Ethereum Overview and what are Smart Contracts and what are their limitations. A Solidity programming language Crash Course. How to create an ERC20 token. What is an ICO on Ethereum.

1. PROGRAMMING SMART CONTRACTS WITH
SOLIDITY
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2. EMANUEL MOTA - @EMOTA7
Founder of Yari Labs
emanuel@yarilabs.com
@yarilabs
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3. ABOUT THE TALK
Intro
Ethereum Overview
Smart Contracts
Solidity Crash Course
What is an ERC20 token
ICO on Ethereum
Questions
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4. #BRAGABLOCKCHAIN
TELEGRAM -HTTP://T.ME/BRAGABLOCKCHAIN
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5. SOME STATS ABOUT THE AUDIENCE
Developers: 70% of the audience
Knowledge of Solidity programming language?
Never heard of it before - 53%
I'm a Beginner - 26%
I've tryed it a couple of times - 16%
I'm an Intermediate user - 5%
I'm a Solidity Guru - 0%
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6. Ethereum Network
“Each node of the Ethereum
network hosts a blockchain database
and a node client capable of
executing application code stored on
blockchain. Nodes communicate
through Wire protocol and expose
same interface but can be
implemented in different languages.”
Excerpt From: Roberto Infante. “Building Ethereum ÐApps”
ETHEREUM
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7. ETHEREUM
Bitcoin like distributed ledger
Cryptocurrency (Ether)
Ethereum Virtual Machine (EVM)
Turing complete
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8. ETHEREUM
Ethereum Blockchain
Contracts (code)
Storage
Logs
Events
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9. ETHEREUM
Two kinds of accounts
External Accounts (wallets controlled by humans)
Contract Accounts (controlled by code)
every account has a balance
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10. ETHEREUM
Code execution costs GAS
Transaction is a message sent from one account to
another and can have a data payload
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11. SMART CONTRACTS
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12. SMART CONTRACTS
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13. SMART CONTRACTS
Contract = code (i.e. functions) + data (i.e. state) and
resides on the blockchain
EVM is the runtime for Smart Contracts on
Ethereum
Accounts have a persistent memory area which is
called storage
Contracts can neither read nor write to any storage
apart from their own
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14. SMART CONTRACTS
Contracts can call other contracts
1024 max call stack depth
Support Events
Contracts can purge themselves from the blockchain
(OPCODE selfdestruct)
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15. SOLIDITY PROGRAMMING LANGUAGE
https://solidity.readthedocs.io/
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16. SOLIDITY
Solidity is a statically typed, contract programming
language that has similarities to Javascript, Java and C.
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17. SOLIDITY
Has some contract-speci c features like:
modi er (guard) clauses
event noti ers for listeners
custom global variables.
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18. SOLIDITY
Hello World
version pragma — declares the version of the compiler to be used to avoid breaking changes
introduced on future versions
pragma solidity ^0.4.19;
contract HelloWorld {
}
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19. SOLIDITY
Statically typed language
uint data type is an unsigned integer (non-
negative number)
int data type is used for signed integers
uint has 256 bits we can also have uint8, uint16,
uint32
contract Example {
// This will be stored permanently in the blockchain
uint myUnsignedInteger = 100;
string name;
}
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20. SOLIDITY
More complex data types - Structs
struct TokenHolder {
uint age;
string obs;
}
// Arrays
string[5] stringArray;
// a dynamic Array - has no fixed size, can keep growing:
uint[] dynamicArray;
// Public array
TokenHolder[] public shareHolders;
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21. SOLIDITY
Mappings and data type address
rst example, the key is an address and the value is a uint
second example, the key is a uint and the value is a string
// For a financial app, storing a uint that holds the user's account balance:
mapping (address => uint) public accountBalance;
// Or could be used to store / lookup usernames based on userId
mapping (uint => string) userIdToName;
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22. SOLIDITY
Function declarations
uint[] scores;
function addNewScore(string _clientId, uint _score) public {
...
_updateScores(_score);
}
function _updatesScores(string _clientId, uint _number) private {
...
scores.push(_number) {
...
