Custom error types in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a custom error type's Error() method

What is the output when running this Go program?

Go

package main
import (
	"errors"
	"fmt"
)
type MyError struct {
	Code int
	Message string
}
func (e MyError) Error() string {
	return fmt.Sprintf("Error %d: %s", e.Code, e.Message)
}
func main() {
	err := MyError{Code: 404, Message: "Not Found"}
	fmt.Println(err.Error())
}

A Error 404: Not Found

B MyError{Code:404, Message:"Not Found"}

C 404 Not Found

D runtime error: invalid memory address or nil pointer dereference

💡 Hint

Look at how the Error() method formats the string.

The Error() method returns a formatted string with the code and message. Printing err.Error() outputs exactly that string.

❓ Predict Output

intermediate

Output when returning a custom error type as error interface

What will this program print?

Go

package main
import (
	"fmt"
)
type MyError struct {
	Msg string
}
func (e *MyError) Error() string {
	return e.Msg
}
func doSomething() error {
	return &MyError{Msg: "something went wrong"}
}
func main() {
	err := doSomething()
	fmt.Println(err)
}

A panic: runtime error: invalid memory address or nil pointer dereference

B &{something went wrong}

C <nil>

D something went wrong

💡 Hint

Printing an error interface calls its Error() method.

The doSomething function returns a pointer to MyError which implements Error(). Printing err calls Error() and prints the message.

🔧 Debug

advanced

Identify the error in custom error type implementation

This code is supposed to implement a custom error type but causes a compile error. What is the cause?

Go

package main
import "fmt"
type MyError struct {
	Msg string
}
func (e MyError) Error() {
	fmt.Println(e.Msg)
}
func main() {
	var err error = MyError{Msg: "fail"}
	fmt.Println(err.Error())
}

A Cannot assign MyError to error interface without pointer receiver

B Error() method must return a string, but here it returns nothing

C MyError struct must embed error interface explicitly

D fmt.Println cannot print error interface values

💡 Hint

Check the signature of the Error() method in the error interface.

The error interface requires Error() to return a string. Here Error() returns nothing, causing a compile error.

🧠 Conceptual

advanced

Why use pointer receiver for custom error types?

Why is it common to define the Error() method with a pointer receiver for custom error types in Go?

A To avoid copying the error struct and allow modifying fields if needed

B Because only pointer receivers satisfy the error interface

C To prevent the error from being nil

D Because Go requires all methods on structs to use pointer receivers

💡 Hint

Think about efficiency and mutability when using pointer receivers.

Pointer receivers avoid copying the struct and allow methods to modify the receiver if needed. Value receivers copy the struct.

❓ Predict Output

expert

Output of type assertion with custom error type

What is the output of this program?

Go

package main
import (
	"errors"
	"fmt"
)
type MyError struct {
	Code int
}
func (e MyError) Error() string {
	return fmt.Sprintf("code %d", e.Code)
}
func main() {
	err := errors.New("standard error")
	var myErr MyError
	if errors.As(err, &myErr) {
		fmt.Println("Matched MyError")
	} else {
		fmt.Println("Did not match MyError")
	}
}

A panic: interface conversion: error is *errors.errorString, not MyError

B Matched MyError

C Did not match MyError

D code 0

💡 Hint

errors.As tries to find the target type in the error chain.

The error returned by errors.New is not of type MyError, so errors.As returns false and prints "Did not match MyError".

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