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.

Practice

1. What is the main purpose of creating a custom error type in Go?

easy

A. To add extra information and context to errors

B. To make errors print in uppercase letters

C. To automatically fix errors in the code

D. To prevent errors from being returned

Solution

1. Step 1: Understand the role of custom error types

   Custom error types allow programmers to include more details about what caused the error, beyond just a simple message.

2. Step 2: Compare options with this understanding

   Only To add extra information and context to errors correctly describes adding extra information and context. Other options describe unrelated or incorrect behaviors.

3. Final Answer:

   To add extra information and context to errors -> Option A

4. Quick Check:

   Custom error types add context = B [OK]

Hint: Custom errors add details, not fix or hide errors [OK]

Common Mistakes:

• Thinking custom errors fix problems automatically
• Believing custom errors change error printing style
• Assuming custom errors stop errors from returning

2. Which of the following is the correct way to define a custom error type in Go?

easy

A. type MyError string func (e MyError) Print() { fmt.Println(e) }

B. func MyError() string { return "error" }

C. type MyError struct { msg string } func (e MyError) Error() string { return e.msg }

D. var MyError = errors.New("error")

Solution

1. Step 1: Recall how to create custom error types

   In Go, a custom error type is a struct that implements the Error() string method.

2. Step 2: Check each option

   type MyError struct { msg string } func (e MyError) Error() string { return e.msg } defines a struct and implements Error() correctly. Options A, B, and D do not implement the Error() method properly or define a struct.

3. Final Answer:

   type MyError struct { msg string } func (e MyError) Error() string { return e.msg } -> Option C

4. Quick Check:

   Custom error = struct + Error() method [OK]

Hint: Custom error needs struct + Error() string method [OK]

Common Mistakes:

• Defining error as a function without Error() method
• Using string type without Error() method
• Using errors.New instead of custom struct

3. What will be the output of this Go code?

package main
import (
  "errors"
  "fmt"
)
type MyError struct {
  Code int
}
func (e MyError) Error() string {
  return fmt.Sprintf("Error code: %d", e.Code)
}
func main() {
  var err error = MyError{Code: 404}
  if errors.As(err, &MyError{}) {
    fmt.Println("Custom error detected")
  } else {
    fmt.Println("Unknown error")
  }
}

medium

A. Unknown error

B. Custom error detected

C. Compilation error

D. Runtime panic

Solution

1. Step 1: Understand errors.As usage

   errors.As(err, targetPtr) returns true if err matches the type pointed to by targetPtr.

2. Step 2: Analyze the code's errors.As call

   &MyError{} is a pointer to type MyError. err is MyError{Code: 404}, which matches MyError exactly. errors.As succeeds and prints "Custom error detected".

3. Final Answer:

   Custom error detected -> Option B

4. Quick Check:

   errors.As(MyError, &MyError{}) matches [OK]

Hint: errors.As works with &MyError{} (pointer to literal) [OK]

Common Mistakes:

• Passing &MyError{} instead of pointer to variable
• Assuming errors.As works with value types
• Confusing errors.Is with errors.As

4. Identify the error in this Go code snippet that tries to use a custom error type:

type MyError struct {
  Msg string
}
func (e *MyError) Error() string {
  return e.Msg
}

func doSomething() error {
  return MyError{Msg: "fail"}
}

medium

A. MyError should not have a pointer receiver for Error()

B. Error() method must return an int, not string

C. MyError struct must embed error interface

D. doSomething returns MyError value but Error() has pointer receiver

Solution

1. Step 1: Check method receiver and returned type

   Error() is defined with pointer receiver (*MyError), but doSomething returns a value (MyError), not a pointer.

2. Step 2: Understand interface satisfaction rules

   Because Error() has pointer receiver, only *MyError implements error interface, not MyError value. Returning MyError value causes a type mismatch.

3. Final Answer:

   doSomething returns MyError value but Error() has pointer receiver -> Option D

4. Quick Check:

   Pointer receiver means only pointer type implements error [OK]

Hint: Pointer receiver means return pointer, not value [OK]

Common Mistakes:

• Returning value when method has pointer receiver
• Thinking struct must embed error interface
• Expecting Error() to return int

5. You want to create a custom error type that holds an error code and message, and you want to check for this error type using errors.As. Which of the following code snippets correctly implements this pattern?

hard

A. type CodeError struct { Code int; Msg string } func (e *CodeError) Error() string { return e.Msg } func main() { var err error = &CodeError{Code: 123, Msg: "fail"} var target *CodeError if errors.As(err, &target) { fmt.Println(target.Code) } }

B. type CodeError struct { Code int; Msg string } func (e CodeError) Error() string { return e.Msg } func main() { var err error = CodeError{Code: 123, Msg: "fail"} var target *CodeError if errors.As(err, &target) { fmt.Println(target.Code) } }

C. type CodeError struct { Code int; Msg string } func (e CodeError) Error() string { return e.Msg } func main() { var err error = &CodeError{Code: 123, Msg: "fail"} var target CodeError if errors.As(err, &target) { fmt.Println(target.Code) } }

D. type CodeError struct { Code int; Msg string } func (e *CodeError) Error() string { return e.Msg } func main() { var err error = CodeError{Code: 123, Msg: "fail"} var target *CodeError if errors.As(err, &target) { fmt.Println(target.Code) } }

Solution

1. Step 1: Check receiver type and error assignment

   type CodeError struct { Code int; Msg string } func (e *CodeError) Error() string { return e.Msg } func main() { var err error = &CodeError{Code: 123, Msg: "fail"} var target *CodeError if errors.As(err, &target) { fmt.Println(target.Code) } } uses pointer receiver for Error() and assigns &CodeError{} to err, which matches the pointer receiver.

2. Step 2: Check errors.As usage

   type CodeError struct { Code int; Msg string } func (e *CodeError) Error() string { return e.Msg } func main() { var err error = &CodeError{Code: 123, Msg: "fail"} var target *CodeError if errors.As(err, &target) { fmt.Println(target.Code) } } passes &target where target is *CodeError, which is correct usage for errors.As to detect the custom error type.

3. Final Answer:

   custom error with pointer receiver and errors.As check -> Option A

4. Quick Check:

   Pointer receiver + pointer error + errors.As pointer variable = A [OK]

Hint: Match pointer receiver with pointer error and pointer target in errors.As [OK]

Common Mistakes:

• Using value receiver but returning pointer or vice versa
• Passing non-pointer target to errors.As
• Assigning value error but using pointer receiver