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Many minor updates
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Nemil Dalal
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@ -14,7 +14,7 @@ As Solidity and Ethereum are under active development, experimental or beta feat
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```javascript
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// Let's start with a simple Bank contract, before diving into to the key components of the language
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// START EXAMPLE
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// ** START EXAMPLE **
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// Start with a Natspec comment (the three slashes) that can be used
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// for documentation - and as descriptive data for UI elements
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/// @title A simple deposit/withdrawal bank built on Bitcoin
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@ -22,31 +22,30 @@ As Solidity and Ethereum are under active development, experimental or beta feat
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// All contracts are declared and named (in CamelCase)
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// They are similar to 'class' in other languages (and allow capabilities like inheritance)
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contract AcmeBank {
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// Declare state variables outside a function,
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// Declare state variables outside a function,
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// these are persistent throughout the life of the contract
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// a dictionary that maps addresses to balances
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mapping (address => uint) balances;
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mapping (address => uint) balances;
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// the 'public' makes 'owner' externally readable by users or contracts
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// (but not writeable) -
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address public owner;
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// the 'public' makes 'owner' externally readable by users or contracts
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// (but not writeable), the 'constant' means this value to be
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// changed after initialization
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address public owner;
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// Constructor, can receive one or many variables here
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function AcmeBank() {
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// msg is a default variable that provides both the
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// msg is a default variable that provides both the
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// contract messager's address and amount
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owner = msg.sender; // msg.sender refers to the address of the contract creator
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}
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function deposit(uint balance) {
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function deposit(uint balance) public {
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balances[msg.sender] += msg.value; // no need for "this." or "self." in front of the state variable
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return balances[msg.sender]; // msg.sender refers to the contract caller
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}
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function withdraw(uint withdrawAmount) returns (uint remainingBalance) {
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function withdraw(uint withdrawAmount) public returns (uint remainingBalance) {
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if(balances[msg.sender] >= withdrawAmount) {
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balances[msg.sender] -= withdrawAmount;
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@ -58,8 +57,9 @@ contract AcmeBank {
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}
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}
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// It's good practice to have a remove function, which disables this contract - but does mean that users have to trust the owner
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function remove () {
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// It's good practice to have a remove function, which disables this
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// contract - but does mean that users have to trust the owner
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function remove() {
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if(msg.sender == owner) { // Only let the contract creator do this
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suicide(owner); // suicide makes this contract inactive, and returns funds to the owner
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}
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@ -71,29 +71,35 @@ contract AcmeBank {
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}
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// Fallback function
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// This function is called if invalid data is sent or ether without data;
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// The fallback function is called if none of the other functions matches the given function identifier.
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// It is often meant to be called when invalid data is sent or ether without data.
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// Added so that ether sent to this contract is reverted if the contract fails
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// otherwise, the sender loses their money; you should add this in most contracts
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function () { throw; }
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}
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// END EXAMPLE
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// ** END EXAMPLE **
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// Now let's go through the basics of Solidity
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// 1. DATA TYPES
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// 1. DATA TYPES AND ASSOCIATED METHOD
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// uint is the data type typically used for currency (there are no doubles
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// or floats) and for dates
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uint x;
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int constant a = 8; // int of 256 bits, cannot be changed after instantiation
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uint x;
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// with 'constant', the compiler replaces each occurence with the acutal value
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// int of 256 bits, cannot be changed after instantiation
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int constant a = 8;
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int256 constant a = 8; // same effect as line above, here the 256 is explict
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// For both int and uint, you can explicitly set space in steps of 8
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// e.g., int8, int16
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uint8 b;
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int64 c;
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// int256 is same as int
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// For both int and uint, you can explicitly set space in steps of 8,
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// e.g., int8, int16
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uint248 e;
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// Type casting
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int x = int(b)
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int x = int(b);
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bool b = true; // or do 'var b = true;' for inferred typing
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@ -115,21 +121,22 @@ bytes m; // A special array, same as byte[] (but packed tightly)
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string n = 'hello';
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// Type inference
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// var does inferred typing based on first assignment,
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// var does inferred typing based on first assignment,
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// can't be used in functions parameters
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var a = true;
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// there are edge cases where inference leads to a value being set (e.g., an uint 8)
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// there are edge cases where inference leads to a value being set (e.g., an uint 8)
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// that is different from what the user wanted (uint16), so use carefully
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// by default, all values are set to 0 on instantiation
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// Delete can be called on most types, and will set the values to 0
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// Delete can be called on most types, and will set the values to 0 by assignment
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uint x = 5;
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delete(x); // x is now 0
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// 2. DATA STRUCTURES
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// Arrays
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bytes32[] names;
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bytes32[5] nicknames; // static array
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bytes32[] names; // dynamic array
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uint newLength = names.push("John"); // adding returns new length of the array
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// Length
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names.length; // get length
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@ -153,13 +160,13 @@ delete(balances); // deletes all elements
