fp-ts

https://github.com/gcanti/fp-ts

Although the `fp-ts` ecosystem seems to be relatively mature. It doesn’t need to be `fp-ts` specifically.

https://github.com/nullpub/fun may be promising as long as long-term support is scheduled.

An official or community supported functional programming library, maybe in the standard library, plays a significant role over the code quality of Deno applications and libraries in the wild.