When a value might be missing

A list of three names has no fourth one. winners[3] would stop the program, because there is nothing to give back. winners.get(3) asks politely instead: it answers either the value or nothing, and you decide what to do.

?? is the simplest way to decide: winners.get(3) ?? "nobody" is the value when there is one, and "nobody" when there is not.

const winners = ["Ada", "Alan", "Grace"]
const third = winners.get(2) ?? "nobody"
const fourth = winners.get(3) ?? "nobody"
print "Third place: {third}"
print "Fourth place: {fourth}"
// prints Third place: Grace
// prints Fourth place: nobody

Something or nothing#

The answer of get is one of two cases, like the weather of the last lesson: Some(name) when there is a value, and None when there is not. match can take it apart, and has to handle both.

Many languages have a special value for "nothing" that fits everywhere, and a program crashes when it forgets to check for it. In TorbScript "maybe nothing" is a type of its own, an Option, so the computer notices when a check is missing.

const winners = ["Ada", "Alan", "Grace"]
match winners.get(0) {
  Some(name) => print "The winner is {name}!"
  None => print "Nobody won."
}
// prints The winner is Ada!

Exercise#

Print the third guest, or nobody when there is no third one. With two guests, the program prints nobody.

const guests = ["Ada", "Alan"]
print guests[0]

Expected output

nobody

Run it on your computer: torb run missing-values.trb

Hint

Ask with guests.get(2), and use ?? "nobody" for the case that it answers nothing. Give the result a name before you print it.

Solution

const guests = ["Ada", "Alan"]
const third = guests.get(2) ?? "nobody"
print third
// prints nobody

Recap#

A value that might be missing is Some(value) or None, and ?? or match decide what happens when it is None.