Before learning about different ways of creating am an error in Golang lets first understand about the error. Error is an interface type with below signature

type error interface {
Error() string
}

Any by definition of interface any type which implements the Error() method becomes of type error.

Let’s see now different methods of creating an error

1. Using errors.New(“some_error_message”)

package main
import (
"errors"
"fmt"
)
func main() {
sampleErr := errors.New("error occured")
fmt.Println(sampleErr)
}

Output:

error occured

2. Using fmt.Errorf(“error is %s”, “some_error_message”). This way creates error with formatting of error messages

package main
import (
"fmt"
)
func main() {
sampleErr := fmt.Errorf("Err is: %s", "database connection issue")
fmt.Println(sampleErr)
}

Output:

Err is: database connection issue

3. Creating Custom error

The below example illustrates the use of custom error. In below example

inputError is a struct that has the Error() method hence it is of type error
You can also add additional information to the custom error by extending its fields or by adding new methods. inputError has an additional field named missingFields and a function getMissingFields function.
We can use type assertion to convert from error to inputError

Example:

package main
import "fmt"
type inputError struct {
message string
missingField string
}
func (i *inputError) Error() string {
return i.message
}
func (i *inputError) getMissingField() string {
return i.missingField
}
func main() {
err := validate("", "")
if err != nil {
if err, ok := err.(*inputError); ok {
fmt.Println(err)
fmt.Printf("Missing Field is %s\n", err.getMissingField())
}
}
}
func validate(name, gender string) error {
if name == "" {
return &inputError{message: "Name is mandatory", missingField: "name"}
}
if gender == "" {
return &inputError{message: "Gender is mandatory", missingField: "gender"}
}
return nil
}

Output:

Name is mandatory
Missing Field is name