething() function and checked if the returned error is nil. If it’s not nil, we print the error message using err.Error(). Otherwise, we print a message indicating that no error occurred.
Save the main.go file.

Now you can run your program to see the custom error handling in action:
```
 go run .
```
You should see the custom error message printed if an error occurs.

Conclusion

In this tutorial, you learned how to create and handle custom error types in Go. Custom error types allow you to encapsulate specific error scenarios and provide more context for debugging and error reporting. By defining your own error types, you can make your code more expressive and improve error handling throughout your Go applications.

Throughout this tutorial, you created a custom error type, implemented the Error() method, and handled the error using conditional statements. You can now apply these concepts to your own projects and create custom error types tailored to your application’s needs.

Remember to practice error handling diligently in your Go code to ensure your programs are reliable and robust.

Published: 13 October 2019