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// Unlike languages like Javascript, you cannot iterate through all elements in
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// a map, without knowing the source keys
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// Structs and enums
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// Structs and enums
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struct Bank { // note the capital
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address owner;
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uint balance;
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}
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Bank b = Bank({
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owner: msg.sender,
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owner: msg.sender,
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balance: 5
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});
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delete(b); // set all values to 0, except any mappings
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@ -168,29 +175,39 @@ delete(b); // set all values to 0, except any mappings
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enum State { Created, Locked, Inactive };
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State public state; // Declare variable from enum
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state = State.Created;
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// enums can be explicitly converted to ints
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// Data locations: Memory vs. storage - all complex types (arrays, structs) have a data location
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// 'memory' does not persist, 'storage' does
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// Default is 'storage' for local and state variables; 'memory' for function parameters
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// 3. Variables of note
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// storage - A persistent storage hash (does not need to be declared)
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storage['abc'] = 'def'; // maps 256 bit words to 256 bit words
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// ** this **
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this; // the address of the current contract
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// 'balance' often used at the end of a contracts life to send the
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// remaining balance to a party
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this.balance;
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this.someFunction(); // calls a function externally (via a message call, not via an internal jump)
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// tx - This transaction
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tx.origin // address, sender of the transaction
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tx.gasprice // uint, gas price of the transaction
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// msg - The current message received by the contract
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// ** msg - The current message received by the contract ** **
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msg.sender; // address, The address of the sender
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msg.value; // uint, The amount of gas provided to this contract in wei
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msg.data // bytes, complete call data
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msg.data; // bytes, complete call data
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msg.gas; // remaining gas
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// balance of the current contract (both contract and external accounts
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// have balances) - often used at the end of a contracts life to send the
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// remaining balance to a party
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this.balance
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// block
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// ** tx - This transaction **
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tx.origin; // address, sender of the transaction
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tx.gasprice; // uint, gas price of the transaction
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// ** block - Information about the current block **
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now // uint, current time, alias for block.timestamp
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block.number // uint, current block number
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block.difficulty // uint, current block difficulty
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block.blockhash(1) // returns bytes32, only provides for most recent 256 block
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block.number; // uint, current block number
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block.difficulty; // uint, current block difficulty
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block.blockhash(1); // returns bytes32, only provides for most recent 256 blocks
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block.gasLimit();
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// ** storage - A persistent storage hash (does not need to be declared) **
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storage['abc'] = 'def'; // maps 256 bit words to 256 bit words
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// 4. FUNCTIONS AND MORE
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// A. Functions
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@ -236,13 +253,13 @@ function b() {
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// B. Events
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// Events are an easy way to notify external listeners that something changed
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// You typically decalre them after your contract parameters
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// You typically declare them after your contract parameters
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event Sent(address from, address to, uint amount);
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// You then call it in a function, when you want to trigger it
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sent(from, to, amount);
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// For an external party (a contract or external entity), to watch
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// For an external party (a contract or external entity), to watch
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// for an event, you write the following:
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Coin.Sent().watch({}, '', function(error, result) {
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if (!error) {
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@ -254,53 +271,89 @@ Coin.Sent().watch({}, '', function(error, result) {
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"Receiver: " + Coin.balances.call(result.args.to));
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}
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}
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// This is a common paradigm for one contract to depend on another (e.g., a
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// This is a common paradigm for one contract to depend on another (e.g., a
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// contract that depends on the current exchange rate provided by another
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// contract)
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// C. Modifiers
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// Modifiers let you validate inputs to functions
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// The '_' (underscore) must be included, and is an indicator that the
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// Modifiers let you validate inputs to functions such as a minimal balance or user authentication
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// It's similar to a guard clause in other languages
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// The '_' (underscore) must be included as the last line in the function body, and is an indicator that the
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// function being called should be placed there
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modifier onlyBefore(uint _time) { if (now >= _time) throw; _ }
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// You can then append it right after the function declaration
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function test()
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onlyBefore()
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{
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function changeOwner(newOwner)
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onlyBefore(someTime)
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{
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owner = newOwner;
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}
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// 5. BRANCHING AND LOOPS
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// All basic logic blocks work - including if/else, for, while, break, continue, return
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// switch is not provided
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// Syntax is the same as javascript, but there is no type conversion from
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// non-boolean to boolean
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// 6. CALLING AN EXTERNAL CONTRACT
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// Syntax is the same as javascript, but there is no type conversion from
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// non-boolean to boolean, so comparison operators must be used to get the boolean value
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// 6. OBJECTS/CONTRACTS
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// A. Calling an external contract
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contract infoFeed {
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function info() returns (uint ret) { return 42; }
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}
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contract Consumer {
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InfoFeed feed; // create a variable that will point to a contract on the blockchain
