Assertions

Balzac supports assertions. The statement assert exp, where exp is an expression of type boolean, checks that the given expression evaluates to true. If the expression evaluates to false, it throws an error.

const a = 42

assert a == 42  // ok
assert a != 0   // ok
assert a == 1   // Assertion error

For example, we can check that the derived address and pubkey from k are equal to some literals.

const k = key:cSyt51FL53tLuG9TGSLxtTapDWquwT6XN6NSbqh6PdquYXCmscXN

assert k.toAddress == address:moytPifb6CmkaPZZ5c85HN553fwsgVro8M
assert k.toPubkey == pubkey:02a599b9400ff0ac9f2eb37c444162d5296e78acefc8e7d216581c16916668396b

A variant of the assert statement allows specifying an error message. The syntax is assert exp : msg, where msg has type string.

The following example checks that the ID of the transaction T corresponds to a given literal hash. This is useful when we publish a transaction on the blockchain and we want to be sure that our transaction (modeled in Balzac) corresponds to the real one.

const kA = key:cSFqKAaCUGHZoGDrcreo3saZpMv9NvcVmRZVbVddbodEuzWNCDNt
const kB = key:cSyt51FL53tLuG9TGSLxtTapDWquwT6XN6NSbqh6PdquYXCmscXN

transaction T_deposit {
    input = _
    output = 1BTC: fun(x). versig(kA; x)
}

assert T.txid == hash:df2ad5bdc765e7c0be1a6964f67c71d9be11e635cfeadc6920ff0973433dc464
       : "Found: " + T.txid

transaction T {
        input = T_deposit: sig(kA)
        output = this.input.value : fun(x). versig(kB; x)
}