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diff --git a/solidity.html.markdown b/solidity.html.markdown new file mode 100644 index 00000000..9bf5bf4d --- /dev/null +++ b/solidity.html.markdown @@ -0,0 +1,856 @@ +--- +language: Solidity +filename: learnSolidity.sol +contributors: + - ["Nemil Dalal", "https://www.nemil.com"] + - ["Joseph Chow", ""] +--- + +Solidity lets you program on [Ethereum](https://www.ethereum.org/), a +blockchain-based virtual machine that allows the creation and +execution of smart contracts, without requiring centralized or trusted parties. + +Solidity is a statically typed, contract programming language that has +similarities to Javascript and C. Like objects in OOP, each contract contains +state variables, functions, and common data types. Contract-specific features +include modifier (guard) clauses, event notifiers for listeners, and custom +global variables. + +Some Ethereum contract examples include crowdfunding, voting, and blind auctions. + +There is a high risk and high cost of errors in Solidity code, so you must be very careful to test +and slowly rollout. WITH THE RAPID CHANGES IN ETHEREUM, THIS DOCUMENT IS UNLIKELY TO STAY UP TO +DATE, SO YOU SHOULD FOLLOW THE SOLIDITY CHAT ROOM AND ETHEREUM BLOG FOR THE LATEST. ALL CODE HERE IS +PROVIDED AS IS, WITH SUBSTANTIAL RISK OF ERRORS OR DEPRECATED CODE PATTERNS. + +Unlike other code, you may also need to add in design patterns like pausing, deprecation, and +throttling usage to reduce risk. This document primarily discusses syntax, and so excludes many +popular design patterns. + +As Solidity and Ethereum are under active development, experimental or beta +features are typically marked, and subject to change. Pull requests welcome. + +```javascript +// First, a simple Bank contract +// Allows deposits, withdrawals, and balance checks + +// simple_bank.sol (note .sol extension) +/* **** START EXAMPLE **** */ + +// Start with Natspec comment (the three slashes) +// used for documentation - and as descriptive data for UI elements/actions + +/// @title SimpleBank +/// @author nemild + +/* 'contract' has similarities to 'class' in other languages (class variables, +inheritance, etc.) */ +contract SimpleBank { // CamelCase + // Declare state variables outside function, persist through life of contract + + // dictionary that maps addresses to balances + // always be careful about overflow attacks with numbers + mapping (address => uint) private balances; + + // "private" means that other contracts can't directly query balances + // but data is still viewable to other parties on blockchain + + address public owner; + // 'public' makes externally readable (not writeable) by users or contracts + + // Events - publicize actions to external listeners + event LogDepositMade(address accountAddress, uint amount); + + // Constructor, can receive one or many variables here; only one allowed + function AcmeBank() { + // msg provides details about the message that's sent to the contract + // msg.sender is contract caller (address of contract creator) + owner = msg.sender; + } + + /// @notice Deposit ether into bank + /// @return The balance of the user after the deposit is made + function deposit() public returns (uint) { + balances[msg.sender] += msg.value; + // no "this." or "self." required with state variable + // all values set to data type's initial value by default + + LogDepositMade(msg.sender, msg.value); // fire event + + return balances[msg.sender]; + } + + /// @notice Withdraw ether from bank + /// @dev This does not return any excess ether sent to it + /// @param withdrawAmount amount you want to withdraw + /// @return The balance remaining for the user + function withdraw(uint withdrawAmount) public returns (uint remainingBal) { + if(balances[msg.sender] >= withdrawAmount) { + // Note the way we deduct the balance right away, before sending - due to + // the risk of a recursive call that allows the caller to request an amount greater + // than their balance + balances[msg.sender] -= withdrawAmount; + + if (!msg.sender.send(withdrawAmount)) { + // increment back only on fail, as may be sending to contract that + // has overridden 'send' on the receipt end + balances[msg.sender] += withdrawAmount; + } + } + + return balances[msg.sender]; + } + + /// @notice Get balance + /// @return The balance of the user + // 'constant' prevents function from editing state variables; + // allows function to run locally/off blockchain + function balance() constant