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function setFeed(address addr) {
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// Set feed to an existing contract
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function setFeed(address addr) {
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// Link to the contract by creating on the address
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feed = InfoFeed(addr);
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feed = InfoFeed(addr);
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}
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function callFeed() {
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// T final parentheses call the contract, optionally adding
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// Set feed based to a new instance of the contract
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function createNewFeed() {
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feed = new InfoFeed();
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}
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function callFeed() {
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// T final parentheses call the contract, optionally adding
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// custom value or gas numbers
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feed.info.value(10).gas(800)();
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}
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}
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// B. Inheritance
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// Order matters, last inherited contract (i.e., 'def') can override parts of the previously
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// inherited contracts
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contact MyContract is abc, def("a custom argument to def") {
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// Override function
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function z() {
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if (msg.sender == owner) {
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def.z(); // call overridden function
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}
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}
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};
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// C. Import
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import "filename";
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import "github.com/ethereum/dapp-bin/library/iterable_mapping.sol";
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// Importing is under active development and will change
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// Importing cannot currently be done at the command line
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// 7. CONTRACT DESIGN PATTERNS
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// A. Obfuscation
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// Remember that all variables are publicly viewable on the blockchain, so
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// Remember that all variables are publicly viewable on the blockchain, so
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// anything that needs some privacy needs to be obfuscated (e.g., hashed)
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// B. Throwing
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@ -317,26 +370,34 @@ if (!addr.send(123)) {
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// Suicide
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suicide(SOME_ADDRESS); // suicide the current contract, sending funds to the address (often the creator)
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// This is a common pattern that lets the owner end the contract
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function remove() {
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if(msg.sender == owner) { // Only let the contract creator do this
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suicide(owner); // suicide makes this contract inactive, and returns funds to the owner
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}
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}
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// D. Storage optimization
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// Reading and writing can be expensive, as data needs to be stored in the
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// blockchain forever - this encourages smart ways to use memory (eventually,
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// compilation may better handle this, but for now there are benefits to
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// Reading and writing can be expensive, as data needs to be stored in the
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// blockchain forever - this encourages smart ways to use memory (eventually,
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// compilation may better handle this, but for now there are benefits to
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// planning your data structures)
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// *** EXAMPLE: Let's do a more complex example ***
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// ** START EXAMPLE **
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// [TODO: Decide what a more complex example looks like, needs a few characteristics:
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// - has a 'constant' state variable
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// - has a state machine (uses modifier)
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// - has a state machine (and uses modifier)
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// - sends money to an address
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// - gets information from another contract (we'll show code for both contracts)
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// - Shows inheritance
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// - show variables being passed in on instantiation (and guard code to throw if variables not provided)
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// - Shows the swapping out of a contract
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// - Shows the swapping out of a contract address
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// Ideas:
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// - crowdfunding?
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// - Peer to peer insurance
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// ]
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// *** END EXAMPLE ***
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// Some final points
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@ -353,7 +414,7 @@ uint a = 1 finney; // 1 ether = 1000 finney
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1 minutes == 60 seconds
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// You typically multiply a variable times the unit, as these units are not
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// You typically multiply a variable times the unit, as these units are not
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// directly stored in a variable
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uint x = 5;
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(x * 1 days); // 5 days
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@ -366,22 +427,22 @@ sha3("ab", "cd");
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ripemd160("abc");
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sha256("def");
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// These are natspec comments, when a user is asked to confirm a transaction
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///
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/** */
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// 8. COMMON MISTAKES/MISCONCEPTIONS
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// A few common mistakes and misconceptions:
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// 8. COMMON MISTAKES
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// A few common mistakes
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// You cannot restrict a human or computer from reading the content of
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// A. You cannot restrict a human or computer from reading the content of
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// your transaction or a transaction's state
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// All data to the start of time is stored in the blockchain, so you and
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// anyone can observe all previous data states
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// All data to the start of time is stored in the blockchain, so you and
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// anyone can observe all previous data stats
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// 9. TESTING
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// When you don't specify public on a variable, you are indicating that other *contracts* can't
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// read the data - but any person can still read the data
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// 10. STYLE NOTES
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// Use 4 spaces for indentation
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// TODO
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// 9. STYLE NOTES
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// Use 4 spaces for indentation
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// (Python's PEP8 is used as the baseline style guide, including its general philosophy)
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```
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@ -391,4 +452,7 @@ sha256("def");
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- [Browser-based Solidity Editor](http://chriseth.github.io/browser-solidity/)
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- [Gitter Chat room](https://gitter.im/ethereum/go-ethereum)
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Feel free to send a pull request with any edits - or email nemild -/at-/ gmail
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## Information purposefully excluded
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- Libraries
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Feel free to send a pull request with any edits - or email nemild -/at-/ gmail
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Loading…
Reference in New Issue
Block a user