Balzac supports assertions. The statement assert exp, where exp is an expression of type boolean, checks that the given expression evaluates to true, otherwise 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 to specify 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 (modelled 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)