Jack and Jill can’t decide on dinner so they’re bringing in an arbiter to hear their dispute. The problem is they don’t trust the other to independently choose an arbiter. Let’s write an Ethereum contract to handle their dispute! This article assumes you have a basic understanding of Ethereum. If you’re still shaky, start with this by . fantastic article Preethi Kasireddy How Our Contract Will Work Person One creates a new arbitration instance and gives it a description. 🌟 1. Person One and Person Two submit their differing opinions. 😠 2. Person One and Person Two also choose an arbiter. The process doesn’t go forward until both agree on the same account address. 👍 3. The arbiter reviews the two points of view and decide the winner. 🤔 ➡ 🏆 4. Setup Feel free to move to the next section if you’ve run a truffle project before We’ll be using a framework called to develop our smart contract. It’s a one stop shop for handling compilation, testing, deployment, migrations and much more. We’ll only be using certain features in this post but I encourage you to dig deeper. Truffle Before we dive in, globally install truffle and testrpc. npm install -g truffle Then, in a new directory run: truffle init You should see truffle’s folder structure inflate with some simple examples in the folder. contracts/ Get Your Feet Wet Let’s get a feel for the workflow. Kickstart our test client with . You’ll see some of the accounts you’ll be using to simulate the interaction: 1. testrpc In run . This generates a corresponding JSON artifacts from the contracts and drops them into a build folder. 2. separate tab truffle compile Now run to generate files to facilitate contract deployment to the blockchain. 3. truffle migrate Important, testrpc must be running when you migrate, otherwise your contracts won’t deploy to that testing instance. Run which will connect to our running Ethereum client and allow us to interact with our contracts. 4. truffle console Poke Around We’re going to modify one of the existing contracts (technically a … works for our purposes though) . It provides a single method to multiply two numbers. library [ConvertLib.sol](https://github.com/ConsenSys/truffle-webpack-demo/blob/master/contracts/ConvertLib.sol) A method that changes state on the chain is a (costs gas) while any method that only returns data is a (free). transaction call Truffle’s console has and our contracts ready to go, give it a go: web3 _// get the deployed contract by its class name, returns a promise_ > const contract = ConvertLib.deployed() // invoke the 'convert' method with 'call', also returns a promise > const response = contract.then(c => c.convert.call(5, 20)) **> response.then(res => res.toNumber())** // print out our result!! 100 Great, we’ve interacted with a to multiply two numbers! deployed contract Let’s Write Some Solidity Ethereum contracts are written in a high level language called . If you’re familiar with or object oriented you’ll notice a lot of similarities. Solidity Javascript programming With Solidity we’ll define state variables and establish our rules of arbitration. Afterwards we’ll use Truffle to compile the contract and deploy it to our testing environment. The state and code we define in our contract will then live at a specific address on the Ethereum blockchain. Let’s start by cleaning house. Remove and but so we can perform migrations. contracts/ConvertLib.sol contracts/MetaCoin.sol keep contracts/Migrations.sol Then, create a file in the folder called . Now go into your folder and modify : contracts/ Arbitration.sol migrations/ 2_deploy_contracts.js var Arbitration = artifacts.require('./Arbitration.sol'); module.exports = function(deployer) {deployer.deploy(Arbitration);}; Open and begin by defining the solidity version and contract name. contracts/Arbitration.sol You’ll be adding public variables and as well as the transaction method to set the description. description winningOpinion setDescription pragma solidity ^0.4.11; contract Arbitration {string public description;string public winningOpinion; function setDescription(string _description) {description = _description;}} Next use the type to define how our claimants’ data should look. Then, we’ll use that type we made to declare an array called . Finally, create a method that adds a new claimant. [struct](http://solidity.readthedocs.io/en/develop/types.html#structs) Claimant claimants contract Arbitration {string public description; struct Claimant {string opinion;address addr;address arbiter;} function setDescription(string _description) {description = _description;} Claimant[] public claimants; } function addClaimant(string opinion, address arbiter) {if (claimants.length < 2) {claimants.push(Claimant(opinion, msg.sender, arbiter));} } One thing to note, will give us the address of the account sending the transaction. has other that it passes along as well. msg.sender addClaimant msg valuable info After claimants have added their proposals, the arbiter will be able to review the disagreement by calling . getProposals contract Arbitration { ... function getProposals()returns (string _description,string _proposalOne,string _proposalTwo){require(claimants.length == 2);return (description,claimants[0].opinion,claimants[1].opinion);} } In the function declaration you’ll see that we define what will be returned with the keyword. We use to allow the code to continue only if two claimants have submitted. returns require Next, we’ll create a method that allows to choose a winner. First we’ll create a helper method that will use. cycles through the claimants and makes sure that the sender’s address matches what each claimant has selected. If require doesn’t throw an error, it then sets the contract’s state variable . only the arbiter verifyArbiter selectWinner verifyArbiter winningOpinion contract Arbitration { ... function verifyArbiter() returns (bool isArbiter) {for (uint i = 0; i < claimants.length; i++) {if (msg.sender != claimants[i].arbiter) {return false;}} **return true; }** function selectWinner(uint proposalNumber) {require(verifyArbiter()); **winningOpinion = claimants\[proposalNumber\].opinion; }** } Interacting With the Contract Open terminal and run . Then, in a new tab run , and as we did in the section above. Let’s start by getting a reference to our contract and adding a description to it: testrpc truffle compile truffle migrate truffle console > const contract = Arbitration.deployed() > _''_> contract.then(c => c.description()) contract.then(c => c.setDescription('What to eat for dinner')) > contract.then(c => c.description()) 'What to eat for dinner' As you can see, we can access state by calling the variable name like a function: . description() Now, go back to your testrpc output and look at the ten addresses it gives you. Set to any of the addresses in the list other than the first because truffle by default uses the first account as the sender. _ARBITER_ADDRESS_ > contract.then(c => c.addClaimant('burgers', ' ')) ARBITER_ADDRESS > contract.then(c => c.claimants(0)) [ 'burgers','FIRST_ADDRESS_LISTED','ARBITER_ADDRESS' ] To add the second claimant, we’ll modify how we call the addClaimant method. The last parameter will take another address from the testrpc account list and simulate sending a call from there. > contract.then(c => c.addClaimant('pizza', ' ', {from: ' '})) ARBITER_ADDRESS SECOND_CLAIMANT_ADDRESS > contract.then(c => c.claimants(1)) [ 'pizza','SECOND_CLAIMANT_ADDRESS','ARBITER_ADDRESS' ] Now imagine the arbiter would like to see the current state of the proposals. Assuming both claimants have voted (otherwise it will throw an error) they’ll get a response like: > **contract.then(c => c.getProposals.call())** [ 'What to eat for dinner', 'burgers', 'pizza' ] Using the arbiter’s address, let’s select a winner using either 0 for burgers or 1 for pizza. > contract.then(c => c.selectWinner(1, {from: ' '})) ARBITER_ADDRESS After the arbiter has selected a winner, anyone can consult the contract for the winner. > **contract.then(c => c.winningOpinion())** 'pizza' We knew pizza was going to win but writing this contract was nonetheless a fruitful exercise in understanding Ethereum. Check out the code ! here

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