Wrapping errors: %w, Is, As, AsType and Join

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7 questions about 7 ideas of this lesson. Each answer is explained at once, and what you get right is kept in this browser.

1. Question 1 of 7 Which fmt.Errorf verb makes the new error keep the error it formats?

   Choose one answer.

   %e %v %w %s

   Show the answer: Question 1 of 7

   Answer: %w

   Only %w wraps; every other verb turns the error into text.

2. Question 2 of 7 A payment package returns fmt.Errorf("charge: %v", err) for every gateway error, and its callers now need to retry gateway timeouts. What should change?

   Choose one answer.

   Log every error inside the package instead of returning it Retry inside fmt.Errorf Return the timeout with %w, so callers can test errors.Is(err, ErrGatewayTimeout) Nothing: callers can search the message for timeout

   Show the answer: Question 2 of 7

   Answer: Return the timeout with %w, so callers can test errors.Is(err, ErrGatewayTimeout)

   Wrap with %w the errors that callers need to recognise.

3. Question 3 of 7 Which function finds an error of a given type, such as *QuotaError, anywhere in a chain and returns it?

   Choose one answer.

   errors.Unwrap errors.AsType errors.Is errors.New

   Show the answer: Question 3 of 7

   Answer: errors.AsType

   Is answers "is this value there?"; As and AsType answer "is an error of this type there, and which is it?".

4. Question 4 of 7 The gateway exports both ErrQuota and a *QuotaError type with Used and Limit. A login screen must say how many messages over the limit the user is. Which test does it need?

   Choose one answer.

   err.(*QuotaError) errors.Is(err, ErrQuota) errors.AsType[*QuotaError](err), to read Used and Limit Parse the numbers out of err.Error()

   Show the answer: Question 4 of 7

   Answer: errors.AsType[*QuotaError](err), to read Used and Limit

   Choose errors.Is when the kind of failure is enough, and errors.AsType when you need its data.

5. Question 5 of 7 How does an error made by errors.Join format its message?

   Choose one answer.

   Each non-nil error's text, in order, with a newline between them The first error's text only A count, such as 2 errors The texts separated by commas

   Show the answer: Question 5 of 7

   Answer: Each non-nil error's text, in order, with a newline between them

   A joined error reads as one line per failure.

6. Question 6 of 7 join.go validates a form with an empty name, the phone 98765 and the PIN code 560001, then a valid form. What does it print?

   What does this program print? Choose one answer.

   // join.go: every failure of a signup form returned at once with errors.Join, and each found with errors.Is.
   package main
   
   import (
   	"errors"
   	"fmt"
   )
   
   var (
   	ErrNameMissing = errors.New("name is missing")
   	ErrPhoneDigits = errors.New("phone needs 10 digits")
   	ErrPINDigits   = errors.New("PIN code needs 6 digits")
   )
   
   type Signup struct {
   	Name, Phone, PIN string
   }
   
   // validate checks every field and returns all the failures it found, or nil.
   func validate(s Signup) error {
   	var name, phone, pin error
   	if s.Name == "" {
   		name = ErrNameMissing
   	}
   	if len(s.Phone) != 10 {
   		phone = fmt.Errorf("phone %q: %w", s.Phone, ErrPhoneDigits)
   	}
   	if len(s.PIN) != 6 {
   		pin = fmt.Errorf("PIN code %q: %w", s.PIN, ErrPINDigits)
   	}
   	return errors.Join(name, phone, pin)
   }
   
   func main() {
   	err := validate(Signup{Name: "", Phone: "98765", PIN: "560001"})
   	fmt.Println(err)
   	fmt.Println("name missing:", errors.Is(err, ErrNameMissing))
   	fmt.Println("phone digits:", errors.Is(err, ErrPhoneDigits))
   	fmt.Println("PIN digits:  ", errors.Is(err, ErrPINDigits))
   
   	ok := validate(Signup{Name: "Asha", Phone: "9876543210", PIN: "560001"})
   	fmt.Println("a valid form:", ok, ok == nil)
   }

   name is missing name missing: true phone digits: false PIN digits: false a valid form: <nil> true name is missing phone "98765": phone needs 10 digits name missing: true phone digits: false PIN digits: false a valid form: <nil> true name is missing phone "98765": phone needs 10 digits name missing: true phone digits: true PIN digits: false a valid form: <nil> true name is missing phone "98765": phone needs 10 digits name missing: true phone digits: true PIN digits: true a valid form: <nil> true

   Show the answer: Question 6 of 7

   Answer: it prints

   name is missing
   phone "98765": phone needs 10 digits
   name missing: true
   phone digits: true
   PIN digits:   false
   a valid form: <nil> true

   errors.Join drops nil values, writes each remaining error on its own line, lets errors.Is reach every part, and returns nil when nothing failed.

7. Question 7 of 7 A bulk upload of three files must report every file that failed, and the caller must still notice if one of them failed with ErrDiskFull. What should the upload return?

   Choose one answer.

   A slice of strings with the messages One error whose text lists the files, made with %v errors.Join of one error per failed file, each made with fmt.Errorf("upload %s: %w", name, err) The first error only

   Show the answer: Question 7 of 7

   Answer: errors.Join of one error per failed file, each made with fmt.Errorf("upload %s: %w", name, err)

   Join keeps every failure and every wrapped value; each part should carry its own context.