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Kavachकवच

Optionals

Also known as: null nil none undefined nullable maybe option optional chaining null pointer exception safe navigation elvis coalescing

● Intermediate5 minBefore this: Records, If LetSpec 2.7

There is no null. A value that may be missing has a type ending in ?, and you must say what happens when it is.

type User {
    name: String
}

fn find_user(id: Int) -> User? {
    if id == 7 { User(name: "Asha") } else { none }
}

fn main() {
    const found = find_user(7)
    const missing = find_user(8)
    print(found?.name ?? "guest")
    print(missing?.name ?? "guest")
    if let u = find_user(7) {
        print("Found {u.name}")
    }
}

The program ran and printed:

kavach run optionals-basic.kav

AshaguestFound Asha
optionals-basic.kav: Kavach code, then what running it prints
  • User? is a User or none.
  • ?. reads through: found?.name is the name, or none when there is no user.
  • ?? gives a value to use instead of none.
  • if let runs a block only when the value is there.

Using an optional as if it were surely there is a compile error. The "null pointer exception" of other languages can't happen.

One page for each form

What an optional is →Reading one safely →

What Kavach does for you

  • There is no null

    A value that may be missing says so in its type, User?, and the compiler makes you handle the missing case. A null pointer error can't happen.

    spec 2.7