Whitepaper - Ethereum Classic With Cover

Ethereum Classic Documentation, Release 0.1
Interacting with a contract
Interaction with a contract is typically done using an abstraction layer such as the eth.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 = eth.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
ether 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 ether.
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
1.7. Contracts and Transactions 83