What is a contract?

A contract is a collection of code (its functions) and data (its state) that resides at a specific address on the Expanse blockchain. Contract accounts are able to pass messages between themselves as well as doing practically Turing complete computation. Contracts live on the blockchain in a Expanse-specific binary format called Expanse Virtual Machine (EVM) bytecode.
Contracts are typically written in some high level language such as Solidity and then compiled into bytecode to be uploaded on the blockchain.
See also
Other languages also exist, notably Serpent and LLL, which are described further in the expanse-high-level-languages section of this documentation.
​Dapp development resources lists the integrated development environments, developer tools that help you develop in these languages, offering testing, and deployment support among other features.
EXPANSE HIGH LEVEL LANGUAGES
Contracts live on the blockchain in an Expanse-specific binary format (EVM bytecode) that is executed by the Expanse Virtual Machine (EVM). However, contracts are typically written in a higher level language and then compiled using the EVM compiler into byte code to be deployed to the blockchain.
Below are the different high level languages developers can use to write smart contracts for Expanse.
SOLIDITY
Solidity is a language similar to JavaScript which allows you to develop contracts and compile to EVM bytecode. It is currently the flagship language of Expanse and the most popular.
​Solidity Documentation – Solidity is the flagship Expanse high level language that is used to write contracts.
​Solidity online realtime compiler​
​Standardized Contract APIs​
​Useful Ðapp Patterns – Code snippets which are useful for Ðapp development.
SERPENT
Serpent is a language similar to Python which can be used to develop contracts and compile to EVM bytecode. It is intended to be maximally clean and simple, combining many of the efficiency benefits of a low-level language with ease-of-use in programming style, and at the same time adding special domain-specific features for contract programming. Serpent is compiled using LLL.
​Serpent on the expanse wiki​
​Serpent EVM compiler​
LLL
​Lisp Like Language (LLL) is a low level language similar to Assembly. It is meant to be very simple and minimalistic; essentially just a tiny wrapper over coding in EVM directly.
​LIBLLL in GitHub​
​Examples of LLL​
MUTAN (DEPRECATED)
​Mutan is a statically typed, C-like language designed and developed by Jeffrey Wilcke. It is no longer maintained.
WRITING A CONTRACT
No language would be complete without a Hello World program. Operating within the Expanse environment, Solidity has no obvious way of “outputting” a string. The closest we can do is to use a log event to place a string into the blockchain:
contract HelloWorld {
       event Print(string out);
       function() { Print(“Hello, World!”); }
}
This contract will create a log entry on the blockchain of type Print with a parameter “Hello, World!” each time it is executed.
See also
​Solidity docs has more examples and guidelines to writing Solidity code.
COMPILING A CONTRACT
Compilation of solidity contracts can be accomplished via a number of mechanisms.
Using the solc compiler via the command line.
Using web3.exp.compile.solidity in the javascript console provided by gexp or exp (This still requires the solc compiler to be installed).
The online Solidity realtime compiler.
The Meteor dapp Cosmo for building solidity contracts.
The Mix IDE.
The Expanse Wallet.
Note
More information on solc and compiling Solidity contract code can be found here.
SETTING UP THE SOLIDITY COMPILER IN GEXP
If you start up your gexp node, you can check which compilers are available.
> web3.exp.getCompilers();
[“lll”, “solidity”, “serpent”]
This command returns an array of strings indicating which compilers are currently available.
Note
The solc compiler is installed with cpp-expanse. Alternatively, you can build it yourself.
If your solc executable is in a non-standard location you can specify a custom path to the solcexecutable using th –solc flag.
$ gexp –solc /usr/local/bin/solc
Alternatively, you can set this option at runtime via the console:
> admin.setSolc(“/usr/local/bin/solc”)
solc, the solidity compiler commandline interface
Version: 0.2.2-02bb315d/.-Darwin/appleclang/JIT linked to libethereum-1.2.0-8007cef0/.-Darwin/appleclang/JIT
path: /usr/local/bin/solc
COMPILING A SIMPLE CONTRACT
Let’s compile a simple contract source:
> source = “contract test { function multiply(uint a) returns(uint d) { return a * 7; } }”
This contract offers a single method multiply which is called with a positive integer a and returns a * 7.
You are ready to compile solidity code in the gexp JS console using exp.compile.solidity():
> contract = exp.compile.solidity(source).test
{
 code: ‘605280600c6000396000f3006000357c010000000000000000000000000000000000000000000000000000000090048063c6888fa114602e57005b60376004356041565b8060005260206000f35b6000600782029050604d565b91905056’,
 info: {
   language: ‘Solidity’,
   languageVersion: ‘0’,
   compilerVersion: ‘0.9.13’,
   abiDefinition: [{
     constant: false,
     inputs: [{
       name: ‘a’,
       type: ‘uint256’
     } ],
     name: ‘multiply’,
     outputs: [{
       name: ‘d’,
       type: ‘uint256’
     } ],
     type: ‘function’
   } ],
   userDoc: {
     methods: {
     }
   },
   developerDoc: {
     methods: {
     }
   },
   source: ‘contract test { function multiply(uint a) returns(uint d) { return a * 7; } }’
 }
}
Note
The compiler is also available via RPC and therefore via web3.js to any in-browser Ðapp connecting to gexp via RPC/IPC.
