Solidity in 5 Minutes Creating Decentralized Apps on Ethereum Blockchain U-Zyn Chua 
 @uzyn JSConf.Asia 2016

@uzyn Smart contracts are computer protocols that facilitate, verify, or enforce the negotiation or performance of a contract, or that make a contractual clause unnecessary. @uzyn

@uzyn Smart contracts are computer protocols that facilitate, verify, or enforce the negotiation or performance of a contract, or that make a contractual clause unnecessary.

@uzyn Smart contract is 
 application logic
 with trustless execution

@uzyn a single globally highly available 
 virtual machine that
 executes instructions.

@uzyn is thus slow, inefficient and expensive

@uzyn a price to pay for trustless decentralization

@uzyn open execution open source

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

@uzyn Solidity actual contract code of Singapore-based DigixDAO ABI Bytecode

@uzyn Price to pay for carrying out operation and storage. Gas

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

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

@uzyn 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.

@uzyn Function Visibilities 1.public a. Visible externally and interally b. Automatically creates accessor. 2.private a. Only visible in current contract Remember, on blockchain, all data is publicly viewable! also internal and external.

@uzyn 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; } Hash Table

@uzyn Ready?
 Let’s try it out!

@uzyn What shall we create?

@uzyn How about... we print some cash

@uzyn Code is available at http://bit.ly/SGDT-sol

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

@uzyn Roles 1. Issuer a. Able to issue SGDT, after putting up equivalent SGD collateral. b. SGDT <==> SGD exchanger & guarantor. 2. User a. Able to freely transact SGDT. 3.Authority a. Grant license to issuers. http://bit.ly/SGDT-sol

@uzyn pragma solidity ^0.4.2; contract SGDT { /* Public variables of the token */ 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 http://bit.ly/SGDT-sol

@uzyn Modifiers modifier onlyIssuer { if (isIssuer[msg.sender] != true) { throw; } _; } modifier onlyAuthority { if (msg.sender != authority) { throw; } _; } http://bit.ly/SGDT-sol

@uzyn Events event Transfer(address from, address to, uint value); event TokenIssued(address by); http://bit.ly/SGDT-sol Notify your JS daemon 
 on contract updates

@uzyn Issuer functions function issue(uint _value) public onlyIssuer { issuedBy[msg.sender] += _value; balanceOf[msg.sender] += _value; TokenIssued(msg.sender, _value); // Event } Can you work on the redeem() function? http://bit.ly/SGDT-sol

@uzyn User functions // Transfer SGDT 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 // Notify anyone listening that this transfer took place Transfer(msg.sender, _to, _value); } http://bit.ly/SGDT-sol

@uzyn Authority functions function addIssuer(address _newIssuer) public onlyAuthority { isIssuer[_newIssuer] = true; } Can you add removeIssuer() function? http://bit.ly/SGDT-sol

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

@uzyn Interface with Smart Contract Ethereum node web3.js Frontend JS Node.JS

@uzyn Ethereum node web3.js Frontend JS Node.JS Dapp Dapp! Interface with Smart Contract

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

@uzyn Smart Contract & JS code more examples available at http://bit.ly/SGDT-sol

@uzyn Say hi! Twitter · GitHub
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