}
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23. SOLIDITY
More about functions
string greeting = "Whazaaa ?";
function sayHello() public returns (string) {
return greeting;
}
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24. SOLIDITY
Function Modi ers
function sayHello() public view returns (string) {
function _multiply(uint a, uint b) private pure returns (uint) {
return a * b;
}
// Functions can return many arguments, and by specifying returned arguments
// names we don't need to explicitly return
function increment(uint x, uint y) returns (uint x, uint y) {
x += 1;
y += 1;
}
// when a function returns multiple values we need to parallel assign
(x1, y1) = increment(1,2);
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25. SOLIDITY
More on functions Modi ers
modifier onlyAfter(uint _time) { require (now >= _time); _; }
modifier onlyOwner { require(msg.sender == owner) _; }
// Append right after function declaration
function changeOwner(newOwner) onlyAfter(someTime) onlyOwner() {
owner = newOwner;
}
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26. SOLIDITY
Payable function Modi er
// All functions that receive ether must be marked 'payable'
function depositEther() public payable {
balances[msg.sender] += msg.value;
}
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27. SOLIDITY
Events
Events are a way for a contract to communicate that something happened on the blockchain
to a front-end client that is 'listening' for events
A javascript implementation would look something like:
// declare the event
event IntegersAdded(uint x, uint y, uint result);
function add(uint _x, uint _y) public returns(uint) {
uint result = _x + _y;
// fire an event to let the app know the function was called:
IntegersAdded(_x, _y, result);
return result;
}
YourContract.IntegersAdded(function(error, result) {
// do something with result
}
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28. SOLIDITY
Important global variables
this; // address of contract
this.balance; // often used at end of contract life to transfer balance
// ** msg - Current message received by the contract ** **
msg.sender; // address of sender
msg.value; // amount of eth sent to contract (in wei) function should be "payabl
msg.data; // bytes, complete call data
msg.gas; // remaining gas
now; // current time (approximately) - uses Unix time
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29. IMPORTANT DESIGN NOTES
Obfuscation: All variables are publicly viewable on
blockchain, so anything that is private needs to be
obfuscated
Storage optimization: Writing to blockchain is
expensive, as data is stored forever
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30. IMPORTANT DESIGN NOTES
Cron Job: Contracts must be manually called to
handle time-based scheduling
Cost of Gas: the fuel Ethereum DApps run on
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31. SOME USEFULL LINKS
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32. SOME USEFULL LINKS
Ethereum website
Online compiler
Another online compiler
Online tools
(block explorer, tx submit)
EthFiddle
https://www.ethereum.org/
https://remix.ethereum.org/
https://etherchain.org/solc
https://testnet.etherscan.io
https://etherscan.io
https://eth ddle.com/
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33. SOME USEFULL LINKS
(so you don’t have to run your own
node/s)
Truf e .
Embark
Open Zeppelin
https://etherchain.org/
http://infura.io
https://github.com/ConsenSys/truf e
https://github.com/iurimatias/embark-
framework
https://openzeppelin.org/
https://metamask.io/
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34. WHAT IS AN ERC20 TOKEN?
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35. ERC20 TOKEN
ERC stands for Ethereum Request for Comments
contract MyToken {
/* This creates an array with all balances */
mapping (address => uint256) public balanceOf;
/* Initializes contract with initial supply tokens to the creator */
function MyToken(
uint256 initialSupply
) {
// Give the creator all initial tokens
balanceOf[msg.sender] = initialSupply;
}
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36. ERC20 TOKEN (CONTINUATION)
(from ethereum.org)
/* Send coins */
function transfer(address _to, uint256 _value) {
// Check if the sender has enough
require(balanceOf[msg.sender] >= _value);
// Check for overflows
require(balanceOf[_to] + _value >= balanceOf[_to]);
balanceOf[msg.sender] -= _value;
balanceOf[_to] += _value;
}
}
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37. ERC20 TOKEN
An ERC20 token implements the following API
name
symbol
decimals
transfer(to, value)
transferFrom(from, to, value)
approve(spender, value)
approveAndCall(spender, value, extraData)
burn(value)
burnFrom(from, value)
plus trigger a set of events
a complete spec of a ERC20 Token check andhttps://ethereum.org/token
https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20.md
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38. ERC20 TOKEN
contract ERC20Interface {
function totalSupply() public constant returns (uint);
function balanceOf(address tokenOwner)
public constant returns (uint balance);
function allowance(address tokenOwner, address spender)
public constant returns (uint remaining);
function transfer(address to, uint tokens)
public returns (bool success);
function approve(address spender, uint tokens)
public returns (bool success);
function transferFrom(address from, address to, uint tokens)
public returns (bool success);
event Transfer(address indexed from, address indexed to, uint tokens);
event Approval(address indexed tokenOwner, address indexed spender, uint toke
}
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39. HOW TO MAKE AN ICO (CROWDSALE)
CONTRACT
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40. WHAT REALLY IS AN ICO ?
An ICO - Initial Coin Offer should really be called:
token generation event (TGE) as ICO is a term
initially coined for currencies and now its being used
for selling ERC20 (ethereum tokens).
And ICO should be done automatically via a
SmartContract that implements the rules of the
token sale.
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41. ICO
To buy tokens from a smart contract participants
should transfer ethereum directly from their wallets to
the smart contract address so that the smartcontract
assigns them the tokens automatically.
To see an example of a crowdsale contract check:
https://ethereum.org/crowdsale
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42. NON-FUNGIBLE TOKENS ERC-721
https://www.cryptokitties.co/marketplace
https://cryptozombies.io
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43. QUESTIONS ?
EMANUEL MOTA
@YARILABS
emanuel@yarilabs.com
twitter: @emota7
github: emanuel
HTTP://YARILABS.COM
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44. HANDS ON IN THE AFTERNOON :)
Prepare your rinkeby testnet wallets!
And join BragaBlockchain telegram
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