returns (uint) { + return balances[msg.sender]; + } + + // Fallback function - Called if other functions don't match call or + // sent ether without data + // Typically, called when invalid data is sent + // Added so ether sent to this contract is reverted if the contract fails + // otherwise, the sender's money is transferred to contract + function () { + throw; // throw reverts state to before call + } +} +// ** END EXAMPLE ** + + +// Now, the basics of Solidity + +// 1. DATA TYPES AND ASSOCIATED METHODS +// uint used for currency amount (there are no doubles +// or floats) and for dates (in unix time) +uint x; + +// int of 256 bits, cannot be changed after instantiation +int constant a = 8; +int256 constant a = 8; // same effect as line above, here the 256 is explicit +uint constant VERSION_ID = 0x123A1; // A hex constant +// with 'constant', compiler replaces each occurrence with actual value + + +// For int and uint, can explicitly set space in steps of 8 up to 256 +// e.g., int8, int16, int24 +uint8 b; +int64 c; +uint248 e; + +// Be careful that you don't overflow, and protect against attacks that do + +// No random functions built in, use other contracts for randomness + +// Type casting +int x = int(b); + +bool b = true; // or do 'var b = true;' for inferred typing + +// Addresses - holds 20 byte/160 bit Ethereum addresses +// No arithmetic allowed +address public owner; + +// Types of accounts: +// Contract account: address set on create (func of creator address, num transactions sent) +// External Account: (person/external entity): address created from public key + +// Add 'public' field to indicate publicly/externally accessible +// a getter is automatically created, but NOT a setter + +// All addresses can be sent ether +owner.send(SOME_BALANCE); // returns false on failure +if (owner.send) {} // REMEMBER: wrap in 'if', as contract addresses have +// functions executed on send and these can fail +// Also, make sure to deduct balances BEFORE attempting a send, as there is a risk of a recursive +// call that can drain the contract + +// can override send by defining your own + +// Can check balance +owner.balance; // the balance of the owner (user or contract) + + +// Bytes available from 1 to 32 +byte a; // byte is same as bytes1 +bytes2 b; +bytes32 c; + +// Dynamically sized bytes +bytes m; // A special array, same as byte[] array (but packed tightly) +// More expensive than byte1-byte32, so use those when possible + +// same as bytes, but does not allow length or index access (for now) +string n = "hello"; // stored in UTF8, note double quotes, not single +// string utility functions to be added in future +// prefer bytes32/bytes, as UTF8 uses more storage + +// Type inferrence +// var does inferred typing based on first assignment, +// can't be used in functions parameters +var a = true; +// use carefully, inference may provide wrong type +// e.g., an int8, when a counter needs to be int16 + +// var can be used to assign function to variable +function a(uint x) returns (uint) { + return x * 2; +} +var f = a; +f(22); // call + +// by default, all values are set to 0 on instantiation + +// Delete can be called on most types +// (does NOT destroy value, but sets value to 0, the initial value) +uint x = 5; + + +// Destructuring/Tuples +(x, y) = (2, 7); // assign/swap multiple value + + +// 2. DATA STRUCTURES +// Arrays +bytes32[5] nicknames; // static array +bytes32[] names; // dynamic array +uint newLength = names.push("John"); // adding returns new length of the array +// Length +names.length; // get length +names.length = 1; // lengths can be set (for dynamic arrays in storage only) + +// multidimensional array +uint x[][5]; // arr with 5 dynamic array elements (opp order of most languages) + +// Dictionaries (any type to any other type) +mapping (string => uint) public balances; +balances["charles"] = 1; +console.log(balances["ada"]); // is 0, all non-set key values return zeroes +// 'public' allows following from another contract +contractName.balances("claude"); // returns 1 +// 'public' created a getter (but not setter) like the following: +function balances(address _account) returns (uint balance) { + return balances[_account]; +} + +// Nested mappings +mapping (address => mapping (address => uint)) public custodians; + +// To delete +delete balances["John"]; +delete balances; // sets all elements to 0 + +// Unlike other languages, CANNOT iterate through all elements in +// mapping, without knowing