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2026-05-17 | 🏁 Word Meter PureScript Port: Finish It 🎙️

🧭 Why this post

🎙️ The Word Meter is the little single-button page that counts every word spoken around you in your browser, and over the last several weeks it has been growing a second implementation, written in PureScript, slice by slice, behind the production JavaScript build.

🏁 This wrap-up PR closes out the port and lands the cutover in the same PR: every slice from one through 9c is shipped, the v0.1.1 live-tick and post-reload-sanity fixes are in, the recognizer pre-flight is one-shot, the two builds reached feature parity through the same Playwright contract, and then the JavaScript build was retired in favor of the PureScript bundle at quartz/static/word-meter.js.

🪞 What follows is a short tour of how the wrap-up shook out: the comment cleanup that came first, the comparative analysis that confirmed parity, the migration reflections, and the cutover itself which turned out to be small enough to do in the same PR.

🧹 Cleaning up the historical comments

📝 The first pass was a sweep across the PureScript source for any comment that still talked about slice numbers, the historical journey of getting there, or the wiring that was about to land in the next slice.

🔍 Half a dozen modules carried prose like “the slice-9b orchestrator picks OnDevicePath” or “slice 9 will wire the real recognition.onerror callback”. Useful while the work was in flight, noise once everything is in tree. Each one got rewritten to describe what the code does today rather than how it got there.

🪶 The result is a codebase a new reader can pick up without a map of when each piece arrived. The historical journey lives in the spec and in this post; the source code talks about behavior.

📊 What the comparative analysis turned up

🔬 The second pass was a careful walk through the legacy word-meter.js against the current PureScript bundle, function by function, branch by branch, looking for any product-visible gap.

✅ The good news is that every user-visible feature has parity. Start and stop counting, the live word count, the live rate tiles over short, long, and overall windows, the captions strip with its thirty-second decay, the event log capped at two hundred sessions, the wake-lock and keep-awake toggle, the recognition error banner with the same code classification, the cloud-path recognizer with two-hundred-and-fifty-millisecond auto-restart, the on-device pre-flight with transparent cloud fallback, the one-shot retry on runtime language-not-supported — all of them behave the same way in both builds.

⚠️ The differences that survive the cutover are catalogued in the spec, and the most pleasant surprise is the local-storage one. During the dual-build era the PureScript build used its own key so the two implementations could coexist without stepping on each other. At cutover the question was whether to migrate the legacy data into a new key or to just point the PureScript codec at the legacy key. With essentially one user — the person who already resets daily to get the new daily count — the answer was the second one. The PureScript persistence module now reads and writes the legacy key directly. No migration code, no parallel key, no scheduled cleanup task.

📦 Two smaller differences are intentional improvements the PureScript build carries: it persists active-listening milliseconds and the cloud-fallback flag across reloads, which the legacy build silently dropped. After a reload the legacy build’s rate tiles divided by a denominator of about one millisecond and produced absurd numbers; the PureScript build does not. Both new fields are decoded with optional defaults, so any legacy payload that already exists in local storage continues to load cleanly.

🕊️ The remaining differences are tone and minor surface area. The copy-status text uses sentence case in PureScript and lower case in JavaScript. The clipboard fallback path that uses an off-screen textarea and the deprecated execCommand call is absent in PureScript. The legacy build mirrored every diagnostic entry to the developer console, the PureScript build does not. None of these are regressions; they are listed as optional follow-up.

🪞 What the migration taught us

🧠 The headline win was a vocabulary of impossible states. The legacy build had a wake-lock-held boolean and a separate status-text string, and they could disagree if anyone forgot to update both. The PureScript build has a single algebraic data type with three constructors, and that whole class of bug is gone. The recognition path is a typed value with two constructors instead of a string compared with double-equals. The recognition error code is a closed sum type with a predicate for transient and a predicate for permission-denied, instead of string comparisons against an array of magic constants.

