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Predicates /
Instantiating a Predicate

Icon LinkInstantiating predicates

A predicate in Sway can be as simple as the following: 

predicate;
 
fn main() -> bool {
    true
}

In this minimal example, the main function does not accept any parameters and simply returns true.

Just like contracts in Sway, once you've created a predicate, you can compile it using forc build. For more information on working with Sway, refer to the Sway documentation.

After compiling, you will obtain the binary of the predicate and its JSON ABI (Application Binary Interface). Using these, you can instantiate a predicate in TypeScript as shown in the code snippet below: 

import { Provider } from 'fuels';
 
import { LOCAL_NETWORK_URL } from '../../env';
import { ReturnTruePredicate } from '../../typegend';
 
const provider = await Provider.create(LOCAL_NETWORK_URL);
 
const predicate = new ReturnTruePredicate({
  provider,
});

The created Predicate instance, among other things, has three important properties: the predicate bytes (byte code), the chainId, and the predicate address.

This address, generated from the byte code, corresponds to the Pay-to-Script-Hash (P2SH) address used in Bitcoin.

Icon LinkPredicate with multiple arguments

You can pass more than one argument to a predicate. For example, this is a predicate that evaluates to true if the two arguments are not equal: 

predicate;
 
fn main(arg1: u64, arg2: u64) -> bool {
    return arg1 != arg2;
}

You can pass the two arguments to this predicate like this: 

import { Provider } from 'fuels';
 
import { LOCAL_NETWORK_URL } from '../../env';
import { PredicateMultiArgs } from '../../typegend';
 
const provider = await Provider.create(LOCAL_NETWORK_URL);
 
const predicate = new PredicateMultiArgs({ provider, data: [20, 30] });

Icon LinkPredicate with a Struct argument

You can also pass a struct as an argument to a predicate. This is one such predicate that expects a struct as an argument: 

predicate;
 
struct Validation {
    has_account: bool,
    total_complete: u64,
}
 
fn main(received: Validation) -> bool {
    let expected_has_account: bool = true;
    let expected_total_complete: u64 = 100;
 
    received.has_account == expected_has_account && received.total_complete == expected_total_complete
}

You can pass a struct as an argument to this predicate like this: 

import { Provider } from 'fuels';
 
import { LOCAL_NETWORK_URL } from '../../env';
import { PredicateMainArgsStruct } from '../../typegend';
 
const provider = await Provider.create(LOCAL_NETWORK_URL);
 
const predicate = new PredicateMainArgsStruct({
  provider,
  data: [{ has_account: true, total_complete: 100 }],
});