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# Overview
Leo is a functional, statically-typed programming language built for writing private applications.
## Compiler Architecture
## <a name='TableofContents'></a>Table of Contents
<!-- generated by mermaid compile action - START -->
![~mermaid diagram 1~](/.resources/README-md-1.png)
<details>
<summary>Mermaid markup</summary>
* [1. Overview](#1-overview)
* [2. Build Guide](#2-build-guide)
* [2.1 Install Rust](#21-install-rust)
* [2.2a Build from Crates.io](#22a-build-from-cratesio)
* [2.2b Build from Source Code](#22b-build-from-source-code)
* [3. Quick Start](#3-quick-start)
* [4. Flying Tour](#4-flying-tour)
* [4.1 Functions](#41-functions)
* [4.2 Testing](#42-testing)
* [4.3 Data Types](#43-data-types)
* [4.4 Circuits](#44-circuits)
* [4.5 Imports](#45-imports)
* [5. Contributing](#5-contributing)
* [6. License](#6-license)
```mermaid
graph LR
Pass1(Syntax Parser) -- ast --> Pass2(Type Resolver)
Pass2 -- imports --> Pass3(Import Resolver)
Pass3 -- statements --> Pass4
## 1. Overview
Welcome to the Leo programming language.
Pass2 -- statements --> Pass4(Synthesizer)
Leo exists to provide a simple high-level language that compiles to a rank one constraint system (R1CS) circuit.
With Leo, you can write circuits to support zero-knowledge tokens, private stable coins, and decentralized marketplaces.
Pass4 -- constraints --> Pass5(Program)
The syntax of Leo is influenced by JavaScript, Python, Scala, and Rust with a strong emphasis on readability and ease-of-use.
## 2. Build Guide
### 2.1 Install Rust
We recommend installing Rust using [rustup](https://www.rustup.rs/). You can install `rustup` as follows:
- macOS or Linux:
```bash
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
```
- Windows (64-bit):
Download the [Windows 64-bit executable](https://win.rustup.rs/x86_64) and follow the on-screen instructions.
- Windows (32-bit):
Download the [Windows 32-bit executable](https://win.rustup.rs/i686) and follow the on-screen instructions.
### 2.2a Build from Crates.io
We recommend installing Leo this way. In your terminal, run:
```bash
cargo install leo
```
</details>
<!-- generated by mermaid compile action - END -->
Now to use Leo, in your terminal, run:
```bash
leo
```
## Language Specification
### 2.2b Build from Source Code
* Programs should be formatted:
1. Import definitions
2. Circuit definitions
3. Function definitions
Alternatively, you can install Leo by building from the source code as follows:
## Defining Variables
Leo supports `let` and `const` keywords for variable definition.
```bash
# Download the source code
git clone https://github.com/AleoHQ/leo
cd leo
```let a = true;``` defines an **allocated** program variable `a` with boolean value `true`.
# Build in release mode
$ cargo build --release
```
```const a = true;``` defines a **constant** program variable `a` with boolean value `true`.
This will generate an executable under the `./target/release` directory. To run snarkOS, run the following command:
```bash
./target/release/leo
```
**Allocated** variables define private variables in the constraint system. Their value is constrained in the circuit on initialization.
## 3. Quick Start
**Constant** variables do not define a variable in the constraint system. Their value is constrained in the circuit on computation with an **allocated** variable.
**Constant** variables cannot be mutable. They have the same functionality as `const` variables in other languages.
```js
function add_one() -> {
let a = 0u8; // allocated, value enforced on this line
const b = 1u8; // constant, value not enforced yet
Use the Leo CLI to create a new project
return a + b // allocated, computed value is enforced to be the sum of both values
```bash
# create a new `hello_world` Leo project
leo new hello_world
cd hello_world
# build & setup & prove & verify
leo run
```
The `leo new` command creates a new Leo project with a given name.
The `leo run` command will compile the main program, generate keys for a trusted setup, fetch inputs, generate a proof and verify it.
Congratulations! You've just run your first Leo program.
## 4. Flying Tour
The best way to get to know Leo is by writing some code. We will fly through a high level overview of a Leo file.
