Io is a dynamic, prototype-based language built on a few simple ideas taken from Smalltalk (all values are objects), Self (prototypes, not classes), NewtonScript (differential inheritance), Act1 (actors and futures for concurrency), Lisp (code is a runtime-inspectable message tree), and Lua (small and embeddable).

Its guiding design principle is simplicity and power through conceptual unification — a handful of primitives stand in for features most languages keep separate:

The VM is written in portable C and now targets WebAssembly (WASI) by default. The same binary runs under wasmtime, Node.js, or directly in the browser via a bidirectional Io↔JavaScript bridge.

• Minimal Syntax — Expressions only. No keywords, no statements, no special forms. Every expression is a message send, including assignment, operators, and control flow.
• Prototype-Based Objects — No classes. New objects are clones of existing ones, slots are created on write, and any object can be used as a prototype for any other.
• Differential Inheritance — Clones inherit by reference until a slot is assigned, so most objects carry only the deltas that distinguish them from their parents.
• Multiple Inheritance — Every object has a list of protos; lookup is depth-first with runtime loop detection.
• Messages as Code — Messages form trees that can be inspected and rewritten at runtime. Argument evaluation can be deferred, so if, while, and for are implementable in Io itself.
• Runtime Introspection — Slots, protos, message trees, and block source code are all inspectable and modifiable at runtime.
• Actors and Futures — Any object can receive asyncSend or futureSend messages and run its own coroutine. Futures are transparent (they become their result) and detect deadlocks automatically.
• Coroutines Everywhere — Concurrency uses cooperative user-level coroutines rather than OS threads, scaling to thousands of active tasks without thread-level overhead.
• First-Class Continuations — the VM is callcc-capable: the current computation can be captured as a first-class value and later resumed, stored, or serialized.
• Unified Sequences — Strings, buffers, and numeric vectors share one Sequence primitive, with multiple item types, text encodings, and SIMD acceleration on float vectors.
• WASM/WASI Target — The default build produces a single WebAssembly module that runs under wasmtime, Node.js, or in browsers, with full access to the DOM when hosted in a page.
• Io↔JS Bridge — Host integration on the browser target is bidirectional: Io can call JavaScript functions and JavaScript can call Io methods, replacing the native-addon model of earlier Io releases.
• Embeddable Core — The VM ships as a single WebAssembly module under a BSD license, embeddable in any WASM host — browsers, Node.js, or native apps via wasmtime or wasmer.

For AI agents: llms.txt (curated index) and llms-full.txt (full content).