JuniorTheoryCommon

What does the program print?

package main

import (
    "errors"
    "fmt"
)

func main() {
    errNotFound := errors.New("not found")
    err := fmt.Errorf("load user: %w", errNotFound)
    fmt.Println(err == errNotFound, errors.Is(err, errNotFound))
}

false true

Expecting true from ==: fmt.Errorf with %w returns a new error that holds errNotFound, so the two values differ and only the chain links them.
Expecting errors.Is to compare like ==: it walks the chain that %w builds, finds errNotFound inside err and returns true.

What would errors.Is return if the code used %v instead of %w?
Why is a sentinel error usually declared as a package-level var?

Back to questions Open topic in roadmap →Answer it yourself — Quiz

Back to questions

JuniorTheoryCommon

What does the program print?

package main

import (
    "errors"
    "fmt"
)

func main() {
    errNotFound := errors.New("not found")
    err := fmt.Errorf("load user: %w", errNotFound)
    fmt.Println(err == errNotFound, errors.Is(err, errNotFound))
}

false true

Expecting true from ==: fmt.Errorf with %w returns a new error that holds errNotFound, so the two values differ and only the chain links them.
Expecting errors.Is to compare like ==: it walks the chain that %w builds, finds errNotFound inside err and returns true.

What would errors.Is return if the code used %v instead of %w?
Why is a sentinel error usually declared as a package-level var?

Back to questions Open topic in roadmap →Answer it yourself — Quiz