Introduction to Solidity

Introduction to programming smart contract on Ethereum with Solidity.

Presented at a sharing session at GovTech on 15 November 2016.

Companion code can be downloaded at http://bit.ly/SGDT-sol

Transcript

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

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2. 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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4. Smart contract is
   application logic layer
   with trustless execution
   

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

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6. a single globally highly available
   virtual machine that
   executes instructions.
   

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7. is thus
   slow, inefficient and expensive
   

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8. a price to pay for
   trustless decentralization
   

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

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10. Solidity
    actual contract code of Singapore-based DigixDAO
    ABI
    Bytecode
    

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11. Price to pay for carrying out operation
    and storage.
    Gas
    

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

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

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14. 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.
    

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15. 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!
    

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16. 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;
    }
    

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

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18. Ready?
    Let’s try it out!
    

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

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20. What shall we create?
    

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21. How about...
    we print some cash
    

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

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23. 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.
    

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24. 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
    

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25. 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;
    }
    _;
    }
    

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26. 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?
    

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27. 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
    }
    

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

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

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30. 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
    

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31. 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();
    

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32. 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!
    

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33. 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
    

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

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