From the perspective of Sway, predicates are programs that return a Boolean value and which represent ownership of some resource upon execution to true. They have no access to contract storage. Here is a trivial predicate, which always evaluates to true:


// All predicates require a main function which returns a Boolean value.
fn main() -> bool {

The address of this predicate is 0xd19a5fe4cb9baf41ad9813f1a6fef551107c8e8e3f499a6e32bccbb954a74764. Any assets sent to this address can be unlocked or claimed by executing the predicate above as it always evaluates to true.

It does not need to be deployed to a blockchain because it only exists during a transaction. That being said, the predicate address is on-chain as the owner of one or more UTXOs.

Transfer Coins to a Predicate

In Fuel, coins can be sent to a predicate's address(the bytecode root, calculated here).

Spending Predicate Coins

The coin UTXOs become spendable not on the provision of a valid signature, but rather if the supplied predicate both has a root that matches their owner, and evaluates to true.

If a predicate reverts, or tries to access impure VM opcodes, the evaluation is automatically false.

An analogy for predicates is rather than a traditional 12 or 24 word seed phrase that generates a private key and creates a valid signature, a predicate's code can be viewed as the private key. Anyone with the code may execute a predicate, but only when the predicate evaluates to true may the assets owned by that address be released.

Spending Conditions

Predicates may introspect the transaction spending their coins (inputs, outputs, script bytecode, etc.) and may take runtime arguments, either or both of which may affect the evaluation of the predicate.

It is important to note that predicates cannot read or write memory. They may however check the inputs and outputs of a transaction. For example in the OTC Predicate Swap Example, a user may specify they would like to swap asset1 for asset2 and with amount of 5. The user would then send asset1 to the predicate. Only when the predicate can verify that the outputs include 5 coins of asset2 being sent to the original user, may asset1 be transferred out of the predicate.

Debugging Predicates

Because they don't have any side effects (they are pure), predicates cannot create receipts. Therefore, they cannot have logging or create a stack backtrace. This means that there is no native way to debug them aside from using a single-stepping debugger.

As a workaround, the predicate can be written, tested, and debugged first as a script, and then changed back into a predicate.