source keys - can build data structure +// on top to do this + +// Structs and enums +struct Bank { + address owner; + uint balance; +} +Bank b = Bank({ + owner: msg.sender, + balance: 5 +}); +// or +Bank c = Bank(msg.sender, 5); + +c.amount = 5; // set to new value +delete b; +// sets to initial value, set all variables in struct to 0, except mappings + +// Enums +enum State { Created, Locked, Inactive }; // often used for state machine +State public state; // Declare variable from enum +state = State.Created; +// enums can be explicitly converted to ints +uint createdState = uint(State.Created); // 0 + +// Data locations: Memory vs. storage vs. stack - all complex types (arrays, +// structs) have a data location +// 'memory' does not persist, 'storage' does +// Default is 'storage' for local and state variables; 'memory' for func params +// stack holds small local variables + +// for most types, can explicitly set which data location to use + + +// 3. Simple operators +// Comparisons, bit operators and arithmetic operators are provided +// exponentiation: ** +// exclusive or: ^ +// bitwise negation: ~ + + +// 4. Global Variables of note +// ** this ** +this; // address of contract +// often used at end of contract life to send remaining balance to party +this.balance; +this.someFunction(); // calls func externally via call, not via internal jump + +// ** msg - Current message received by the contract ** ** +msg.sender; // address of sender +msg.value; // amount of ether provided to this contract in wei +msg.data; // bytes, complete call data +msg.gas; // remaining gas + +// ** tx - This transaction ** +tx.origin; // address of sender of the transaction +tx.gasprice; // gas price of the transaction + +// ** block - Information about current block ** +now; // current time (approximately), alias for block.timestamp (uses Unix time) +block.number; // current block number +block.difficulty; // current block difficulty +block.blockhash(1); // returns bytes32, only works for most recent 256 blocks +block.gasLimit(); + +// ** storage - Persistent storage hash ** +storage['abc'] = 'def'; // maps 256 bit words to 256 bit words + + +// 4. FUNCTIONS AND MORE +// A. Functions +// Simple function +function increment(uint x) returns (uint) { + x += 1; + return x; +} + +// Functions can return many arguments, and by specifying returned arguments +// name don't need to explicitly return +function increment(uint x, uint y) returns (uint x, uint y) { + x += 1; + y += 1; +} +// Call previous functon +uint (a,b) = increment(1,1); + +// 'constant' indicates that function does not/cannot change persistent vars +// Constant function execute locally, not on blockchain +uint y; + +function increment(uint x) constant returns (uint x) { + x += 1; + y += 1; // this line would fail + // y is a state variable, and can't be changed in a constant function +} + +// 'Function Visibility specifiers' +// These can be placed where 'constant' is, including: +// public - visible externally and internally (default) +// external +// private - only visible in the current contract +// internal - only visible in current contract, and those deriving from it + +// Functions hoisted - and can assign a function to a variable +function a() { + var z = b; + b(); +} + +function b() { + +} + + +// Prefer loops to recursion (max call stack depth is 1024) + +// B. Events +// Events are notify external parties; easy to search and +// access events from outside blockchain (with lightweight clients) +// typically declare after contract parameters + +// Typically, capitalized - and add Log in front to be explicit and prevent confusion +// with a function call + +// Declare +event LogSent(address indexed from, address indexed to, uint amount); // note capital first letter + +// Call +Sent(from, to, amount); + +// For an external party (a contract or external entity), to watch: +Coin.Sent().watch({}, '', function(error, result) { + if (!error) { + console.log("Coin transfer: " + result.args.amount + + " coins were sent from " + result.args.from + + " to " + result.args.to + "."); + console.log("Balances now:\n" + + "Sender: " + Coin.balances.call(result.args.from) + + "Receiver: " + Coin.balances.call(result.args.to)); + } +} +// Common paradigm for one contract to depend on another (e.g., a +// contract that depends on current exchange rate provided by another) + +// C. Modifiers +// Modifiers validate inputs to functions such as minimal balance or user auth; +// similar to guard