The following example shows how you interface gexp via JSON-RPC to use the compiler.
$ gexp –datadir ~/exp/ –loglevel 6 –logtostderr=true –rpc –rpcport 8100 –rpccorsdomain ‘*’ –mine console  2>> ~/exp/exp.log
$ curl -X POST –data ‘{“jsonrpc”:”2.0″,”method”:”exp_compileSolidity”,”params”:[“contract test { function multiply(uint a) returns(uint d) { return a * 7; } }”],”id”:1}’ http://127.0.0.1:8100
The compiler output for one source will give you contract objects each representing a single contract. The actual return value of exp.compile.solidity is a map of contract name to contract object pairs. Since our contract’s name is test, exp.compile.solidity(source).test will give you the contract object for the test contract containing the following fields:
code
The compiled EVM bytecode
info
Additional metadata output from the compiler
source
The source code
language
The contract language (Solidity, Serpent, LLL)
languageVersion
The contract language version
compilerVersion
The solidity compiler version that was used to compile this contract.
abiDefinition
The Application Binary Interface Definition​
userDoc
The NatSpec Doc for users.
developerDoc
The NatSpec Doc for developers.
The immediate structuring of the compiler output (into code and info) reflects the two very different paths of deployment. The compiled EVM code is sent off to the blockchain with a contract creation transaction while the rest (info) will ideally live on the decentralised cloud as publicly verifiable metadata complementing the code on the blockchain.
If your source contains multiple contracts, the output will contain an entry for each contact, the corresponding contract info object can be retrieved with the name of the contract as attribute name. You can try this by inspecting the most current GlobalRegistrar code:
contracts = exp.compile.solidity(globalRegistrarSrc)
CREATE AND DEPLOY A CONTRACT
Before you begin this section, make sure you have both an unlocked account as well as some funds.
You will now create a contract on the blockchain by sending a transaction to the empty address with the EVM code from the previous section as data.
Note
This can be accomplished much easier using the online Solidity realtime compiler or the Mix IDEprogram.
var primaryAddress = exp.accounts[0]
var abi = [{ constant: false, inputs: [{ name: ‘a’, type: ‘uint256’ } ]
var MyContract = exp.contract(abi)
var contract = MyContract.new(arg1, arg2, …, {from: primaryAddress, data: evmByteCodeFromPreviousSection})
All binary data is serialised in hexadecimal form. Hex strings always have a hex prefix 0x.
Note
Note that arg1, arg2, … are the arguments for the contract constructor, in case it accepts any. If the contract does not require any constructor arguments then these arguments can be omitted.
It is worth pointing out that this step requires you to pay for execution. Your balance on the account (that you put as sender in the from field) will be reduced according to the gas rules of the EVM once your transaction makes it into a block. After some time, your transaction should appear included in a block confirming that the state it brought about is a consensus. Your contract now lives on the blockchain.
The asynchronous way of doing the same looks like this:
MyContract.new([arg1, arg2, …,]{from: primaryAccount, data: evmCode}, function(err, contract) {
 if (!err && contract.address)
   console.log(contract.address);
});
INTERACTING WITH A CONTRACT
Interaction with a contract is typically done using an abstraction layer such as the exp.contract()function which returns a javascript object with all of the contract functions available as callable functions in javascript.
The standard way to describe the available functions of a contract is the ABI definition. This object is an array which describles the call signature and return values for each available contract function.
var Multiply7 = exp.contract(contract.info.abiDefinition);
var myMultiply7 = Multiply7.at(address);
Now all the function calls specified in the ABI are made available on the contract instance. You can just call those methods on the contract instance in one of two ways.
> myMultiply7.multiply.sendTransaction(3, {from: address})
“0x12345”
> myMultiply7.multiply.call(3)
21
When called using sendTransaction the function call is executed via sending a transaction. This will cost expanse to send and the call will be recorded forever on the blockchain. The return value of calls made in this manner is the hash of the stransaction.
When called using call the function is executed locally in the EVM and the return value of the function is returned with the function. Calls made in this manner are not recorded on the blockchain and thus, cannot modify the internal state of the contract. This manner of call is referred to as a constant function call. Calls made in this manner do not cost any expanse.
You should use call if you are interested only in the return value and use sendTransaction if you only care about side effects on the state of the contract.
In the example above, there are no side effects, therefore sendTransaction only burns gas and increases the entropy of the universe.
CONTRACT METADATA
In the previous sections we explained how you create a contract on the blockchain. Now we will deal with the rest of the compiler output, the contract metadata or contract info.
