errors.Is and errors.As

Learn error wrapping and errors.Is, errors.As usage.

errors.Is and errors.As

Wrapped errors can't be compared with == because the outer error is a different value. To look inside the chain:

err := fmt.Errorf("profil: %w", ErrBulunamadi)
fmt.Println(err == ErrBulunamadi)          // false
fmt.Println(errors.Is(err, ErrBulunamadi)) // true

var dh *DogrulamaHatasi
if errors.As(err, &dh) {
    fmt.Println("alan:", dh.Alan)
}

errors.Is searches for a specific error value, errors.As for a specific error type.

Comparing correctly

Rule: in error checks always use errors.Is instead of ==; then your code keeps working even if the error is wrapped later. The standard library does the same: errors.Is(err, fs.ErrNotExist), errors.Is(err, context.DeadlineExceeded), errors.Is(err, sql.ErrNoRows).

Exercise Search the chain

Create ErrYok := errors.New("yok") and err := fmt.Errorf("katman 2: %w", fmt.Errorf("katman 1: %w", ErrYok)). Print err == ErrYok and errors.Is(err, ErrYok) on one line.

Show example solution

package main

import (
    "errors"
    "fmt"
)

func main() {
    ErrYok := errors.New("yok")
    err := fmt.Errorf("katman 2: %w", fmt.Errorf("katman 1: %w", ErrYok))
    fmt.Println(err == ErrYok, errors.Is(err, ErrYok))
}

Test yourself

Why is a pointer (<code>&dh</code>) passed to <code>errors.As</code>?
- So it can write the found error into that variable
- Because it's faster
- It isn't required
- For a nil check
Show answer

So it can write the found error into that variable — If As finds the requested type in the chain, it assigns the value to the target variable; that needs the target's address.

Search for the type

var dh *DogrulamaHatasi
if errors.____(err, &dh) {
    fmt.Println(dh.Alan)
}

Show answer

var dh *DogrulamaHatasi
if errors.As(err, &dh) {
    fmt.Println(dh.Alan)
}

Question and tool

Is this error ErrX?

errors.Is
Is this error of type *MyErr?

errors.As
Unwrap one level

errors.Unwrap
err == ErrX

Breaks once wrapped

Next lesson: Summary - Interface and error

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