# Probl

> Probl is a probabilistic programming language for game odds, forecasts and decisions under uncertainty. Describe random choices, evidence and the questions to report; Probl calculates or estimates the probabilities of the outcomes.

Programs use the `.probl` extension. Run them in the browser playground, or install the CLI with `cargo install probl-cli --locked` (Rust 1.85 or later) and run `probl run model.probl`.

A first model:

```probl
let total ~ 2d6
report total >= 10 as "Rolling 10 or more"
```

Key language rules:

- `=` binds a value or distribution; `~` draws an outcome. Reusing a drawn value refers to the same event; drawing again makes a fresh choice.
- `observe` conditions the model on evidence. Place observations before reports. `report` summarizes weighted outcomes.
- Enumeration is the default: Probl follows possible outcomes and combines equivalent states. Sampling must be selected explicitly; there is no automatic fallback to sampling.
- Reports identify the calculation method and any unresolved probability. Calculations use floating-point arithmetic; sampling results are estimates.

The language guide describes the website's engine version. Upstream documentation and the hosted playground may use a different revision. Consult the reference semantics for precise rules and supported operations.

## Documentation

- [Language guide](https://probl.dev/guide/): Syntax, distributions, evidence, reports, execution modes, examples and built-in functions.
- [Changelog](https://probl.dev/changelog/): Published release notes from the Probl repository, including new features, breaking changes and fixes.
- [Reference semantics](https://github.com/garlab/probl/blob/main/docs/semantics.md): Detailed language contracts, inference behavior, limits and error handling; takes precedence over the guide where they disagree.
- [Project README](https://github.com/garlab/probl/blob/main/README.md): Installation, command-line usage and project overview.

## Tools and examples

- [Playground](https://playground.probl.dev/): Run and share models in a browser without installation.
- [Example models](https://probl.dev/#examples): Models for games, forecasting, calendars and decisions, with links to run them in the playground.
- [Source repository](https://github.com/garlab/probl): Language implementation, example programs and development documentation.
- [Rust API](https://docs.rs/probl): Documentation for using Probl from Rust.

## Optional

- [Privacy policy](https://probl.dev/privacy/): Hosting, analytics, browser storage, shared models and data protection rights for the website and playground.
- [Content policy](https://probl.dev/content-policy/): Model ownership, sharing, open-source licences and use of the hosted sites.
- [Testing guide](https://github.com/garlab/probl/blob/main/docs/testing.md): How numerical results and language behavior are checked.