To gain a deeper understanding of the Leo language, then check out the [developer documentation](https://developer.aleo.org/developer/getting_started/overview)
**Square Root Example**: Let's prove that we know the square root of a number.
**`src/main.leo`**
```rust // change this to leo
function main(a: u32, b: u32) -> bool {
return square_root(a, b)
}
```
Computations are expressed in terms of arithmetic circuits, in particular rank-1 quadratic constraint systems. Thus computing on an allocated variable always results in another allocated variable.
## Mutability
* All defined variables in Leo are immutable by default.
* Variables can be made mutable with the `mut` keyword.
function square_root(a: u32, b: u32) -> bool {
return a * a == b
}
```js
function main() {
let a = 0u32;
//a = 1 <- Will fail
test function test_square_root() {
let a: u32 = 5;
let b: u32 = 25;
let mut b = 0u32;
b = 1; // <- Ok
let result = square_root(a, b);
console.assert(result == true);
}
```
## Addresses
### 4.1 Functions
The `main` function is the entrypoint of a Leo program.
`leo run` will provide private inputs directly to the function for proving and store the program result in an output file.
Addresses are defined to enable compiler-optimized routines for parsing and operating over addresses. These semantics will be accompanied by a standard library in a future sprint.
The `square_root` function is called by `main` with private inputs `a` and `b` which are both unsigned `u32` integers.
```js
function main(owner: address) {
let sender = address(aleo1qnr4dkkvkgfqph0vzc3y6z2eu975wnpz2925ntjccd5cfqxtyu8sta57j8);
let receiver: address = aleo1qnr4dkkvkgfqph0vzc3y6z2eu975wnpz2925ntjccd5cfqxtyu8sta57j8;
assert_eq!(owner, sender);
assert_eq!(sender, receiver);
}
```
### 4.2 Testing
## Booleans
A naive way to test `square_root` would be to execute `leo run` several times on different inputs and check the output of the program each time.
Explicit types are optional.
```js
function main() -> bool {
let a: bool = true || false;
let b = false && false;
let c = 1u32 == 1u32;
return a
}
```
Luckily, we can write unit tests in Leo using the `test function` syntax.
In `test_square_root` we can sanity check our code without having to load in private inputs from a file every time.
Want to upgrade your test function into an integration test?
In Leo you can add a test context annotation that loads different sets of private inputs to make your test suite even more robust.
## Numbers
* The definition of a number must include an explicit type.
* After assignment, you can choose to explicitly add the type or let the compiler interpret implicitly.
* Type casting is not supported.
* Comparators are not supported.
The last line of `test_square_root` uses the console function `console.assert`.
This function along with `console.log`, `console.debug`, and `console.error` provide developers with tools that are run without
affecting the underlying constraint system.
### Integers
Supported integer types: `u8`, `u16`, `u32`, `u64`, `u128`
```js
function main() -> u32 {
let a = 2u32; // explicit type
let a: u32 = 1 + 1; // explicit type
### 4.3 Data Types
let b = a - 1; // implicit type
let c = b * 4;
let d = c / 2;
let e = d ** 3;
return e
}
```
Leo supports boolean, unsigned integer, signed integer, field, group element, and address data types.
Collections of data types can be created in the form of static arrays and tuples.
### Field Elements
```js
function main() -> field {
let a = 1000field; // explicit type
let a: field = 21888242871839275222246405745257275088548364400416034343698204186575808495617; // explicit type
let b = a + 1; // implicit type
let c = b - 1;
let d = c * 4;
let e = d / 2;
return e
}
```
### 4.4 Circuits
### Group Elements
An affine point on the elliptic curve passed into the Leo compiler forms a group.
Leo supports this set as a primitive data type.
**Circuits Example**
```js
function main() -> group {
let a = 1000group; // explicit type
let a = (21888242871839275222246405745257275088548364400416034343698204186575808495617, 21888242871839275222246405745257275088548364400416034343698204186575808495617)group; // explicit type
let b = a + 0; // implicit type
let c = b - 0;
return c
}
```
### Operator Assignment Statements
```js
function main() -> u32 {
let mut a = 10;
a += 5;
a -= 10;
a *= 5;
a /= 5;
a **= 2;
return a
}
```
## Arrays
Leo supports static arrays with fixed length.
