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2026-05-15 | 🎙️ Word Meter Slice 9b — On-Device Pre-Flight 📦

🎯 What Shipped

• 📦 Slice 9b of the Word Meter PureScript port teaches the recognition layer to prefer the new on-device speech model whenever a recent Chromium build exposes the static availability and install methods on the SpeechRecognition constructor.
• 🌐 When the on-device path is not viable, the orchestrator falls back to the existing cloud path silently — counts and behavior look identical, but speech stays off the network for everyone whose browser already has the language pack ready.
• 🔍 A new diagnostic line in the drawer records which path was chosen on every Toggle-to-start, so curious users can verify that their session ran locally.

🧠 Why The Pre-Flight Belongs In Its Own Slice

• 🪜 The original slice 9 in the spec was deliberately split into three smaller end-to-end slices so each step is independently user-visible and reviewable.
• 🛡️ Slice 9a established the real SpeechRecognition wiring on the cloud path; everything works in any browser exposing the Web Speech API.
• 🧪 Slice 9b adds the pre-flight as a layer on top of that wiring, never the other way around — if the static availability check is missing, the entire pre-flight is skipped and the orchestrator behaves exactly like slice 9a.
• 🩹 Slice 9c is reserved for the one runtime failure mode that the pre-flight cannot catch: browsers that resolve availability to ready and then reject start with a language-not-supported error.

🧩 The Typed Surface Area

• 🆕 The recognition FFI module grew two new shims. The first checks whether the constructor exposes the static availability and install methods. The second wraps the asynchronous availability and install round trip in a continuation-passing API that the PureScript layer translates into an Either with a typed unavailable reason.
• 🎁 The unavailable reason is a closed sum type with four variants: API absent, unsupported language, install failed with a detail string, and availability rejected with a detail string. Each variant maps to exactly one observable failure mode from the browser, and a rendering function projects each variant into a stable diagnostic string.
• 📜 The recognition capability gained two new methods that mirror the FFI: prepare the on-device language pack with a progress callback, and start the recognizer with the process-locally hint set to true. Both retain the callback-shaped error surface that the rest of the recognition layer already uses.

🎼 Orchestration In Main

• 🎬 When the user toggles on, the orchestrator first asks whether the SpeechRecognition constructor exists at all. If not, the cloud-path code is never reached, and a recognition-unavailable diagnostic is recorded.
• 🛰️ Otherwise, the orchestrator asks whether the on-device pre-flight API is exposed. If not, it records that the on-device API was absent and starts the cloud recognizer immediately.
• 🚀 If the pre-flight API is exposed, the orchestrator hands off to prepare the language pack. While the install is in flight, the status row temporarily reads downloading on-device language pack to give the user a hint that the slight delay is intentional.
• 🪞 When the pre-flight resolves, the orchestrator clears the status override, re-reads the session, and proceeds with the on-device recognizer only if the user has not hit Stop in the meantime. Any unavailable outcome logs a falling-back-to-cloud diagnostic and starts the cloud recognizer instead.

🧪 Tests Drive Confidence

• 🔬 New unit tests cover the typed rendering of every unavailable variant, the recording recognition capability surfacing the prepare-then-start call sequence, and the reducer treatment of the new recognition status override field — including that any Toggle-stop, Reset, or empty-string set returns the override to the idle empty value.
• 🌐 New Playwright tests confirm two end-to-end invariants. The diagnostic drawer carries the cloud-fallback line whenever the pre-flight is short-circuited for test determinism. The recognition status override is always empty in the idle state.

🪤 A Surprise From Headless Chromium

• 💥 Headless Chromium actually exposes the new on-device API, but the underlying model is not installed, and calling the availability method crashes the renderer rather than rejecting cleanly.
• 🚧 To keep the Playwright suite deterministic without painting the production code with test-only branches, the FFI checks one window flag — disable on-device pre-flight — and skips the real availability call when set. The e2e fixture sets that flag before loading the bundle, so every test exercises the cloud path through the diagnostic that says the on-device API was absent.
• 🏭 Production browsers never see the flag and always run the full pre-flight. Unit tests cover every typed branch of the on-device path with the recording recognition capability, so the test-only flag does not leave a coverage hole.

🪞 What The User Sees

• 👤 To the user, almost nothing changes. The button still says start counting, the count still rolls forward, the captions still decay over thirty seconds, and the keep-awake toggle still requests a wake lock.
• 🔔 The one small new affordance is the temporary status line announcing the language pack download. After the download finishes, the status returns to the familiar listening text.
• 📒 For anyone who opens the diagnostics drawer, a single line tells the truth: this session ran on-device, or this session fell back to the cloud, with the typed reason in tow.

🚦 What Comes Next

• 🛣️ Slice 9c is the last recognition slice on the roadmap. It teaches the runtime to handle one stubborn class of misbehavior: browsers that say the on-device path is ready but reject start at runtime with language-not-supported. A new field on the session guards a one-shot retry on the cloud path so that a misbehaving browser cannot loop forever.
• 🎯 After 9c the port reaches parity with the legacy build, and slice 10 — the cutover that points the actual word-meter page at the PureScript bundle — becomes safe to land.

📚 Book Recommendations

📖 Similar

• Designing Data-Intensive Applications by Martin Kleppmann is relevant because the pre-flight pattern in this slice is a small example of the same idea Kleppmann discusses at scale: probe a system’s actual capabilities at runtime and fall back gracefully when an optimistic path turns out not to be supported.
• Site Reliability Engineering by Betsy Beyer and colleagues is relevant because the silent cloud fallback in the orchestrator embodies the SRE principle of degrading gracefully rather than presenting the user with an error from a path they never knew existed.

↔️ Contrasting

• The Pragmatic Programmer by David Thomas and Andrew Hunt offers a contrasting voice: it warns against speculative generality, and slice 9b deliberately leaves the runtime retry path for slice 9c rather than building both at once because the failure mode that 9c addresses is narrow and best implemented when its triggering signal is concrete.

🔗 Related

• Type-Driven Development with Idris by Edwin Brady explores how typed sum types like the new unavailable reason eliminate entire classes of bugs at compile time, which is exactly the rule the FFI shim follows when it refuses to silently swallow any of the four observable browser failure modes.