Contracts

Some quick examples for techniques with Contracts.

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Return a Value from a State-Changing Method

Since a state-changing (non-constant) function can take a non-trivial amount of time to mine, the response is a transaction, and cannot return values directly.

Using events, we can simulate a return value from a non-constant function.

Solidity

contract Example {
    event Return(uint256);

    uint256 _accum = 0;

    function increment() returns (uint256 sum) {
        _accum++;
        Returns(_accum);
    }
}

JavaScript

const assert = require('assert')

const {
    Contract,
    Wallet,
    getDefaultProvider
} = require('ethers')

const provider = getDefaultProvider('ropsten')

const wallet = new Wallet(privateKey, provider)

const abi = [
    "event Return(uint256)",
    "function increment() returns (uint256 sum)"
]

const contractAddress = "0x..."

const contract = new Contract(contractAddress, abi)

async function increment() {

    // Call the contract, getting back the transaction
    let tx = await contract.increment()

    // Wait for the transaction to have 2 confirmations.
    // See the note below on "Economic Value" for considerations
    // regarding the number of suggested confirmations
    let receipt = await tx.wait(2)

    // The receipt will have an "events" Array, which will have
    // the emitted event from the Contract. The "Return(uint256)"
    // call is the last event.
    let sumEvent = receipt.events.pop()

    // Not necessary; these are just for the purpose of this
    // example
    assert.equal(sumEvent.event, 'Return')
    assert.equal(sumEvent.eventSignature, 'Return(uint256)')

    // The sum is the first (and in this case only) parameter
    // in the "Return(uint256 sum)" event
    let sum = sumEvent.args[0]

    return sum
}

increment.then((value) => {
    console.log(value);
});

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Economic Incentives and Economic Value

A very important aspect of Smart Contracts is consideration of the Economic Value being protected; even a completely logically correct Smart Contract can fall victim to misaligned economic incentives.

Imagine a city with an average parking ticket cost of $60, but the parking enforcement is not effective, allowing a person to park anytime and anywhere with only an average of 3 tickets per year. If the average cost to pay for parking is $10, and you park 100 times per year, the $180 in fines compared to the $1,000 in paying for parking is actually incentivizing illegal parking and disincentivizing paying legitimate parking.

This is a fairly involved topic, which we will write an article for and then link to from here, along with a related topic, “Miner Front-Running”.

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