Parables provides the necessary speed to perform property testing of smart contracts.

We make use of the excellent proptest framework to accomplish this.

Let's rewrite our example from the last chapter to instead of testing that we can get and set some well-defined numeric values, we test a wide range of values.

# #![allow(unused_variables)]
#fn main() {
tests.test("get and increment value randomly", pt!{
    |(x in any::<u64>())| {
        let x = U256::from(x);

        let evm = evm.get()?;
        let contract = simple_contract::contract(&evm, simple, call);

        let out = contract.get_value()?.output;
        assert_eq!(U256::from(0), out);

        contract.set_value(x)?;
        let out = contract.get_value()?.output;
        assert_eq!(x, out);
    }
});
#}

For the heck of it, let's introduce a require that will prevent us from setting the field to a value larger or equal to 1000000.

function setValue(uint update) public ownerOnly() {
    require(value < 1000000);
    value = update;
    emit ValueUpdated(value);
}

What does our test case say?

cargo run

get and increment value randomly in 0.144s: failed at src/main.rs:36:9
Test failed: call was reverted; minimal failing input: x = 1000000
        successes: 0
        local rejects: 0
        global rejects: 0

What's happening here is actually quite remarkable. When proptest notices a failing, random, input, it tries to reduce the value to minimal failing test. The exact strategy is determined by the type being mutated, but for numeric values it performs a binary search through all the inputs.

For more information on property testing, please read the proptest README.

In the next section we will discuss how to expect that a transaction is reverted.