Why Custom error types in Go? - Purpose & Use Cases

The Big Idea

What if your program could tell you exactly what went wrong, instead of just saying "error"?

The Scenario

Imagine you write a program that talks to a bank system. When something goes wrong, you just get a generic error message like "something went wrong". You have to guess what happened and where. This makes fixing problems slow and frustrating.

The Problem

Using only generic errors is like getting a "404" on a website without knowing which page is missing. You waste time checking everything manually. It's easy to miss important details or confuse one error for another. This leads to bugs that are hard to find and fix.

The Solution

Custom error types let you create specific error messages with extra details. It's like having a special label on each problem that tells you exactly what went wrong and why. This makes your program smarter and easier to debug.

Before vs After

✗ Before

return errors.New("something went wrong")

✓ After

type InsufficientFundsError struct { Amount float64 }

func (e *InsufficientFundsError) Error() string {
    return fmt.Sprintf("insufficient funds: need %.2f more", e.Amount)
}

What It Enables

Custom error types let your program handle problems precisely and clearly, making it easier to fix bugs and improve user experience.

Real Life Example

When a bank app detects a withdrawal that exceeds the balance, a custom error can tell the user exactly how much more money is needed instead of a vague failure message.

Key Takeaways

Generic errors hide important details and slow debugging.

Custom error types add clear, specific information to errors.

This helps programs respond better and developers fix issues faster.

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