Intro to Solidity Smart Contract on Blockchain U-Zyn Chua @uzyn

Smart contract is computer protocol that facilitate, verify, or enforce the negotiation or performance of a contract, or that make a contractual clause unnecessary.

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Smart contract is application logic layer with trustless execution

Further explanations will be done in the “Trust Me I’m an Engineer” fashion – for engineers from an engineer

a single globally highly available virtual machine that executes instructions.

is thus slow, inefficient and expensive

a price to pay for trustless decentralization

high level JavaScript-like language that compiles to Ethereum VM machine code Solidity

Solidity actual contract code of Singapore-based DigixDAO ABI Bytecode

Price to pay for carrying out operation and storage. Gas

Solidity Basics based on http://solidity.readthedocs.io/

State variables are stored permanently on EVM. Data types ● Boolean bool ● Integer int / uint (unsigned) ● Ethereum address address ● Bytes and String bytes / string

mapping (address => uint) likes; function like(address _friend) public { likes[_friend]++; } Functions and Modifiers modifier onlyOwner { if (msg.sender != owner) throw; _; } function makeAdmin(_candidate) public onlyOwner { admin = _candidate; } The rest of the function goes here Looks pretty much like a JS function.

Function Visibilities 1. public a. Visible externally and interally b. Automatically creates accessor. 2. private a. Only visible in current contract 3. internal a. Similar to private, but callable by child contracts. 4. external a. Similar to public, different low level handling. Remember, on blockchain, all data is publicly viewable!

Arrays, Mappings & Structs // mapping - non-iterable (recommended) mapping (address => uint) likes; // array - iterable address[] public friends; // struct struct Person { string name; address wallet; uint lastSeen; }

web3.js Ethereum JavaScript API Because Web 2.0 is so year 2000. Allows interaction with JS (Node.JS and browser)

Ready? Let’s try it out!

http://ethereum.github.io/browser-solidity Browser-solidity web-based solidity compiler Solidity Compiler Make sure it’s not HTTPS

What shall we create?

How about... we print some cash

SGD Token (SGDT) Shh… Don’t tell them 1 SGDT == 1 SGD, always.

Roles 1. Issuer a. Able to issue SGDT. b. SGDT <==> SGD exchanger & guarantor. 2. User a. Able to freely transact SGDT. 3. Authority a. Grant license to issuers.

pragma solidity ^0.4.2; contract SGDT { /* Public variables of the token */ string public name = 'Singapore Dollar Token'; string public symbol = 'SGDT'; uint8 public decimals = 2; uint public totalSupply = 0; address public authority; /* This creates an array with all balances */ mapping (address => uint) public balanceOf; mapping (address => uint) public issuedBy; mapping (address => bool) public isIssuer; } Define state variables

Events and Modifiers event Transfer(address from, address to, uint value); event TokenIssued(address by); modifier onlyIssuer { if (isIssuer[msg.sender] != true) { throw; } _; } modifier onlyAuthority { if (msg.sender != authority) { throw; } _; }

Issuer functions function issue(uint _value) public onlyIssuer { issuedBy[msg.sender] += _value; balanceOf[msg.sender] += _value; TokenIssued(msg.sender, _value); } Can you work on the redeem() function?

User functions /* Send coins */ function transfer(address _to, uint _value) public { if (balanceOf[msg.sender] < _value) { throw; // Check if the sender has enough } balanceOf[msg.sender] -= _value; // Subtract from the sender balanceOf[_to] += _value; // Add the same to the recipient Transfer(msg.sender, _to, _value); // Notify anyone listening that this // transfer took place }

Authority functions function addIssuer(address _newIssuer) public onlyAuthority { isIssuer[_newIssuer] = true; } Can you add removeIssuer() function?

Contract is available at http://bit.ly/SGDT-sol

Deploy and Test Quick test via NodeJS console 1. mkdir sgdt 2. cd sgdt 3. yarn add web3 or npm install web3 4. node const Web3 = require('web3'); const web3 = new Web3( new Web3.providers.HttpProvider( 'http://localhost:8545 ' ) ); > web3 > web3.eth > web3.eth.accounts

Deploy and Test Instantiate contract const address = 'ADDRESS'; const abi = ABI_OBJECT; const SGDT = web3.eth.contract(abi).at(address); > SGDT; > SGDT.authority(); // Read some static values > SGDT.totalSupply(); // BigNumber.js Object > SGDT.totalSupply.toNumber();

Deploy and Test const authority = SGDT.authority(); SGDT.isIssuer(web3.eth.accounts[1]); // false SGDT.addIssuer(web3.eth.accounts[1], { from: authority, gas: 1000000 }); SGDT.isIssuer(web3.eth.accounts[1]); // true Let’s do some magic!

SGDT.balanceOf(web3.eth.accounts[1]).toNumber(); // 0 SGDT.issue(1000, { from: web3.eth.accounts[1], gas: 1000000 }); SGDT.balanceOf(web3.eth.accounts[1]).toNumber(); // 1000 ($10.00) SGDT.balanceOf(web3.eth.accounts[2]).toNumber(); // 0 SGDT.transfer(web3.eth.accounts[2], 499, { from: web3.eth.accounts[1], gas: 1000000, }); SGDT.balanceOf(web3.eth.accounts[2]).toNumber(); // 499 ($4.99) SGDT.balanceOf(web3.eth.accounts[1]).toNumber(); // 501 ($5.01) Print some cash and transact

Feel free to play around with other commands. Questions? Thank you.