Introduction

Crow is a programming language for people who work with data and ship what they find. It looks like Python, compiles to Rust, and makes three promises:

1. Parallel by default. A for loop runs its iterations concurrently. You never write threads, async, or locks — you write loops.
2. Errors are values. No exceptions, no try/catch, no null. A failed operation returns a value that carries its own story (.ok, .reason), propagates safely, and refuses to be silently misread.
3. One way to do things. One call syntax (f[x]), one string quote ('), one loop, one way to build a map. When Crow rejects a feature you expect, a Design Note in the specification explains why — and the compile error tells you what to write instead.

The language ships as a single binary containing an interpreter (for immediate feedback), a compiler to Rust (for production speed; the Rust toolchain it needs is fetched once, on the first native build), tensors, data frames, a database layer, and a reactive web framework. The final chapter of this book builds a live dashboard using all of them.

Crow is in alpha: the language still changes between releases, the changelog in the repository says what moved, and this book is rebuilt for every change, so what you read matches the release you install.

Who This Book Is For

You've written some Python, R, MATLAB, or JavaScript. You don't need to know Rust — Crow compiles to Rust, you never read the output.

How to Read It

Two parts, one language. Part I is the book: read it in order. Getting Started installs Crow and meets the toolchain, the guessing game builds a small program before any theory, and the chapters that follow build the language up one idea at a time — concepts, functions and scope, collections, parallelism, errors, modules, testing — each using only what came before it. Then the applied layers: a hands-on data project, databases, web UIs, and a final project that uses everything. Crow by Example closes Part I with one runnable program per area of the language. Whatever language you're arriving from — Python, JavaScript, R, MATLAB, Julia, Go, Rust, Elixir, Ruby, Swift, Zig — Appendix B lists the habits to unlearn, and Appendix A is the language on one page.

Part II is the reference: the standard library module by module, every function linked to the example that shows it, and the language specification — the source of truth (docs/language-spec.md in the repository), with a Design Note for every feature Crow deliberately leaves out. Consult it whenever the book says "the spec covers the rest", and read its Design Notes whenever you wonder "why can't I…". The book teaches; the spec specifies.

Every Block Runs

Every code block in this book is executed by crow test --book in both of Crow's execution modes on every change: what you read is what runs. And every block can be run where it stands. From a checkout of the repository run crow book and open the printed address: each block gets ▶ Run (the interpreter), ⚙ Compile (the native binary), ✓ Test (its test blocks), ↺ Reset, and ■ Stop. The block itself is editable — change it and run again. The same binary that verifies the book serves it.