```js
function main() -> u32[2] {
// initialize an integer array with integer values
let mut a: u32[3] = [1, 2, 3];
// set a mutable member to a value
a[2] = 4;
// initialize an array of 4 values all equal to 42
let b = [42u8; 4];
// initialize an array of 5 values copying all elements of b using a spread
let c = [1, ...b];
// initialize an array copying a slice from `c`
let d = c[1..3];
// initialize a field array
let e = [5field; 2];
// initialize a boolean array
let f = [true, false || true, true];
return d
}
```
### Multidimensional Arrays
```js
function main() -> u32[3][2] {
let m = [[0u32, 0u32], [0u32, 0u32]];
let m: u32[3][2] = [[0; 3]; 2];
return m
}
```
## Conditionals
Branching in Leo is different than traditional programming languages. Leo developers should keep in mind that every program compiles to a circuit which represents
all possible evaluations.
### If Else Ternary Expression
Ternary `if [cond] ? [first] : [second];` expressions are the cheapest form of conditional.
Since `first` and `second` are expressions, we can resolve their values before proceeding execution.
In the underlying circuit, this is a single bit multiplexer.
```js
function main() -> u32 {
let y = if 3==3 ? 1 : 5;
return y
}
```
### If Else Conditional Statement
Leo supports the traditional `if [cond] { [first] } else { [second] }` which can be chained using `else if`.
Since `first` and `second` are one or more statements, they resolve to separate circuits which will all be evaluated.
In the underlying circuit this can be thought of as a demultiplexer.
```js
function main(a: bool, b: bool) -> u32 {
let mut res = 0u32;
if a {
res = 1;
} else if b {
res = 2;
} else {
res = 3;
}
return res
}
```
### For loop
```js
function main() -> fe {
let mut a = 1field;
for i in 0..4 {
a = a + 1;
}
return a
}
```
## Functions
```js
function test1(a : u32) -> u32 {
return a + 1
}
function test2(b: fe) -> field {
return b * 2field
}
function test3(c: bool) -> bool {
return c && true
}
function main() -> u32 {
return test1(5)
}
```
### Function Scope
```js
function foo() -> field {
// return myGlobal <- not allowed
return 42field
}
function main() -> field {
let myGlobal = 42field;
return foo()
}
```
### Multiple returns
Functions can return tuples whose types are specified in the function signature.
```js
function test() -> (u32, u32[2]) {
return (1, [2, 3])
}
function main() -> u32[3] {
let (a, b) = test();
// (a, u32[2] b) = test() <- explicit type also works
return [a, ...b]
}
```
### Function inputs
Main function inputs are allocated private variables in the program's constraint system.
`a` is implicitly private.
```js
function main(a: field) -> field {
return a
}
```
Normal function inputs are passed by value.
```js
function test(mut a: u32) {
a = 0;
}
function main() -> u32 {
let a = 1;
test(a);
return a // <- returns 1
}
```
## Circuits
Circuits in Leo are similar to classes in object oriented langauges. Circuits are defined above functions in a Leo program. Circuits can have one or more members.
#### Circuit member values
Members can be defined as fields which hold primitive values.
```js
**`src/main.leo`**
```rust
circuit Point {
x: u32
y: u32
}
function main() -> u32 {
let p = Point {x: 1, y: 0};
return p.x
}
```
x: u32,
y: u32,
#### Circuit member functions
Members can also be defined as functions.
```js
circuit Foo {
function echo(x: u32) -> u32 {
return x
static function new() -> Self {
return Self {
x: 0,
y: 0,
}
}
function main() -> u32 {
let c = Foo { };
return c.echo(1u32)
}
```
#### Circuit member static functions
Circuit functions can be made static, enabling them to be called without instantiation.
```js
circuit Foo {
static function echo(x: u32) -> u32 {
return x
}
}
function main() -> u32 {
return Foo::echo(1u32)
}
```
#### `Self` and `self`
The `Self` keyword is supported in circuit functions.
```js
circuit Circ {
b: bool
static function new() -> Self { // Self resolves to Foo
return Self { b: true }
}
}
function main() -> bool {
let c = Foo::new();
return c.b
}
```
The `self` keyword references the circuit's members.