🧪 The second win was a capability stack that is genuinely swappable. Every effect the app needs lives behind a typeclass with a production AppM instance and at least one test newtype. The test suite drives the entire orchestrator under deterministic test newtypes that never touch the browser, which means whole code paths — visibility re-acquisition of the wake lock, recognition auto-restart on onend, the one-shot cloud fallback on a runtime language-not-supported — are unit-tested. In JavaScript these paths were untestable without a real browser, so they shipped on hope.

🧰 The third win was a typed FFI boundary. Every JavaScript shim in the port is thin: no module state, no decisions, no silent failures. Every fallible operation returns an either with a domain-specific error type, and every left ends up in the diagnostics drawer verbatim. The “never silently swallow errors” rule that lives in AGENTS.md is now enforced by the FFI contract, not by reviewer vigilance.

🛠️ The compiler is a refactoring tool. Splitting the recording module into four files, introducing an instant type for every timestamp in the program, introducing a locale newtype, replacing two boolean flags with the wake-lock state algebraic data type — each one of these landed without a single runtime regression because the compiler walked us to every call site. The legacy build’s equivalent refactor would have required a global search and a prayer.

🍰 Slicing vertically pays. Every slice from one through 9c delivered end-to-end user-visible functionality. We never built a horizontal layer like a virtual DOM library or a capability stack or a persistence module as a slice on its own. Each one grew in service of the feature that needed it. That kept the port shippable on every Friday.

🪧 The cutover, in the same PR

🪜 Once the comparative analysis came back clean, the cutover was small enough to land in the same PR. There was no real user other than the repo owner, and the daily-reset workflow meant that even if the local-storage question went badly, the worst case was a one-day setback. So the cutover happened immediately: the PureScript persistence module now reads and writes the legacy local-storage key directly, the PureScript bundle now compiles to quartz/static/word-meter.js overwriting the location the legacy IIFE used to occupy, the staging file at quartz/static/word-meter-ps.js is gone, and the node-vm sandbox suite that exercised the legacy IIFE is gone.

✂️ The Playwright fixture lost its conditional loader and now points at the single bundle. The end-to-end spec lost its build parameter. The continuous-integration workflow lost its references to the staging artifact. The spec lost its slice-ten plan and gained a slice-ten “shipped” section that narrates what landed. The content page that loads the bundle from static/word-meter.js was untouched, because that path now resolves to the PureScript build.

🧷 The acceptance criteria collapsed to a one-line check: on a fresh page load, the PureScript bundle is the one running, and a localStorage key written by the legacy build before the cutover continues to load. Both held.

📌 The port is done. The next feature added to the Word Meter — whatever it is — adds to one codebase instead of two.

📚 Book Recommendations

📖 Similar

• Domain Modeling Made Functional by Scott Wlaschin is relevant because it is the clearest articulation in print of the move from “stringly typed” branches to algebraic data types that make impossible states unrepresentable, which is exactly what the Word Meter port did with wake-lock state, recognition path, and recognition error codes.
• Type-Driven Development with Idris by Edwin Brady is relevant because it shows how letting the type system guide a refactor — exactly the experience of replacing two booleans with an algebraic data type and watching the compiler list every call site — scales from toy examples to whole programs.

↔️ Contrasting

• A Philosophy of Software Design by John Ousterhout is relevant because it argues for deep modules with simple interfaces hiding a lot of complexity, and a strict capability-pattern port like this one deliberately exposes the typeclass interface to make swapping implementations possible, accepting more surface area in exchange for testability.

🔗 Related

• Working Effectively with Legacy Code by Michael Feathers is relevant because the playbook here — keep the legacy build alive, add a parallel build behind a feature flag, drive both through the same selector contract, then subtract the legacy build once the parallel build has parity — is the textbook seam strategy applied to a browser tool.
• Designing Data-Intensive Applications by Martin Kleppmann is relevant because the question the cutover faced — migrate the legacy storage payload into a new key, or point the new codec at the old key and accept it via a forgiving decoder — is exactly the on-the-wire compatibility tradeoff Kleppmann walks through for evolving schemas in production systems, and choosing the forgiving codec is one of his recurring recommendations when there is no audience constraint forcing the other choice.