# 3.4 谓词 (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:

从Sway的角度来看，谓词是返回一个布尔值的程序，它在执行时表示对某些资源的所有权为真。它们不能访问合约存储。下面是一个简单的谓词，它总是被评估为真：

```
predicate;

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

```

### 调试谓词 (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](https://github.com/FuelLabs/fuel-debugger/pull/1)).

因为它们没有任何副作用（它们是 _纯_ 的），谓词不能创建收据。因此，它们不能有日志记录或创建堆栈来回溯。这意味着除了使用单步调试器（这是一个[正在进行的工作](https://github.com/FuelLabs/fuel-debugger/pull/1)）外，没有原生的方法来调试它们。

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

作为一种变通方法，谓词可以先作为 `脚本 script`编写、测试和调试，然后再改回为 `谓词 predicate`。
