Write a Module

Create and deploy your first move module.

If you have not already, open your newly initiatlized project within your code editor of choice on your host machine. Remember, the move_workspace directory within the container is bound to the /supra/move_workspace directory on your host machine. Any changes made to these files on the host machine will be reflected within the container.

If you are using VS Code, we highly recommend that you install the Supra Move extension.

1

Create a new move file within the sources directory.

In Move, global storage is a forest with trees rooted at an account address. Accounts can store both resource data and module code. Modules are deployed to an account, rather than a unique address being generated at deployment as you may be custom to with other VMs.

The sources directory holds the move modules that you will be working on within your project. Within the sources directory of your exampleContract package, create the hello_blockchain.move file.

execute me!

echo > /supra/move_workspace/exampleContract/sources/hello_blockchain.move

Once created, open the hello_blockchain.move file within your code editor and add the following code.

hello_blockchain.move

module hello_blockchain::message {
    use std::error;
    use std::signer;
    use std::string;
    use supra_framework::event;

    //:!:>resource
    struct MessageHolder has key {
        message: string::String,
    }
    //<:!:resource

    #[event]
    struct MessageChange has drop, store {
        account: address,
        from_message: string::String,
        to_message: string::String,
    }

    /// There is no message present
    const ENO_MESSAGE: u64 = 0;

    #[view]
    public fun get_message(addr: address): string::String acquires MessageHolder {
        assert!(exists<MessageHolder>(addr), error::not_found(ENO_MESSAGE));
        borrow_global<MessageHolder>(addr).message
    }

    public entry fun set_message(account: signer, message: string::String)
    acquires MessageHolder {
        let account_addr = signer::address_of(&account);
        if (!exists<MessageHolder>(account_addr)) {
            move_to(&account, MessageHolder {
                message,
            })
        } else {
            let old_message_holder = borrow_global_mut<MessageHolder>(account_addr);
            let from_message = old_message_holder.message;
            event::emit(MessageChange {
                account: account_addr,
                from_message,
                to_message: copy message,
            });
            old_message_holder.message = message;
        }
    }

    #[test(account = @0x1)]
    public entry fun sender_can_set_message(account: signer) acquires MessageHolder {
        let addr = signer::address_of(&account);
        supra_framework::account::create_account_for_test(addr);
        set_message(account, string::utf8(b"Hello, Blockchain"));

        assert!(
            get_message(addr) == string::utf8(b"Hello, Blockchain"),
            ENO_MESSAGE
        );
    }
}