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