meet it

Your own functions can hand back errors too.

Your functions can fail as well. Say the cinema sells at most 10 tickets per order. A buy function can return two results, (int, error): the price when all is well, or an error saying what went wrong.

makes an error value with a fixed message. works like Sprintf and builds an error from placeholders: fmt.Errorf("%v is too many", tickets).

Check the bad cases first and , handing back the zero value with the error: return 0, err. At the end, once every check has passed, return the real answer and nil.

package main
import "errors"
import "fmt"
func buy(tickets int) (int, error) {
	if tickets < 1 {
		return 0, errors.New("buy at least one ticket")
	}
	if tickets > 10 {
		return 0, fmt.Errorf("%v is too many, the limit is 10", tickets)
	}
	return tickets * 4, nil
}
func main() {
	for _, n := range []int{2, 0, 12} {
		price, err := buy(n)
		fmt.Println(price, err)
	}
}

Error messages start with a small letter and end without a full stop, because they often get joined onto other text.

guess first

A long name for the class register. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

package main
import "errors"
import "fmt"
func checkName(name string) error {
	if name == "" {
		return errors.New("name is empty")
	}
	if len(name) > 8 {
		return fmt.Errorf("%v is longer than 8 letters", name)
	}
	return nil
}
func main() {
	fmt.Println(checkName("Bartholomew"))
}

your turn

Your turn. Guard the big rollercoaster.

Choose the smallest height in cm allowed on the ride. Fill in what rideCheck returns when all is well. Then pick a height that lets you on.

Your code

output

press run to see what happens

fix it

Sharing sweets with nobody. Can you fix it?

Broken code to fix

output

press run to see what happens

Read the error on line 6. Which type does Go expect first in that return?
The function promises (int, error): the number first, then the error.
Swap them so line 6 says return 0, errors.New("nobody to share with"), then run.

nailed it

your functions can say what went wrong. :D

errors.New makes an error with a fixed message.
fmt.Errorf builds an error message with placeholders.
Check for problems first, and return early with the zero value.

new words in your collection

errors.New
Errorf
return early

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