Error interface in Go - Mini Project: Build & Apply

Understanding the Error Interface in Go

📖 Scenario: Imagine you are building a simple program that checks if a user is old enough to vote. If the user is too young, the program should return an error message.

🎯 Goal: You will create a custom error using Go's error interface, check the user's age, and print the error message if the user is not old enough.

📋 What You'll Learn

Create a custom error type that implements the Error() method

Write a function checkAge that returns an error if age is less than 18

Call checkAge with a specific age and handle the error

Print the error message if an error occurs

💡 Why This Matters

🌍 Real World

Custom errors help programs give clear messages when something goes wrong, like invalid user input.

💼 Career

Understanding error handling is essential for writing reliable Go programs used in backend services and tools.

1

Create a custom error type

Create a struct called AgeError with a field message of type string.

Go

# Your code here
// Create a struct called AgeError with a string field message

Hint

Use type AgeError struct { message string } to define the struct.

2

Implement the Error() method

Add a method Error() to AgeError that returns the message field as a string.

Go

package main

type AgeError struct {
    message string
}

// Add the Error() method to AgeError
# Your code here

Hint

The method signature should be func (e AgeError) Error() string and return e.message.

3

Write the checkAge function

Write a function called checkAge that takes an age of type int and returns an error. If age is less than 18, return an AgeError with the message "Age is less than 18". Otherwise, return nil.

Go

package main

type AgeError struct {
    message string
}

func (e AgeError) Error() string {
    return e.message
}

// Write the checkAge function below
# Your code here

Hint

Use an if statement to check the age and return the custom error or nil.

4

Call checkAge and print the error

In the main function, call checkAge with the value 16. If an error is returned, print the error message using fmt.Println(err).

Go

package main

import "fmt"

type AgeError struct {
    message string
}

func (e AgeError) Error() string {
    return e.message
}

func checkAge(age int) error {
    if age < 18 {
        return AgeError{message: "Age is less than 18"}
    }
    return nil
}

func main() {
    // Call checkAge with 16 and print error if any
    # Your code here
}

Hint

Use err := checkAge(16) and then check if err != nil to print the error.