clause in other languages + +// '_' (underscore) often included as last line in body, and indicates +// function being called should be placed there +modifier onlyAfter(uint _time) { if (now <= _time) throw; _ } +modifier onlyOwner { if (msg.sender == owner) _ } +// commonly used with state machines +modifier onlyIfState (State currState) { if (currState != State.A) _ } + +// Append right after function declaration +function changeOwner(newOwner) +onlyAfter(someTime) +onlyOwner() +onlyIfState(State.A) +{ + owner = newOwner; +} + +// underscore can be included before end of body, +// but explicitly returning will skip, so use carefully +modifier checkValue(uint amount) { + _ + if (msg.value > amount) { + uint amountToRefund = amount - msg.value; + if (!msg.sender.send(amountToRefund)) { + throw; + } + } +} + + +// 6. BRANCHING AND LOOPS + +// All basic logic blocks work - including if/else, for, while, break, continue +// return - but no switch + +// Syntax same as javascript, but no type conversion from non-boolean +// to boolean (comparison operators must be used to get the boolean val) + +// For loops that are determined by user behavior, be careful - as contracts have a maximal +// amount of gas for a block of code - and will fail if that is exceeded +// For example: +for(uint x = 0; x < refundAddressList.length; x++) { + if (!refundAddressList[x].send(SOME_AMOUNT)) { + throw; + } +} + +// Two errors above: +// 1. A failure on send stops the loop from completing, tying up money +// 2. This loop could be arbitrarily long (based on the amount of users who need refunds), and +// therefore may always fail as it exceeds the max gas for a block +// Instead, you should let people withdraw individually from their subaccount, and mark withdrawn + + +// 7. OBJECTS/CONTRACTS + +// A. Calling external contract +contract infoFeed { + function info() returns (uint ret) { return 42; } +} + +contract Consumer { + InfoFeed feed; // points to contract on blockchain + + // Set feed to existing contract instance + function setFeed(address addr) { + // automatically cast, be careful; constructor is not called + feed = InfoFeed(addr); + } + + // Set feed to new instance of contract + function createNewFeed() { + feed = new InfoFeed(); // new instance created; constructor called + } + + function callFeed() { + // final parentheses call contract, can optionally add + // custom ether value or gas + feed.info.value(10).gas(800)(); + } +} + +// B. Inheritance + +// Order matters, last inherited contract (i.e., 'def') can override parts of +// previously inherited contracts +contract MyContract is abc, def("a custom argument to def") { + +// Override function + function z() { + if (msg.sender == owner) { + def.z(); // call overridden function from def + super.z(); // call immediate parent overriden function + } + } +} + +// abstract function +function someAbstractFunction(uint x); +// cannot be compiled, so used in base/abstract contracts +// that are then implemented + +// C. Import + +import "filename"; +import "github.com/ethereum/dapp-bin/library/iterable_mapping.sol"; + +// Importing under active development +// Cannot currently be done at command line + + +// 8. OTHER KEYWORDS + +// A. Throwing +// Throwing +throw; // reverts unused money to sender, state is reverted +// Can't currently catch + +// Common design pattern is: +if (!addr.send(123)) { + throw; +} + +// B. Selfdestruct +// selfdestruct current contract, sending funds to address (often creator) +selfdestruct(SOME_ADDRESS); + +// removes storage/code from current/future blocks +// helps thin clients, but previous data persists in blockchain + +// Common pattern, lets owner end the contract and receive remaining funds +function remove() { + if(msg.sender == creator) { // Only let the contract creator do this + selfdestruct(creator); // Makes contract inactive, returns funds + } +} + +// May want to deactivate contract manually, rather than selfdestruct +// (ether sent to selfdestructed contract is lost) + + +// 9. CONTRACT DESIGN NOTES + +// A. Obfuscation +// All variables are publicly viewable on blockchain, so anything +// that is private needs to be obfuscated (e.g., hashed w/secret) + +// Steps: 1. Commit to something, 2. Reveal commitment +sha3("some_bid_amount", "some secret"); // commit + +// call contract's reveal function in the future +// showing bid plus secret that hashes to SHA3 +reveal(100, "mySecret"); + +// B. Storage optimization +// Writing to blockchain can be