Custom error types in Go - Mini Project: Build & Apply

Custom error types

📖 Scenario: You are building a simple program that processes orders. Sometimes, orders can have errors like invalid IDs or negative quantities. To handle these clearly, you want to create your own error types.

🎯 Goal: Learn how to create custom error types in Go and use them to return meaningful error messages.

📋 What You'll Learn

Create a custom error type struct

Implement the Error() method for the custom error type

Use the custom error type in a function to return errors

Print the error message from the custom error

💡 Why This Matters

🌍 Real World

Custom error types help you give clear, specific error messages in programs that handle many different error situations.

💼 Career

Understanding custom errors is important for writing clean, maintainable Go code in professional software development.

1

Create a custom error type struct

Create a struct called OrderError with two fields: OrderID of type int and Message of type string.

Go

# Your code here

Hint

Think of OrderError as a box holding the order number and a message explaining the error.

2

Implement the Error() method

Add a method called Error() to the OrderError struct that returns a string. It should return a formatted string like "Order {OrderID}: {Message}" using fmt.Sprintf.

Go

package main

type OrderError struct {
    OrderID int
    Message string
}

// Your code here

Hint

The Error() method lets Go treat your struct like a normal error with a message.

3

Use the custom error type in a function

Write a function called ProcessOrder that takes an orderID int and quantity int. If quantity is less than 1, return an OrderError with OrderID set to orderID and Message set to "invalid quantity". Otherwise, return nil.

Go

package main

import "fmt"

type OrderError struct {
    OrderID int
    Message string
}

func (e OrderError) Error() string {
    return fmt.Sprintf("Order %d: %s", e.OrderID, e.Message)
}

// Your code here

Hint

This function checks if the quantity is valid and returns your custom error if not.

4

Print the error message

Call ProcessOrder with orderID 101 and quantity 0. If it returns an error, print the error using fmt.Println.

Go

package main

import "fmt"

type OrderError struct {
    OrderID int
    Message string
}

func (e OrderError) Error() string {
    return fmt.Sprintf("Order %d: %s", e.OrderID, e.Message)
}

func ProcessOrder(orderID int, quantity int) error {
    if quantity < 1 {
        return OrderError{OrderID: orderID, Message: "invalid quantity"}
    }
    return nil
}

// Your code here

Hint

Use fmt.Println(err) to show the error message from your custom error.