Lesson 4: Failures as part of the contract

Treat errors as data — Option, Result, and the ? operator.

Goal

In one sentence: return and propagate errors explicitly and scannably.

Code

public func fetchBalance(account: ref Account): Result<int, str> {
    if account.active == false {
        return Result.Err("account disabled")
    }
    return Result.Ok(account.balance)
}

The type Result<int, str> says the function either returns an int (Result.Ok(...)) or an error message (Result.Err(...)). No hidden exceptions behind the scenes.

Propagating with ?

public func summary(account: ref Account): Result<str, str> {
    let balance = fetchBalance(account)?
    return Result.Ok("Balance: " + balance)
}

The ? shortens propagation: on Err, the whole function returns that error; on Ok, you get the value.

Try it

Remove the ? from fetchBalance(account) and watch the compiler ask you to handle the Result explicitly. It will not let an error slip through without you deciding what to do.

Errors as values keep the error flow visible in the code itself — you can see exactly where an error might arise.