The error interface

Learn the error interface and the Error() method.

The error interface

In Go an error is any value that satisfies the built-in error interface:

type error interface {
    Error() string
}

Functions return the error as their last value; on success they return nil. Errors aren't exceptions but ordinary values; they are checked with if err != nil.

f, err := os.Open("ayar.json")
if err != nil {
    return err
}
defer f.Close()

A custom error type

Error types that carry extra information let the caller handle the error better:

type DogrulamaHatasi struct {
    Alan  string
    Neden string
}

func (e *DogrulamaHatasi) Error() string {
    return e.Alan + ": " + e.Neden
}

func dogrula(yas int) error {
    if yas < 0 {
        return &DogrulamaHatasi{"yaş", "negatif olamaz"}
    }
    return nil
}

Exercise Validation error

Write the DogrulamaHatasi type and the dogrula(yas int) error function above. Print the results of dogrula(-3) and dogrula(20) on separate lines.

Show example solution

package main

import "fmt"

type DogrulamaHatasi struct {
    Alan  string
    Neden string
}

func (e *DogrulamaHatasi) Error() string {
    return e.Alan + ": " + e.Neden
}

func dogrula(yas int) error {
    if yas < 0 {
        return &DogrulamaHatasi{"yaş", "negatif olamaz"}
    }
    return nil
}

func main() {
    fmt.Println(dogrula(-3))
    fmt.Println(dogrula(20))
}

Test yourself

What is needed to write your own error type?
- Calling panic
- Registering with the errors package
- A type with an Error() string method
- Inheriting from error
Show answer

A type with an Error() string method — Implicit satisfaction applies here too; any type with an Error() string method is an error.

Error method

func (e *DogrulamaHatasi) ____() string {
    return e.Alan + ": " + e.Neden
}

Show answer

func (e *DogrulamaHatasi) Error() string {
    return e.Alan + ": " + e.Neden
}

Concept

error

Error() string
A nil error

Success
if err != nil

Error check
*DogrulamaHatasi

Custom error type

Next lesson: Returning errors

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies