Introduction

Pleno is a dynamically typed language with optional type annotations that are checked when the program runs. It compiles to bytecode for a register-based virtual machine written in Rust, and comes with a package manager, a formatter, a linter, a test runner, a language server, a debugger and a VS Code extension.

class Account {
    private balance: Float = 0.0;

    fn deposit(amount: Float) {
        if (amount <= 0.0) {
            throw Error("deposit must be positive");
        }
        this.balance += amount;
    }

    fn total(): Float {
        return this.balance;
    }
}

let account = Account();
account.deposit(25.0);
print(`Balance: ${account.total()}`);

Run

What it offers

• Annotations you can add gradually. let x = 5 works; let x: Int = 5 is checked, and so are parameters, return values, fields, generics with in/out variance, Result and Option.
• Several styles in one language. Classes with real private fields, interfaces, enums with data and methods, pattern matching, closures, the pipeline operator and partial application.
• Async built in. async/await, promises, spawn, channels and mutexes.
• A standard library for everyday programs. Files, paths, processes, HTTP client and server, JSON, YAML, TOML, XML, CSV, regular expressions, dates, cryptography, SQLite, logging and testing.
• Tools that come together. pleno runs, checks, formats, lints and tests; plm creates projects and manages dependencies; the VS Code extension installs everything.

Status

Pleno is at version 0.1, before 1.0. The language can change between releases, and the changelog lists what did. The 1.0 plan lists what is left, including:

• Speed: the interpreter is currently slower than CPython on simple benchmarks.
• Static checking: pleno check finds syntax errors, undefined names, wrong argument counts and literals that contradict their annotations, but most type errors appear only when the code runs.
• Missing features: destructuring, TCP sockets and a public package registry. Pages that describe features which do not exist yet mark them as Planned.

Ready to try it? Install Pleno.