Predicates

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:

predicate;

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

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 (which is a work-in-progress).

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