```rust
circuit Foo {
b: bool
function bar() -> bool {
return self.b
}
function baz() -> bool {
return self.bar()
function add() -> u32 {
return self.x + self.y
}
}
function main() -> bool {
let c = Foo { b: true };
return c.baz()
}
```
## Imports
Leo supports importing functions
}
} and circuits by name into the current file with the following syntax:
```js
import [package].[name];
```
#### Import Aliases
To import a name using an alias:
```js
import [package].[name] as [alias];
```
#### Import Multiple
To import multiple names from the same package:
```js
import [package].(
[name_1],
[name_2] as [alias],
);
```
#### Import Star
To import all symbols from a package:
Note that this will only import symbols from the package library `lib.leo` file.
```js
import [package].*;
```
### Local
You can import from a local file in the same package using its direct path.
`src/` directory by using its `[file].leo` as the `[package]` name.
```js
import [file].[name];
```
#### Example:
`src/bar.leo`
```js
circuit Bar {
b: u32
}
function baz() -> u32 {
return 1u32
}
```
`src/main.leo`
```js
import bar.(
Bar,
baz
);
function main() {
const bar = Bar { b: 1u32};
const z = baz();
let mut p = Point::new();
p.x = 4u32;
p.y = 6u32;
let sum = p.add();
console.log("The sum is {}", sum);
}
```
### Foreign
You can import from a foreign package in the `imports/` directory using its `[package]` name.
```js
import [package].[name];
```
Circuits in leo are similar to structures in other object-oriented languages.
They provide a composite data type that can store primitive values and provide functions for instantiation and computation.
#### Example:
`imports/bar/src/lib.leo`
```js
circuit Bar {
b: u32
}
```
The `static` keyword modifies the `new` function so it can be called without instantiating the circuit.
`src/main.leo`
```js
import bar.Bar;
Leo introduces `Self` and `self` keywords to access circuit member values.
function main() {
const bar = Bar { b: 1u32 };
}
```
### 4.5 Imports
### Package Paths
Leo treats directories as package names when importing.
```js
import [package].[directory].[file].[name]
```
Imports fetch other circuits and functions and bring them into the current file scope.
Leo supports imports for dependencies that are declared locally or in an imported package.
#### Example:
We wish to import the `Baz` circuit from the `baz.leo` file in the `bar` directory in the `foo` package
Importing packages can be accomplished using the `leo add` command in the CLI.
## 5. Contributing
`imports/foo/src/bar/baz.leo`
```js
circuit Baz {
b: u32
}
```
Please see our guidelines in the [developer documentation](https://developer.aleo.org/developer/additional_material/contributing)
`src/main.leo`
```js
import foo.bar.baz.Baz;
Thank you for helping make Leo better!
function main() {
const baz = Baz { b: 1u32 };
}
```
## Constraints
### Assert Equals
This will enforce that the two values are equal in the constraint system.
```js
function main() {
assert_eq!(45, 45);
assert_eq!(2fe, 2fe);
assert_eq!(true, true);
}
```
## Testing
Use the `test` keyword to add tests to a leo program. Tests must have 0 function inputs and 0 function returns.
```js
function main(a: u32) -> u32 {
return a
}
test function expect_pass() {
let a = 1u32;
let res = main(a);
assert_eq!(res, 1u32);
}
test function expect_fail() {
assert_eq!(1u8, 0u8);
}
```
## Logging
Leo supports `print!`, `debug!`, and `error!` logging macros.
The first argument a macro receives is a format string. This must be a string literal. The power of the formatting string is in the `{}`s contained.
Additional parameters passed to a macro replace the `{}`s within the formatting string in the order given.
#### `print!`
Directly calls the `println!` macro in rust.
```js
function main(a: u32) {
print!("a is {}", a);
}
```
#### `debug!`
Enabled by specifying the `-d` flag after a Leo command.
```js
function main(a: u32) {
debug!("a is {}", a);
}
```
#### `error!`
Prints the error to console.
```js
function main(a: u32) {
error!("a is {}", a);
}
```
# Leo Inputs
Private inputs for a Leo program are specified in the `inputs/` directory. The syntax for an input file is a limited subset of the Leo program syntax. The default inputs file is `inputs/inputs.leo`.
## Sections
A Leo input file is made up of sections. Sections are defined by a section header in brackets followed by one or more input definitions.
Section headers specify the target file which must have a main function with matching input names and types.
