Custom error types in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom error type in Go?

A custom error type in Go is a user-defined type that implements the error interface by having an Error() string method. It allows you to create errors with additional context or behavior beyond the basic error message.

beginner

How do you define a custom error type in Go?

You define a custom error type by creating a struct or type and implementing the Error() string method on it. For example:

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

intermediate

Why use custom error types instead of plain errors?

Custom error types let you add extra information or methods to errors. This helps in identifying error causes, handling errors differently, or adding context like error codes or timestamps.

intermediate

How can you check for a specific custom error type in Go?

You can use a type assertion or errors.As to check if an error is a specific custom error type. For example:

var myErr MyError
if errors.As(err, &myErr) { /* handle MyError */ }

beginner

Show a simple example of a custom error type with additional context.

Example:

type NotFoundError struct { Resource string }
func (e NotFoundError) Error() string {
  return fmt.Sprintf("%s not found", e.Resource)
}

This error tells which resource was not found.

What method must a custom error type implement in Go?

A Error() string

B String() string

C Message() string

D ToString() string

The error interface in Go requires the Error() string method.

Which Go feature helps you check if an error is a specific custom error type?

A Panic recovery

B Type assertion or errors.As

C Reflection only

D Interface embedding

Type assertion or errors.As lets you check and extract specific error types.

Why might you create a custom error type instead of using errors.New?

A To make errors print in uppercase

B To make errors faster

C To avoid using the error interface

D To add extra information or behavior to the error

Custom error types allow adding context or methods to errors.

What is the return type of the Error() method in a custom error type?

A string

B int

C bool

D error

The Error() method returns a string describing the error.

Which of these is a valid custom error type declaration in Go?

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

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

C type MyError struct { Msg string }

D type MyError int

A struct with fields is a common way to define a custom error type.

Explain how to create and use a custom error type in Go.

Think about how Go's error interface works and how you can add extra info.

Why are custom error types useful in Go programming?

Consider real-life situations where knowing more about an error helps.

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