Ethereum contracts are small pieces of code that can be deployed to the Ethereum blockchain to execute autonomously. They are written in the Solidity language and can store and transfer value and data. To develop contracts, they must be compiled from Solidity to bytecode, deployed to the blockchain, and can then be interacted with by users via transactions that call functions defined in the contract's application binary interface (ABI). Examples of suitable contracts include tokens, crowdfunding, voting systems, and decentralized autonomous organizations.

1. Ethereum Contracts
Coinfest 2015 - Decentral Vancouver
Rob Myers - https://robmyers.org

2. ÐApps
Distributed Applications
● Ethereum - Smart contracts
● Whisper - Ephemeral communication
● Swarm - DHT storage

3. How Will We Use ÐApps?

4. How Will We Develop ÐApps?

5. What Is A Contract?
● A small piece of code.
● A self-contained piece of code.
● A small amount of data.
● A public body of data.
● A small amount of Ether.

6. If You Know MVC...

7. What Is A Transaction?
● How we communicate with contracts.
● Data and Ether can be sent via transactions.
● Sent from the JS API or other contracts.
● Transactions cost "gas", the sender pays.
● JavaScript API can use accessors instead.

8. Writing Contracts
● Write contracts in the Solidity language.
● Create the UI for contracts in HTML/JS.
● AlethZero is a good place to start.
● solc / eth / Chrome is the cutting edge.
● Mix IDE & Mist browser are the future.

9. Bad Ideas For Contracts
● Doom on the EVM (too much processing).
● Video on the blockchain (too much storage).
● Ethereum web server (no network access).
● Blockchain DRM (blockchain is public).
● Anything that uses too much processor time,
storage, or secrecy.

10. Good Ideas For Contracts
● Altcoins, tokens, assets.
● Crowdfunding, fan incentive schemes.
● Voting systems, prediction markets, lotteries.
● Access control - sites, games, doors, cars.
● DAOs - Organizations on the blockchain.
● Use your imagination! :-)

11. A Solidity Contract
contract Coin {
address minter;
mapping (address => uint) balances;
function Coin() {
minter = msg.sender;
}
function mint(address owner, uint amount) {
if (msg.sender != minter) return;
balances[owner] += amount;
}
function send(address receiver, uint amount) {
if (balances[msg.sender] < amount) return;
balances[msg.sender] -= amount;
balances[receiver] += amount;

12. Compiling The Contract
$ solc --binary file --json-abi file Coin.sol

13. The Compiled Contract
60056013565b6101e0806100216000396000f35b336000819060000155505b56006000357c010000
00000000000000000000000000000000000000000000000000009004806337f42841146100455780
6340c10f191461005a578063d0679d341461006e57005b6100506004356101a8565b806000526020
6000f35b610068600435602435610082565b60006000f35b61007c6004356024356100fd565b6000
6000f35b60005473ffffffffffffffffffffffffffffffffffffffff163373ffffffffffffffffff
ffffffffffffffffffffff1614156100bd576100c2565b6100f9565b80600160008473ffffffffff
ffffffffffffffffffffffffffffff16815260200190815260200160002090815401908190600001
55505b5050565b80600160003373ffffffffffffffffffffffffffffffffffffffff168152602001
908152602001600020541061013257610137565b6101a4565b80600160003373ffffffffffffffff
ffffffffffffffffffffffff16815260200190815260200160002090815403908190600001555080
600160008473ffffffffffffffffffffffffffffffffffffffff1681526020019081526020016000
209081540190819060000155505b5050565b6000600160008373ffffffffffffffffffffffffffff
ffffffffffff1681526020019081526020016000205490506101db565b91905056

14. The Contract's ABI
[
{
"constant" : true,
"inputs" : [
{
"name" : "addr",
"type" : "address"
}
],
"name" : "queryBalance",
"outputs" : [
{
"name" : "balance",
"type" : "uint256”,

15. Call The Contract From JavaScript
<script type="text/javascript" src="../ext/bignumber.min.js"></script>
<script type="text/javascript" src="../ext/ethereum.js/dist/ethereum.js"></script>
...
var web3 = require('web3');
web3.setProvider(new web3.providers.HttpSyncProvider('http://localhost:8545'));
var contract_address = "0xaf1206fcb32fbdb42878c429e61a49d5143e6f32";
var contract_abi = ... // see previous slide
var contract = web3.eth.contract(contract_address, contract_abi);
...
var accountBalance = parseInt(contract.call().queryBalance(queryAddress));
contract.call().send("0xaf1206fcb32fbdb42878c429e61a49d5143e6f32", accountBalance / 100);

16. Try It Yourself
Online compiler:
https://chriseth.github.io/cpp-ethereum/
Example ÐApps:
https://github.com/ethereum/dapp-bin

17. Where To Learn More
The Wiki:
https://github.com/ethereum/wiki/wiki
The Forum:
https://forum.ethereum.org/

18. Install Party!
AlethZero / solc / eth / mix:
https://github.com/ethereum/cpp-ethereum/wiki
Mist:
https://github.com/ethereum/go-
ethereum/wiki/Building-Ethereum%28Go%29