[lhorie]

Eh. That hasn't been my experience. FWIW, having seen the codebase you're referring to, I don't consider a huge ignore list exactly a success story. The bulk of the work is merely deferred as tech debt at this point.

To be fair, many Flow upgrades are easy, yes. But some are absolutely nightmarish. The 0.85 upgrade was especially painful - it involved some very non-trivial codemods, we couldn't get it right in all cases, and it involved some loss of type safety as well :(

I've also found Flow to be painful to work with when dealing with a multi-repo setup which exports types from source code. It becomes unnecessarily hard to make libraries interoperable because a Flow upgrade in one project can cause a cascade of errors deep in another project's transitive dep. One basically has no actionable recourse other than waiting until all her deps are version-aligned on Flow (or never upgrading).

In that regard, the Typescript pattern of using .d.ts files as a boundary between a library's tsc version and the consumers's tsc version is quite nice and something I'd like to explore more w/ Flow.