expensive, as data stored forever; encourages +// smart ways to use memory (eventually, compilation will be better, but for now +// benefits to planning data structures - and storing min amount in blockchain) + +// Cost can often be high for items like multidimensional arrays +// (cost is for storing data - not declaring unfilled variables) + +// C. Data access in blockchain +// Cannot restrict human or computer from reading contents of +// transaction or transaction's state + +// While 'private' prevents other *contracts* from reading data +// directly - any other party can still read data in blockchain + +// All data to start of time is stored in blockchain, so +// anyone can observe all previous data and changes + +// D. Cron Job +// Contracts must be manually called to handle time-based scheduling; can create external +// code to regularly ping, or provide incentives (ether) for others to + +// E. Observer Pattern +// An Observer Pattern lets you register as a subscriber and +// register a function which is called by the oracle (note, the oracle pays +// for this action to be run) +// Some similarities to subscription in Pub/sub + +// This is an abstract contract, both client and server classes import +// the client should implement +contract SomeOracleCallback { + function oracleCallback(int _value, uint _time, bytes32 info) external; +} + +contract SomeOracle { + SomeOracleCallback[] callbacks; // array of all subscribers + + // Register subscriber + function addSubscriber(SomeOracleCallback a) { + callbacks.push(a); + } + + function notify(value, time, info) private { + for(uint i = 0;i < callbacks.length; i++) { + // all called subscribers must implement the oracleCallback + callbacks[i].oracleCallback(value, time, info); + } + } + + function doSomething() public { + // Code to do something + + // Notify all subscribers + notify(_value, _time, _info); + } +} + +// Now, your client contract can addSubscriber by importing SomeOracleCallback +// and registering with Some Oracle + +// F. State machines +// see example below for State enum and inState modifier + + +// *** EXAMPLE: A crowdfunding example (broadly similar to Kickstarter) *** +// ** START EXAMPLE ** + +// CrowdFunder.sol + +/// @title CrowdFunder +/// @author nemild +contract CrowdFunder { + // Variables set on create by creator + address public creator; + address public fundRecipient; // creator may be different than recipient + uint public minimumToRaise; // required to tip, else everyone gets refund + string campaignUrl; + byte constant version = 1; + + // Data structures + enum State { + Fundraising, + ExpiredRefund, + Successful + } + struct Contribution { + uint amount; + address contributor; + } + + // State variables + State public state = State.Fundraising; // initialize on create + uint public totalRaised; + uint public raiseBy; + uint public completeAt; + Contribution[] contributions; + + event LogFundingReceived(address addr, uint amount, uint currentTotal); + event LogWinnerPaid(address winnerAddress); + + modifier inState(State _state) { + if (state != _state) throw; + _ + } + + modifier isCreator() { + if (msg.sender != creator) throw; + _ + } + + // Wait 6 months after final contract state before allowing contract destruction + modifier atEndOfLifecycle() { + if(!((state == State.ExpiredRefund || state == State.Successful) && + completeAt + 6 months < now)) { + throw; + } + _ + } + + function CrowdFunder( + uint timeInHoursForFundraising, + string _campaignUrl, + address _fundRecipient, + uint _minimumToRaise) + { + creator = msg.sender; + fundRecipient = _fundRecipient; + campaignUrl = _campaignUrl; + minimumToRaise = _minimumToRaise; + raiseBy = now + (timeInHoursForFundraising * 1 hours); + } + + function contribute() + public + inState(State.Fundraising) + { + contributions.push( + Contribution({ + amount: msg.value, + contributor: msg.sender + }) // use array, so can iterate + ); + totalRaised += msg.value; + + LogFundingReceived(msg.sender, msg.value, totalRaised); + + checkIfFundingCompleteOrExpired(); + return contributions.length - 1; // return id + } + + function checkIfFundingCompleteOrExpired() { + if (totalRaised > minimumToRaise) { + state = State.Successful; + payOut(); + + // could incentivize sender who initiated state change here + } else if ( now > raiseBy ) { + state = State.ExpiredRefund; // backers can now collect refunds by calling getRefund(id) + } + completeAt = now; + } + + function payOut() + public + inState(State.Successful) + { + if(!fundRecipient.send(this.balance)) { + throw; + } + + + LogWinnerPaid(fundRecipient); + } + + function getRefund(id) + public + inState(State.ExpiredRefund) + { + if (contributions.length <= id || id < 0 || contributions[id].amount == 0 ) { + throw; + } + + uint amountToRefund = contributions[id].amount; + contributions[id].amount = 0; + + if(!contributions[id].contributor.send(amountToSend)) { + contributions[id].amount = amountToSend; + return false; + } + + return true; + } + + function removeContract() + public + isCreator() + atEndOfLifecycle() + { + selfdestruct(msg.sender); + // creator gets all money that hasn't be claimed + } + + function () { throw; } +} +// ** END EXAMPLE ** + +// 10. OTHER NATIVE FUNCTIONS + +// Currency units +// Currency is defined using wei, smallest unit of Ether +uint minAmount = 1 wei; +uint a = 1 finney; // 1 ether == 1000 finney +// Other units, see: http://ether.fund/tool/converter + +// Time units +1 == 1 second +1 minutes == 60 seconds + +// Can multiply a variable times unit, as units are not stored in a variable +uint x = 5; +(x * 1 days); // 5 days + +// Careful about leap seconds/years with equality statements for time +// (instead, prefer greater than/less than) + +// Cryptography +// All strings passed are concatenated before hash action +sha3("ab", "cd"); +ripemd160("abc"); +sha256("def"); + +// 11. SECURITY + +// Bugs can be disastrous in Ethereum contracts - and even popular patterns in Solidity, +// may be found to be antipatterns + +// See security links at the end of this doc + +// 12. LOW LEVEL FUNCTIONS +// call - low level, not often used, does not provide type safety +successBoolean = someContractAddress.call('function_name', 'arg1', 'arg2'); + +// callcode - Code at target address executed in *context* of calling contract +// provides library functionality +someContractAddress.callcode('function_name'); + + +// 13. STYLE NOTES +// Based on Python's PEP8 style guide + +// Quick summary: +// 4 spaces for indentation +// Two lines separate contract declarations (and other top level declarations) +// Avoid extraneous spaces in parentheses +// Can omit curly braces for one line statement (if, for, etc) +// else should be placed on own line + + +// 14. NATSPEC COMENTS +// used for documentation, commenting, and external UIs + +// Contract natspec - always above contract definition +/// @title Contract title +/// @author Author name + +// Function natspec +/// @notice information about what function does; shown when function to execute +/// @dev Function documentation for developer + +// Function parameter/return value natspec +/// @param someParam Some description of what the param does +/// @return Description of the return value +``` + +## Additional resources +- [Solidity Docs](https://solidity.readthedocs.org/en/latest/) +- [Solidity Style Guide](https://ethereum.github.io/solidity//docs/style-guide/): Ethereum's style guide is heavily derived from Python's [pep8](https://www.python.org/dev/peps/pep-0008/) style guide. +- [Browser-based Solidity Editor](http://chriseth.github.io/browser-solidity/) +- [Gitter Solidity Chat room](https://gitter.im/ethereum/solidity) +- [Modular design strategies for Ethereum Contracts](https://docs.erisindustries.com/tutorials/solidity/) + +## Sample contracts +- [Dapp Bin](https://github.com/ethereum/dapp-bin) +- [Solidity Baby Step Contracts](https://github.com/fivedogit/solidity-baby-steps/tree/master/contracts) +- [ConsenSys Contracts](https://github.com/ConsenSys/dapp-store-contracts) +- [State of Dapps](http://dapps.ethercasts.com/) + +## Security +- [Thinking About Smart Contract Security](https://blog.ethereum.org/2016/06/19/thinking-smart-contract-security/) +- [Smart Contract Security](https://blog.ethereum.org/2016/06/10/smart-contract-security/) +- [Hacking Distributed Blog](http://hackingdistributed.com/) + +## Information purposefully excluded +- Libraries + +## Style +- Python's [PEP8](https://www.python.org/dev/peps/pep-0008/) is used as the baseline style guide, including its general philosophy + +## Editors +- [Vim Solidity](https://github.com/tomlion/vim-solidity) +- Editor Snippets ([Ultisnips format](https://gist.github.com/nemild/98343ce6b16b747788bc)) + +## Future To Dos +- New keywords: protected, inheritable +- List of common design patterns (throttling, RNG, version upgrade) +- Common security anti patterns + +Feel free to send a pull request with any edits - or email nemild -/at-/ gmail |