Release Notes

Io uses date-based versioning. This page collects the notable changes of the last twelve months — a period in which the VM was rewritten around a stackless evaluator and re-targeted from native binaries to WebAssembly. Entries marked tagged correspond to release tags in the repository; the rest are development milestones on the road between them. For the longer arc, see the Timeline.

Changes on master since the WASM merge, not yet part of a tagged release.

• WASI 0.2 component build — make component builds the VM as a WebAssembly component (wasm32-wasip2) alongside the core WASI module.
• Cooperative scheduler timer queue — wait parks the calling coroutine on a timer queue instead of busy-spinning; timed coroutines run concurrently and the VM idles in a single host wait until the next deadline.
• Live Interpreter — an in-browser REPL running the full VM, linked from the home page.
• Performance tracking — a benchmark page recording interpreter, shootout, and SIMD results over time.
• WASI 0.3 notes — design notes on adopting native-async components (future/stream) once host and toolchain support land.

Tagged 2026.04.20-native-final. The WASM branch merged to master (#495); WebAssembly/WASI is now the default and only build target. The final native build — CMake, Eerie, and DynLib — is frozen at this tag, with ongoing native work on the native branch.

• SIMD128 Vector backend — the Vector primitive gains a WebAssembly SIMD128 backend, restoring AltiVec-era vector acceleration on modern hardware and in the browser.
• 64-bit integer literals — integer literals no longer truncate to 32 bits (#420).
• Map equality — Map implements comparison for equality (#28).
• JSON singletons — asJson added to nil, true, and false (#27).
• isKindOf fix — ancestor checks use identity comparison (#201).
• Site and Book restructured for the WASM era; continuous integration rewritten for the WASM toolchain.

Tagged 2026.03.06-stackless-alpha. The first alpha of the stackless VM, together with the initial WebAssembly port and a new JavaScript integration model.

• WASM/WASI cross-compilation — the VM compiles to a single portable WebAssembly module that runs under wasmtime, Node.js, or the browser.
• Bidirectional Io↔JavaScript bridge — Io can call JavaScript functions and JavaScript can call Io methods, replacing the native addon model. DynLib, AddonLoader, and the Eerie addon infrastructure were removed.
• Async/await across the bridge — futures integrate with JavaScript Promises: the eval loop yields and resumes across the JS boundary, and an unrecognized message sent to a pending future implicitly awaits its result.
• BigInt — arbitrary-precision integers with transparent round-tripping across the Io↔JS bridge.
• Browser REPL — an interactive interpreter hosted in a web page, with demos for the bridge, DOM access, async, and callbacks.
• Strings default to UTF-8.

The recursive, C-stack-based evaluator was replaced with a heap-allocated frame state machine — the foundation the WASM port builds on. With no native C stack to switch, the platform-specific coroutine assembly is no longer needed.

• Iterative eval loop — evaluation runs on a frame state machine instead of C recursion.
• First-class continuations — the current computation can be captured, resumed, stored, or serialized (callcc, behind the IO_CALLCC build flag).
• Tail-call optimization — including through if branches.
• Robust exception unwinding — Io-level exceptions are unwound by the eval loop rather than C setjmp/longjmp.
• Resumable exceptions — Common Lisp–style signal / withHandler: a handler can resume at the signal site, fall through, or unwind.
• GC-managed eval frames — frames are pooled, garbage-collected objects; coroutines became frame-based and the old platform-specific coroutine library was removed.

Native ARM64 coroutine support for Apple Silicon and modern Linux (#476) — the last major addition to the native-era coroutine implementation before the stackless rewrite made platform-specific coroutine code unnecessary.