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".