When interacting with a contract you did not create you might want documentation or to look at the source code. Contract authors are encouraged to make such information available by registering it on the blockchain or through a third party service, such as EtherChain. The admin API provides convenience methods to fetch this bundle for any contract that chose to register.
// get the contract info for contract address to do manual verification
var info = admin.getContractInfo(address) // lookup, fetch, decode
var source = info.source;
var abiDef = info.abiDefinition
The underlying mechanism that makes this work is is that:
contract info is uploaded somewhere identifiable by a URI which is publicly accessible
anyone can find out what the URI is only knowing the contracts address
These requirements are achieved using a 2 step blockchain registry. The first step registers the contract code (hash) with a content hash in a contract called HashReg. The second step registers a url with the content hash in the UrlHint contract. These registry contracts were part of the Frontier release and have carried on into Homestead.
By using this scheme, it is sufficient to know a contract’s address to look up the url and fetch the actual contract metadata info bundle.
So if you are a conscientious contract creator, the steps are the following:
1.Deploy the contract itself to the blockchain
2.Get the contract info json file.
3.Deploy contract info json file to any url of your choice
4.Register codehash ->content hash -> url
The JS API makes this process very easy by providing helpers. Call admin.register to extract info from the contract, write out its json serialisation in the given file, calculates the content hash of the file and finally registers this content hash to the contract’s code hash. Once you deployed that file to any url, you can use admin.registerUrl to register the url with your content hash on the blockchain as well. (Note that in case a fixed content addressed model is used as document store, the url-hint is no longer necessary.)
source = “contract test { function multiply(uint a) returns(uint d) { return a * 7; } }”
// compile with solc
contract = exp.compile.solidity(source).test
// create contract object
var MyContract = exp.contract(contract.info.abiDefinition)
// extracts info from contract, save the json serialisation in the given file,
contenthash = admin.saveInfo(contract.info, “~/dapps/shared/contracts/test/info.json”)
// send off the contract to the blockchain
MyContract.new({from: primaryAccount, data: contract.code}, function(error, contract){
 if(!error && contract.address) {
   // calculates the content hash and registers it with the code hash in `HashReg`
   // it uses address to send the transaction.
   // returns the content hash that we use to register a url
   admin.register(primaryAccount, contract.address, contenthash)
   // here you deploy ~/dapps/shared/contracts/test/info.json to a url
   admin.registerUrl(primaryAccount, hash, url)
 }
});
TESTING CONTRACTS AND TRANSACTIONS
Often you need to resort to a low level strategy of testing and debugging contracts and transactions. This section introduces some debug tools and practices you can use. In order to test contracts and transactions without real-word consequences, you best test it on a private blockchain. This can be achieved with configuring an alternative network id (select a unique integer) and/or disable peers. It is recommended practice that for testing you use an alternative data directory and ports so that you never even accidentally clash with your live running node (assuming that runs using the defaults. Starting your gexp with in VM debug mode with profiling and highest logging verbosity level is recommended:
gexp –datadir ~/dapps/testing/00/ –port 30310 –rpcport 8110 –networkid 4567890 –nodiscover –maxpeers 0 –vmdebug –verbosity 6 –pprof –pprofport 6110 console 2>> ~/dapp/testint/00/00.log
Before you can submit any transactions, you need set up your private test chain. See Test Networks.
// create account. will prompt for password
personal.newAccount();
// name your primary account, will often use it
primary = exp.accounts[0];
// check your balance (denominated in expanse)
balance = web3.fromWei(exp.getBalance(primary), “expanse”);
// assume an existing unlocked primary account
primary = exp.accounts[0];

// mine 10 blocks to generate expanse

// starting miner
miner.start(4);
// sleep for 10 blocks (this can take quite some time).
admin.sleepBlocks(10);
// then stop mining (just not to burn heat in vain)
miner.stop();
balance = web3.fromWei(exp.getBalance(primary), “expanse”);
After you create transactions, you can force process them with the following lines:
miner.start(1);
admin.sleepBlocks(1);
miner.stop();
You can check your pending transactions with:
// shows transaction pool
txpool.status
// number of pending txs
exp.getBlockTransactionCount(“pending”);
// print all pending txs
exp.getBlock(“pending”, true).transactions
If you submitted contract creation transaction, you can check if the desired code actually got inserted in the current blockchain:
txhash = exp.sendTansaction({from:primary, data: code})
//… mining
contractaddress = exp.getTransactionReceipt(txhash);
exp.getCode(contractaddress)
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Accessing contracts and transactions
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Outline
EXPANSE HIGH LEVEL LANGUAGES
WRITING A CONTRACT
COMPILING A CONTRACT
CREATE AND DEPLOY A CONTRACT
INTERACTING WITH A CONTRACT
CONTRACT METADATA
TESTING CONTRACTS AND TRANSACTIONS