Fp-ts is one of like, 4 things in life I feel the need to shill for. It's docs are a little rough coming into it for the first time, and I think some of gcanti's tutorials are a little to complex. But I've slipped it into 3 or 4 moderate sized projects. Every time someone goes to touch it there's initial confusion, a 5 minute explanation of Either, 5 minutes of Q and A, and then they love it.

We have, and it's Got The Spirit, but it's still not the same. It's just so much more verbose, so much noisier, and has so much less compiler safety. You don't get things like non-eager evaluation or effect tracking, your stack traces become much messier, and your open source dependencies are still in plain JS.

Overall, we found that fighting against the language this way was the worst of both worlds - you paid a non-trivial cost and did not gain enough benefit. But our original codebase also predated TypeScript (remember TLDRLegal? That's us!), and much of our TS was bolted on after-the-fact on a pretty significant existing codebase that was written in very idiomatic JS. A team with more resources to do technical refactoring (we were quite small and very focused on building product) or starting from scratch may have a different experience.