CosmWasm: A Rust Framework Tutorial/Course 05

As we continue building our “Hello World” contract in CosmWasm, we will discuss the following imports as their usage come up.

use cosmwasm_std::{Binary, Deps, DepsMut, Env, MessageInfo, Response, StdResult};
// use cw2::set_contract_version;

use crate::error::ContractError;
use crate::msg::{ExecuteMsg, InstantiateMsg, QueryMsg};

The next code portion to be examined is

#[cfg_attr(not(feature = "library"), entry_point)]
pub fn instantiate(
    _deps: DepsMut,
    _env: Env,
    _info: MessageInfo,
    _msg: InstantiateMsg,
) -> Result<Response, ContractError> {
    unimplemented!()
}

cfg_attris a different version of cfg macro as it is used to apply multiple attributes conditionally to an item. In the above code, it conditionally applies entry_point attribute to a function when library feature is not enabled in crate’s configuration.

instantiate function takes multiple inputs and returns a single output Resultwhich is an enum type. It represents the result of an operation that can either succeed (Ok) or fail (Err). In case of success, Resposne type is returned which typically contains the response of contract’s execution. In typical cases, it would be success and failure messages, attributes, or some other data. In case of error, ContractError is returned. It is a custom Error type that provides information why some logic in contract failed.

As far as inputs are concerned!

• deps represents the dependencies required by the contract. In CosmWasm, dependencies provide access to various blockchain-related functionality, such as reading and writing data to the contract’s storage. deps of type DepsMut is usually used when the contract needs to modify the state of blockchain.
• env input provides information about the execution environment of the contract. It contains details like the current block height, time, chain ID, and more.
• info of type MessageInfo contains information about the message that triggered the contract execution. This includes the sender’s address, the funds attached to the message, and some other information.
• msg of type InstantiateMsg is a custom message type that carries data specific to contract initialisation.

Last but not least unimplemented!() in function body is a macro placeholder that signifies that the instantiate function has not been implemented yet. Let’s implement a basic contract by replacing the unimplemented!() macro with following code.

Ok(
    Response::new()
		    .add_attribute("message", "Hello, World")
)

That’s is it. We construct a new Response object by calling new method on it. Then we chain the constructor call with an add_attribute method which takes a key and message as inputs. Key in this case is message while message is “Hello, World”. In next chapter, we are going to write test this basic functionality.