`inputs/inputs.leo`
```rust
[main] // <- section header
a: u32 = 1;
b: u32 = 2;
```
`src/main.leo`
```rust
function main(a: u32, b: u32) -> u32 {
let c: u32 = a + b;
return c
}
```
## Input Definitions
### Supported types
```rust
[main]
a: bool = true; // <- booleans
b: u8 = 2; // <- integers
c: field = 0; // <- fields
d: group = (0, 1)group // <- group tuples
```
### Arrays
```rust
[main]
a: u8[4] = [0u8; 4]; // <- single
b: u8[2][3] = [[0u8; 2]; 3]; // <- multi-dimensional
```
# Leo CLI
## Develop
### `leo new`
To setup a new package, run:
```
leo new {$NAME}
```
This will create a new directory with a given package name. The new package will have a directory structure as follows:
```
- inputs # Your program inputs
- inputs.leo # Your program inputs for main.leo
- outputs # Your program outputs
- src
- main.leo # Your program
- tests
- test.leo # Your program tests
- Leo.toml # Your program manifest
```
#### Flags
```rust
leo new {$Name} --bin
```
This will create a new directory with a given package name. The new package will have a directory structure as above.
```rust
leo new {$Name} --lib
```
This will create a new directory with a given package name. The new package will have a directory structure as follows:
```
- src
- lib.leo # Your program library
- Leo.toml # Your program manifest
```
### `leo init`
To initialize an existing directory, run:
```
leo init
```
This will initialize the current directory with the same package directory setup.
#### Flags
`leo init` supports the same flags as `leo new`
```rust
leo init --bin
```
```rust
leo init --lib
```
### `leo build`
To compile your program and verify that it builds properly, run:
```
leo build
```
### `leo test`
To execute unit tests on your program, run:
```
leo test
```
The results of test compilation and the constraint system will be printed:
```
INFO leo Running 2 tests
INFO leo test language::expect_pass compiled. Constraint system satisfied: true
ERROR leo test language::expect_fail errored: Assertion 1u8 == 0u8 failed
```
## Run
### `leo setup`
To perform the program setup, producing a proving key and verification key, run:
```
leo setup
```
Leo uses cryptographic randomness from your machine to perform the setup. The proving key and verification key are stored in the `target` directory as `.leo.pk` and `.leo.vk`:
```
{$LIBRARY}/target/{$PROGRAM}.leo.pk
{$LIBRARY}/target/{$PROGRAM}.leo.vk
```
### `leo prove`
To execute the program and produce an execution proof, run:
```
leo prove
```
Leo starts by checking the `target` directory for an existing `.leo.pk` file. If it doesn't exist, it will proceed to run `leo setup` and then continue.
Next any input files in the `inputs` directory are parsed and all input values are passed to the program.
Once again, Leo uses cryptographic randomness from your machine to produce the proof. The proof is stored in the `target` directory as `.leo.proof`:
```
{$LIBRARY}/target/{$PROGRAM}.leo.proof
```
### `leo verify`
To verify the program proof, run:
```
leo verify
```
Leo starts by checking the `target` directory for an existing `.leo.proof` file. If it doesn't exist, it will proceed to run `leo prove` and then continue.
After the verifier is run, Leo will output either `true` or `false` based on the verification.
## Remote
To use remote compilation features, start by authentication with:
```
leo login
```
You will proceed to authenticate using your username and password. Next, Leo will parse your `Leo.toml` file for `remote = True` to confirm whether remote compilation is enabled.
If remote compilation is enabled, Leo syncs your workspace so when you run `leo build`, `leo test`, `leo setup` and `leo prove`, your program will run the program setup and execution performantly on remote machines.
This speeds up the testing cycle and helps the developer to iterate significantly faster.
## Publish
To package your program as a gadget and publish it online, run:
```
leo publish
```
Leo will proceed to snapshot your directory and upload your directory to the circuit manager. Leo will verify that `leo build` succeeds and that `leo test` passes without error.
If your gadget name has already been taken, `leo publish` will fail.
## Deploy
To deploy your program to Aleo, run:
```
leo deploy
```
# Install
To install Leo from source, in the root directory of the repository, run:
```
cargo install --path .
```
## TODO
- Change `target` directory to some other directory to avoid collision.
- Figure out how `leo prove` should take in assignments.
- Come up with a serialization format for `.leo.pk`, `.leo.vk`, and `.leo.proof`.
## 6. License
[![License: GPL v3](https://img.shields.io/badge/License-GPLv3-blue.svg)](./LICENSE.md)