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Contract Methods

In ParallelChain Smart Contract Programming Model, a contract is like a Rust struct that controls access to persistent storage. Accounts can interact with contracts by submitting transactions that include a call command to invoke methods of the contract. These methods are sometimes called just "methods" for short.

Methods

The model defines Contract Methods as methods with the #[call] macro. Each of these corresponds to a method that can be called via a Call Command in the CBI Subprotocol. To allow methods to be called from the outside world, you must write method definitions within an impl Contract statement marked with the #[contract_methods] macro. The following example illustrates this:

#[contract_methods]
impl PrinceTheDog {

    #[call]
    pub fn eat_food(&mut self, food: DogFood) -> Poop {
        // ...
    }
}

Methods can change Contract Storage. However, mutations made in a Call Transaction only apply if the transaction is successful. This means that the transaction must have enough gas, not have panicked during execution, and so on.

A function can be called an Action Method only if:

The #[call] macro is added above the function declaration. Its arguments (if any) implement BorshDeserialize. Its return value (if any) implements BorshSerialize.

Accepting Parameters and Returning Values

Some of the code snippets provided in this document show Contract Methods that accept arguments (in addition to borrow of the Contract struct) and/or return a value. To enable a contract to receive arguments from and return values to callers, both the contract and the caller need to agree on a serialization format.

The pchain-sdk expects callers to serialize method arguments using the Borsh serialization standard. It generates code to serialize values into borsh for inclusion in a Transaction's Receipt. To be precise, a Call Transaction specifies the Contract Method to call and provide the arguments for the call by including a borsh-serialized data structure Option<Vec<Vec<u8>> in its arguments field, and then returning a borsh-serialized value (if any).