How to code a precompiled contract

A primer on how precompiled contracts should be coded in AppLayer's BDK.

When creating precompiled contracts for AppLayer, there are a few rules that must be followed to ensure they work as intended. While each contract type has its own rules, some other rules apply to both. This will be explained and demonstrated further.

General contract rules

As a general stance, contracts must:

• Inherit from their respective base class, depending on their type (see below) and make sure you're passing the right arguments for their constructors

• Implement, initialize and manage variables within the state and database, as well as the respective view and non-view functions that manage them when required - those variables should be loaded during contract construction and saved by means of an overriden dump function

• Register callbacks for contract functions with their proper functors/signatures (if functions are called by an RPC eth_call or a transaction)

• Ensure that their assigned name and their own class name match - both contract constructors take a contractName string as an argument, i.e. if your contract is called "TestContract", your constructor's definition would be TestContract(...) : DynamicContract(interface, "TestContract", ...) - both names HAVE to match, otherwise a segfault may happen

• Declare view functions, non-view functions and events (if they exist) correctly, accoding to their specific types (explained further)

Rules for Protocol Contracts

Protocol Contracts specifically must:

• Inherit BaseContract from src/contract/contract.h

• Override ethCall() functions to parse transaction arguments, manage state changes during their processing (depending on whether the call is committing or not), and commit/revert variables when necessary

Rules for Dynamic Contracts

Dynamic Contracts specifically must:

• Inherit DynamicContract from src/contract/dynamiccontract.hand any type of SafeVariable used from the src/contract/variables folder

• Provide a ConstructorArguments tuple with the contract's constructor argument types for registering the contract, and two registering functions: registerContract() and registerContractFunctions(), for contract metadata, variables, functions, and events (both of which should be called inside the contract's constructor)

• Provide two constructors: one for creating the contract from scratch within ContractManager, and one for loading the contract from the database

• Only allow contract creation through a transaction call to the ContractManager contract

• Develop functions for handling your contract's creation and logic

• Override ethCall() functions to register and properly call those functions

• Override the dump() function to properly save the contract's variables in the database

• Set all of the contract's internal variables as private, inherit them from one of the many SafeVariable classes, and always reference them with this to ensure correct semantics - e.g. string name and uint256 value in Solidity should be SafeString name and SafeUint256_t value in C++, respectively - referencing them in your definition would be this->name, this->value, so on and so forth

• Allow loops using containers such as SafeUnorderedMap, but keep in mind how safe containers work

  • e.g. when you access a key from a SafeUnorderedMap, it'll check if it exists and copy only the key, not the entire map or its value - thus when iterating a loop, you can't assume the "temporary" value is the original one

  • We recommended you only loop inside view functions to ensure value safety, but you can do it on non-view functions as well, just be careful when doing so

• Trigger state changes only via transaction calls to contract functions

• Call updateState(true) at the end of the contract's constructor

Inherited functions and variables

Every contract within AppLayer's BDK inherits from the following classes, which means they can use their functions anywhere in their logic. Check the Doxygen docs for more details on each function's implementation, parameters, returns and overloads.

• ContractGlobals - global variables accessible through transaction/RPC calls, set by the State when calling the contract

Function	Description
getCoinbase	Get the coinbase address (creator of current block)
getBlockHash	Get the current block hash
getBlockHeight	Get the current block height
getBlockTimestamp	Get the current block timestamp

• ContractLocals - local variables accessible through transaction/RPC calls, set by ContractManager when calling the contract

Function	Description
getOrigin	Get the transaction's origin
getCaller	Get the transaction's caller
getValue	Get the transaction's value

• BaseContract - base class for all contracts, provides and stores the respective contract's information

Function	Description
getContractAddress	Get the contract's address
getContractCreator	Get the contract's owner
getContractChainId	Get the contract's chainId
getContractName	Get the contract's name
getDBPrefix	Get the contract's database prefix
getNewPrefix	Same as getDBPrefix() but with a user-defined prefix appended to it

For Dynamic Contracts specifically, you can also use the following:

Function	Description
emitEvent	Emit an event
isPayableFunction	Check if a function is payable or not
getContract	Get a pointer to a contract
callContractViewFunction	Call a contract's view function
callContractFunction	Call a contract's non-view function
callCreateContract	Call a contract's create function
getBalance	Get the current balance of an address
sendTokens	Send an amount of tokens to an address