Hello, Leo

Initialize the project

Use the Leo Command Line Interface (CLI) to create a new project. In your terminal, run:

leo new hello
cd hello

This creates a directory with the following structure:

hello/
├── .gitignore # A default `.gitignore` file for Leo projects
├── program.json # The manifest for the Leo project
├── tests/
  └── test_hello.leo # The Leo source code for unit tests
└── src/
  └── main.leo # The Leo source code

Unpacking the Project

The Manifest

program.json is the Leo manifest file that configures the package.

program.json

{
  "program": "hello.aleo",
  "version": "0.1.0",
  "description": "",
  "license": "MIT",
  "dependencies": null,
  "dev_dependencies": null
}

The program field contains the official program ID. After network deployment, other developers can use this ID to find the program. It must match the program name in main.leo. Otherwise, compilation fails.

Dependencies will be added to the field of the same name, as they are imported. Dependencies that are only used during development and not in production will be added to the dev_dependencies field.

The Code

The src/main.leo file is the entry point of a Leo project. It initially contains a function named main. The following sections explain the structure of a Leo file.

src/main.leo

// The 'hello' program.
program hello.aleo {
    @noupgrade
    constructor() {}

    fn main(public a: u32, b: u32) -> u32 {
        let c: u32 = a + b;
        return c;
    }
}

The keyword program indicates the name of the program inside the Leo file. In this case, it is hello.aleo. As mentioned before, this program name must match the one in the program.json manifest file.

The keyword fn indicates an entry function definition in Leo. The main function takes an input a with type u32 and public visibility, and an input b with type u32 and private visibility (by default). The function returns one result with type u32. The function body is enclosed in curly braces { }.

fn main(public a: u32, b: u32) -> u32 {

Inside the main function we declare a variable c with type u32 and set it equal to the addition of variables a and b. Leo's compiler will check that the types of a and b are equal and that the result of the addition is type u32.

let c: u32 = a + b;

note

Leo is designed to detect many errors at compile time, via statically checked strong types. Try changing the type of any variable and seeing what Leo recommends with helpful error messages.

Last, we return the variable c. Leo will check that c's type matches the function return type u32.

return c;

The file also contains a constructor function. This function enables program upgrades. An upgrade can change some program logic and content after on-chain deployment.

@noupgrade
constructor() {}

The constructor controls program deployment and upgrades. The network runs its logic before each deployment and upgrade.

note

All programs must have an explicitly declared constructor function.

For now, we will leave it as is, which will prevent upgrades from occurring. For more details on how program upgradability works, and different patterns for upgrading your programs, check out Upgrading Programs.

Now, compile and run the program.

Build and Run

To compile the program, run:

leo build

On invoking the build command, Leo automatically creates a build/⁠ folder in the project directory. Inside it, every program - your own program and each dependency - gets its own build/{program}/ directory containing its compiled .aleo bytecode and ABI.

The leo run command will both compile and run the specified program. In your terminal, run:

leo run main 1u32 2u32

console output:

       Leo     2 statements before dead code elimination.
       Leo     2 statements after dead code elimination.
       Leo ✅ Compiled 'hello.aleo' into Aleo instructions.

⛓  Constraints

 •  'hello.aleo::main' - 33 constraints (called 1 time)

➡️  Output

 • 3u32

       Leo ✅ Finished 'hello.aleo::main' (in "./hello/build")

Deploying and Executing

After local tests, deploy the program and execute functions on-chain. Use leo deploy for deployment. Use leo execute to execute functions and generate a transaction. The transaction contains the required metadata and zero-knowledge proofs.

We have dedicated guides for both Deploying and Executing, so please check those out for more information!

Clean

Finally, you can remove all build artifacts with:

leo clean

console output:

Leo 🧹 Cleaned the build directory ./hello/build

Next Steps

To learn more about the Leo language and its syntax, start with the language overview.

To learn more about how to use the Leo CLI, start with the CLI overview.

To get started with some sample projects, check out the Leo By Example section.