--- title: "Flow ports to Rust in v0.319, Prettier 3.9 ships micromark and a faster Flow parser" url: https://daily.dev/posts/flow-ports-to-rust-in-v0-319-prettier-3-9-ships-micromark-and-a-faster-flow-parser-9kc56acfo source_url: https://daily.dev/posts/flow-ports-to-rust-in-v0-319-prettier-3-9-ships-micromark-and-a-faster-flow-parser-9kc56acfo type: freeform source: "Web Digest" published: 2026-06-28T04:17:50.316Z updated: 2026-06-28T04:18:15.786Z tags: ["webdev", "rust", "nextjs"] reading_time: 4 upvotes: 0 comments: 0 language: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Flow ports to Rust in v0.319, Prettier 3.9 ships micromark and a faster Flow parser **[Web Digest](https://daily.dev/sources/webdev_digest)** · 4 min read · 0 upvotes · 0 comments ## Summary A roundup of major frontend and tooling releases: Flow v0.319 completes its OCaml-to-Rust port with 2x faster type-checking, with AI (Claude) handling most of the mechanical translation. Prettier 3.9 ships a new micromark-based Markdown parser, YAML v2, and a Rust-based Flow parser that's 40% faster. es-toolkit surpasses 20M weekly npm downloads after a lodash compatibility layer drove adoption by Yarn Berry, Storybook, and Recharts. Turbopack is now stable for next dev in Next.js 15, delivering up to 76% faster cold starts and sub-200ms HMR. Also covered: a CLI tool for reducing JS/TS token counts before LLM calls, React 19's useActionState and useOptimistic hooks, and a CSS-only adventure game. ## Content **TLDR:** Flow's full OCaml-to-Rust port shipped in v0.319, delivering 2x faster type-checking with AI agents handling most of the mechanical translation work. Prettier 3.9 is out with a new Markdown parser, YAML v2, and a Rust-based Flow parser that's 40% faster. es-toolkit crossed 20 million weekly npm downloads after adding a lodash compat layer that pulled in Yarn Berry, Storybook, and Recharts. Turbopack is now stable for next dev in Next.js 15, with up to 76% faster cold starts and sub-200ms HMR. --- ## Flow v0.319: full OCaml-to-Rust port ships, 2x faster type-checking Meta's Flow type checker completed its port from OCaml to Rust in v0.319, released June 17. The Rust binary is 2x faster across most type-checking stages. The team picked Rust partly for ecosystem overlap with SWC and oxlint. What's interesting here is the AI story: Claude models handled the parser port in about 4 weeks and the checker port in roughly a month, working line-by-line from OCaml with strict no-unsafe rules and reviewer agents rejecting non-compliant changes. Human engineers still had to step in for larger wins like a work-stealing scheduler, but the mechanical translation was almost entirely AI-driven. [Read more](https://daily.dev/posts/OLU894cu5) ## Prettier 3.9: micromark replaces remark-parse, Flow parser is 40% faster Prettier 3.9 upgrades the Markdown parser from remark-parse v8 to micromark v4 for better CommonMark and GFM compliance, bumps the YAML parser to yaml v2, and adds GraphQL.js v17 support. The Flow parser switches to a Rust-based oxidized parser that's roughly 40% faster. JavaScript and TypeScript formatting gets a batch of bug fixes around comment placement, no-semi mode, union types, and optional chaining. If you're on Prettier and use Flow, the parser speedup alone is worth the upgrade. [Read more](https://daily.dev/posts/dBVcSo6bX) ## es-toolkit hits 20M weekly npm downloads after lodash compat layer drives major project adoption es-toolkit started as an internal lodash replacement at Toss, built around native browser APIs and ESM, achieving 2-10x speed improvements and up to 30x smaller bundle sizes by dropping legacy browser support. The inflection point was adding an es-toolkit/compat layer that made migration from lodash low-friction enough for Yarn Berry, Storybook, Mermaid, and Recharts to adopt it. Weekly npm downloads now exceed 20 million. If you're still shipping lodash in a modern stack, this is worth a look. [Read more](https://daily.dev/posts/Yipmew6kF) ## Turbopack stable for next dev in Next.js 15: 76% faster cold starts, sub-200ms HMR Turbopack is now the stable dev server for Next.js 15, replacing Webpack for local development. Cold starts are up to 76% faster and HMR comes in under 200ms. Migration involves moving custom Webpack config — loaders, path aliases — to the new turbopack key in next.config.js. Production builds via next build --turbo are still experimental, so the current recommendation is Turbopack for dev and Webpack for production until that stabilizes. [Read more](https://daily.dev/posts/zZWrwHi0t) --- ## Also notable - **headroom CLI: 60-94% token reduction for JS/TS files sent to LLMs:** headroom is an open-source CLI that uses AST-level analysis to strip comments and redundant syntax from JS/TS files before sending them to LLM APIs, with three profiles: gentle (60-63%), moderate (75-77%), and aggressive (90-94%) reduction — benchmarked against a Next.js SaaS app, Express API, and React component library at GPT-4o pricing. [Read more](https://daily.dev/posts/WiCtib0Ov) - **React 19 useActionState and useOptimistic collapse form boilerplate from ~50 lines to ~12:** React 19's two stable hooks replace the standard loading/error/data useState trio and manual optimistic state snapshots, with the key gotcha being that addOptimistic must be called synchronously before any await inside a React transition. [Read more](https://daily.dev/posts/weEZlGZ6n) - **CSS-only adventure game: 1,600 lines, no JavaScript, hidden checkbox state machine:** A playable adventure game with inventory management built in ~1,600 lines of CSS using hidden checkboxes and the :checked pseudo-selector to manage game state and viewport navigation — a useful reference for how far CSS state management can actually go. [Read more](https://daily.dev/posts/lswacmIxE) ## Similar posts on daily.dev - [Prettier 3.9: New Parser Foundations and Formatting Improvements · Prettier](https://daily.dev/posts/prettier-3-9-new-parser-foundations-and-formatting-improvements-prettier-dbvcso6bx) · Prettier · 28 upvotes · 4 comments --- Tags: [#webdev](https://daily.dev/tags/webdev), [#rust](https://daily.dev/tags/rust), [#nextjs](https://daily.dev/tags/nextjs) [View this post on daily.dev](https://daily.dev/posts/flow-ports-to-rust-in-v0-319-prettier-3-9-ships-micromark-and-a-faster-flow-parser-9kc56acfo)