Coming From Another Language

Pick the language you know best. Each page shows one program in that language and in Kavach, then a table of what changes, with a link to the page that teaches each part.

• Python: classes, exceptions and None.
• JavaScript: classes, null, === and async.
• C++: pointers, exceptions and undefined behaviour.
• Rust: borrowing, Result and Option.
• Go: if err != nil, nil and goroutines.

What changes whichever language you know

The program each page uses, in Kavach:

type Order {
    item: String
    qty: Int
    cents: Int

    fn cost(self) -> Int {
        self.qty * self.cents
    }
}

type OrderError = BadQty(item: String)

fn total(orders: [Order]) -> Int ! OrderError {
    for o in orders {
        if o.qty <= 0 {
            fail OrderError.BadQty(item: o.item)
        }
    }
    orders.sum(o => o.cost())
}

fn main() {
    const codes = ["SPRING": 10, "VIP": 25]
    const orders = [Order(item: "pen", qty: 3, cents: 150), Order(item: "cup", qty: 1, cents: 900)]

    const sum = total(orders) catch _ => 0
    const off = codes["SPRING"] ?? 0
    print("total {sum}, with {off}% off {sum * (100 - off) / 100}")
    print("WINTER gives {codes["WINTER"] ?? 0}%")

    const broken = [Order(item: "kite", qty: 0, cents: 500)]
    match total(broken) {
        ok(t) => print("total {t}"),
        err(OrderError.BadQty(item)) => print("refused: no quantity for {item}"),
    }
}

total 1350, with 10% off 1215WINTER gives 0%refused: no quantity for kite

• total says in its signature that it can fail with OrderError, and the caller decides what happens then: catch gives a value, match looks at each case.
• codes["WINTER"] may be missing, so its type is Int?, and ?? 0 says what to use then.

What you give up

• Kavach is young. It has far fewer packages than pip, npm, Cargo or Go modules, so you lean on its standard library and on code you write.
• It runs on Linux and Windows on x86-64 today. macOS and ARM64 are planned for a later phase of the roadmap.
• Some habits are refused on purpose: there is no inheritance, no exception that passes up unseen, and no null.

No decorators

Kavach has no @decorator: nothing written above a function can change what it does, so its signature stays the whole story. What decorators do elsewhere is said in the signature or in plain code: