Building a Smart Contract

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Building a Smart Contract

Icon LinkWriting A Sway Smart Contract

Icon LinkInstallation

To install the Fuel toolchain, you can use the fuelup-init script. This will install forc, forc-client, forc-fmt, forc-lsp, forc-wallet as well as fuel-core in ~/.fuelup/bin.

curl https://install.fuel.network | sh

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Having problems? Visit the installation guide or post your question in our forum Icon Link.

If you're using VSCode, we recommend installing the Sway extension Icon Link.

Icon LinkAlready have `fuelup` installed?

If you already have fuelup installed, run the commands below to make sure you are on the most up-to-date toolchain.

fuelup self update
fuelup update
fuelup default latest

Icon LinkYour First Sway Project

Icon BulbWant to skip this guide and just get started? Try our new scaffold CLI tool to generate a full-stack counter dApp in seconds:

pnpm create fuels

We'll build a simple counter contract with two functions: one to increment the counter, and one to return the value of the counter.

Start by creating a new, empty folder. We'll call it fuel-project.

mkdir fuel-project

Icon LinkWriting the Contract

Move inside of your fuel-project folder:

cd fuel-project

Then create a contract project using forc:

forc new counter-contract

You will get this output:

To compile, use `forc build`, and to run tests use `forc test`
----
Read the Docs:
- Sway Book: https://docs.fuel.network/docs/sway
- Forc Book: https://docs.fuel.network/docs/forc
- Rust SDK Book: https://docs.fuel.network/docs/fuels-rs
- TypeScript SDK: https://docs.fuel.network/docs/fuels-ts

Join the Community:
- Follow us @SwayLang: https://twitter.com/SwayLang
- Ask questions on Discourse: https://forum.fuel.network/

Report Bugs:
- Sway Issues: https://github.com/FuelLabs/sway/issues/new

Here is the project that forc has initialized:

tree counter-contract

counter-contract
├── Forc.toml
└── src
    └── main.sw

1 directory, 2 files

forc.toml is the manifest file (similar to Cargo.toml for Cargo or package.json for Node) and defines project metadata such as the project name and dependencies.

Open your project in a code editor and delete everything in src/main.sw apart from the first line.

Every Sway file must start with a declaration of what type of program the file contains; here, we've declared that this file is a contract. You can learn more about Sway program types in the Sway Book .

contract;

Next, we'll define a storage value. In our case, we have a single counter that we'll call counter of type u64 (a 64-bit unsigned integer) and initialize it to 0.

storage {
    counter: u64 = 0,
}

Icon LinkABI

ABI stands for Application Binary Interface. An ABI defines an interface for a contract. A contract must either define or import an ABI declaration.

It is considered best practice to define your ABI in a separate library and import it into your contract. This allows callers of the contract to import and use the ABI more easily.

For simplicity, we will define the ABI directly in the contract file itself.

abi Counter {
    #[storage(read, write)]
    fn increment();
 
    #[storage(read)]
    fn count() -> u64;
}

Icon LinkImplement ABI

Below your ABI definition, you will write the implementation of the functions defined in your ABI.

impl Counter for Contract {
    #[storage(read)]
    fn count() -> u64 {
        storage.counter.read()
    }
 
    #[storage(read, write)]
    fn increment() {
        let incremented = storage.counter.read() + 1;
        storage.counter.write(incremented);
    }
}

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storage.counter.read() is an implicit return and is equivalent to return storage.counter.read();.

Here's what your code should look like so far:

File: ./counter-contract/src/main.sw

contract;
 
storage {
    counter: u64 = 0,
}
 
abi Counter {
    #[storage(read, write)]
    fn increment();
 
    #[storage(read)]
    fn count() -> u64;
}
 
impl Counter for Contract {
    #[storage(read)]
    fn count() -> u64 {
        storage.counter.read()
    }
 
    #[storage(read, write)]
    fn increment() {
        let incremented = storage.counter.read() + 1;
        storage.counter.write(incremented);
    }
}

Icon LinkBuild the Contract

Navigate to your contract folder:

cd counter-contract

Then run the following command to build your contract:

forc build

  Compiled library "core".
  Compiled library "std".
  Compiled contract "counter-contract".
  Bytecode size: 84 bytes.

Let's have a look at the content of the counter-contract folder after building:

tree .

.
├── Forc.lock
├── Forc.toml
├── out
│   └── debug
│       ├── counter-contract-abi.json
│       ├── counter-contract-storage_slots.json
│       └── counter-contract.bin
└── src
    └── main.sw

3 directories, 6 files

We now have an out directory that contains our build artifacts such as the JSON representation of our ABI and the contract bytecode.

Icon LinkTesting your Contract with Rust

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Don't want to test with Rust? Skip this section and jump to Deploy the Contract .

We will start by adding a Rust integration test harness using a Cargo generate template. If you don't already have Rust installed, you can install it by running this command:

curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

Next, if you don't already have it installed, let's install cargo generate Icon Link:

cargo install cargo-generate --locked

Now, let's generate the default test harness with the following command:

cargo generate --init fuellabs/sway templates/sway-test-rs --name counter-contract

⚠️   Favorite `fuellabs/sway` not found in config, using it as a git repository: https://github.com/fuellabs/sway.git
🔧   Destination: /home/user/path/to/counter-contract ...
🔧   project-name: counter-contract ...
🔧   Generating template ...
🔧   Moving generated files into: `/home/user/path/to/counter-contract`...
✨   Done! New project created /home/user/path/to/counter-contract

Let's have a look at the result:

tree .

.
├── Cargo.toml
├── Forc.lock
├── Forc.toml
├── out
│   └── debug
│       ├── counter-contract-abi.json
│       ├── counter-contract-storage_slots.json
│       └── counter-contract.bin
├── src
│   └── main.sw
└── tests
    └── harness.rs

4 directories, 8 files

We have two new files!

The Cargo.toml is the manifest for our new test harness and specifies the required dependencies including fuels (the Fuel Rust SDK).
The tests/harness.rs contains some boilerplate test code to get us started, though doesn't call any contract methods just yet.

Now that we have our default test harness, let's add a useful test to it.

At the bottom of test/harness.rs below the can_get_contract_id() test, add the test_increment test function below to verify that the value of the counter gets incremented:

#[tokio::test]
async fn test_increment() {
    let (instance, _id) = get_contract_instance().await;
 
    // Increment the counter
    instance.methods().increment().call().await.unwrap();
 
    // Get the current value of the counter
    let result = instance.methods().count().call().await.unwrap();
 
    // Check that the current value of the counter is 1.
    // Recall that the initial value of the counter was 0.
    assert_eq!(result.value, 1);
}

Here is what your file should look like:

File: ./counter-contract/tests/harness.rs

use fuels::{prelude::*, types::ContractId};
 
// Load abi from json
abigen!(Contract(
    name = "MyContract",
    abi = "out/debug/counter-contract-abi.json"
));
 
async fn get_contract_instance() -> (MyContract<WalletUnlocked>, ContractId) {
    // Launch a local network and deploy the contract
    let mut wallets = launch_custom_provider_and_get_wallets(
        WalletsConfig::new(
            Some(1),             /* Single wallet */
            Some(1),             /* Single coin (UTXO) */
            Some(1_000_000_000), /* Amount per coin */
        ),
        None,
        None,
    )
    .await
    .unwrap();
    let wallet = wallets.pop().unwrap();
 
    let id = Contract::load_from(
        "./out/debug/counter-contract.bin",
        LoadConfiguration::default(),
    )
    .unwrap()
    .deploy(&wallet, TxPolicies::default())
    .await
    .unwrap();
 
    let instance = MyContract::new(id.clone(), wallet);
 
    (instance, id.into())
}
 
#[tokio::test]
async fn can_get_contract_id() {
    let (_instance, _id) = get_contract_instance().await;
 
    // Now you have an instance of your contract you can use to test each function
}
 
#[tokio::test]
async fn test_increment() {
    let (instance, _id) = get_contract_instance().await;
 
    // Increment the counter
    instance.methods().increment().call().await.unwrap();
 
    // Get the current value of the counter
    let result = instance.methods().count().call().await.unwrap();
 
    // Check that the current value of the counter is 1.
    // Recall that the initial value of the counter was 0.
    assert_eq!(result.value, 1);
}

Run cargo test in the terminal:

cargo test

If all goes well, the output should look as follows:

  ...
  running 2 tests
  test can_get_contract_id ... ok
  test test_increment ... ok
  test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.25s

Icon LinkDeploy the Contract

It's now time to deploy . We will show how to do this using forc from the command line, but you can also do it using the Rust SDK or the TypeScript SDK .

In order to deploy a contract, you need to have a wallet to sign the transaction and coins to pay for gas. Fuelup will guide you in this process.

Icon LinkDeploy To Testnet

Now, you can deploy the contract to the latest testnet with the forc deploy --testnet command.

If you don't have a wallet installed, the terminal will help you to create one. Then you can use the beta-5 faucet Icon Link to get some testnet coins.

forc deploy --testnet

The terminal will ask for the password of the wallet:

Please provide the password of your encrypted wallet vault at "~/.fuel/wallets/.wallet":

Once you have unlocked the wallet, the terminal will show a list of the accounts:

Account 0 -- fuel18caanqmumttfnm8qp0eq7u9yluydxtqmzuaqtzdjlsww5t2jmg9skutn8n:
  Asset ID                                                           Amount
  0000000000000000000000000000000000000000000000000000000000000000 499999940

Just below the list, you'll see this prompt:

Please provide the index of account to use for signing:

Then you'll enter the number of the account of preference and press Y when prompted to accept the transaction.

Finally, you will get back the network endpoint where the contract was deployed, a Contract ID and the block where the transaction was signed. Save the Contract ID, as you'll need this later to connect the frontend.

Contract deploy-to-beta-5 Deployed!

Network: https://beta-5.fuel.network
Contract ID: 0x8342d413de2a678245d9ee39f020795800c7e6a4ac5ff7daae275f533dc05e08
Deployed in block 0x4ea52b6652836c499e44b7e42f7c22d1ed1f03cf90a1d94cd0113b9023dfa636

Icon LinkCongrats, you have completed your first smart contract on Fuel ⛽

Here is the repo for this project Icon Link. If you run into any problems, a good first step is to compare your code to this repo and resolve any differences.

Tweet us @fuel_network Icon Link letting us know you just built a dapp on Fuel, you might get invited to a private group of builders, be invited to the next Fuel dinner, get alpha on the project, or something 👀.

Icon LinkNeed Help?

Get help from the team by posting your question in the Fuel Forum Icon Link.