TL;DR. Types-First will become the only mode in Flow in v0.143 (mid Jan 2021). Classic mode, currently enabled with types_first=false, will no longer be supported.

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In May of 2020 we announced Flow Types-First mode. This new architecture unlocked big gains for Flow’s performance at scale and enabled us to tackle some long-standing reliability issues. To do so, this mode assumes some strong new invariants, most importantly that module boundaries are annotated with types. …


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TL;DR: The types-first architecture unlocks Flow’s potential at scale by leveraging fully typed module boundaries. We plan to migrate to the new architecture over the next months.

Flow is designed to type check extremely large projects. Over the past several years, we’ve introduced some big changes to enable Flow to scale exponentially to 10M+ lines of code, just barely keeping pace with Facebook’s codebases. For example, lazy mode only checks files affected by local changes, instead of the entire codebase. …

Panagiotis Vekris

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