If we're talking HN trends then I'd say Haskell comes back around roughly every 6-7 years. There was a big push around 2008-2010, again in 2016-2017, so... maybe next year we'll start seeing it come back around?
I work in Haskell full time now and have for more than a few years at this point. The ecosystem is small because there aren't any network effects propping up Haskell's popularity. We don't have large corporations like M$, Google, etc pouring dosh into GHC development, tooling, etc. It mainly survives on the community of dedicated folks working in academia, their spare time, and contributions from the small (growing!) pool of companies investing in it. Progress happens but it's slow.
This is one factor that can contribute to the cyclic nature of FP trends, Haskell specifically; a handful of influential people discover it, learn a bunch in their free time, write about it, and then when it fails to catch on they move on.
Another factor that contributes is... well network effects. Potential new programmers aren't rushing out to learn Haskell/OCaml/F# because there aren't a whole lot of jobs using it, there aren't a lot of courses teaching it, and there aren't many people recommending it. It also means that established language ecosystems are free to adopt ideas and features from the FP community in their own languages which further prevents people from leaving their ecosystem and adopting another language. Sure, C# may not be a great functional programming language but it's good enough and you don't have to fully buy in: you can use FP patterns when it feels appropriate and OOP ones when that works better (C# has the advantage of the .NET runtime which F# uses and the two can interop well... further preventing any reason to leave that space).
The set of Haskell programmers isn't empty. It's filled with people who are rather dedicated and passionate! And sometimes people new to Haskell join, learn something, and leave for various reasons. Some stay.
As for Rust well... if you look at just HN, again, I think you see these hype cycles. The early-mid 2010's the big trend was "X written in Go." Now it's, "X written in Rust." You don't see that happen much with OCaml/Haskell/F#... probably, again, because of the aforementioned effects. Either the pool of candidates re-writing existing tools is small enough that they can't break through the current hype-cycle or they're not re-writing those things and are carrying on with their work.
I work in Haskell full time now and have for more than a few years at this point. The ecosystem is small because there aren't any network effects propping up Haskell's popularity. We don't have large corporations like M$, Google, etc pouring dosh into GHC development, tooling, etc. It mainly survives on the community of dedicated folks working in academia, their spare time, and contributions from the small (growing!) pool of companies investing in it. Progress happens but it's slow.
This is one factor that can contribute to the cyclic nature of FP trends, Haskell specifically; a handful of influential people discover it, learn a bunch in their free time, write about it, and then when it fails to catch on they move on.
Another factor that contributes is... well network effects. Potential new programmers aren't rushing out to learn Haskell/OCaml/F# because there aren't a whole lot of jobs using it, there aren't a lot of courses teaching it, and there aren't many people recommending it. It also means that established language ecosystems are free to adopt ideas and features from the FP community in their own languages which further prevents people from leaving their ecosystem and adopting another language. Sure, C# may not be a great functional programming language but it's good enough and you don't have to fully buy in: you can use FP patterns when it feels appropriate and OOP ones when that works better (C# has the advantage of the .NET runtime which F# uses and the two can interop well... further preventing any reason to leave that space).
The set of Haskell programmers isn't empty. It's filled with people who are rather dedicated and passionate! And sometimes people new to Haskell join, learn something, and leave for various reasons. Some stay.
As for Rust well... if you look at just HN, again, I think you see these hype cycles. The early-mid 2010's the big trend was "X written in Go." Now it's, "X written in Rust." You don't see that happen much with OCaml/Haskell/F#... probably, again, because of the aforementioned effects. Either the pool of candidates re-writing existing tools is small enough that they can't break through the current hype-cycle or they're not re-writing those things and are carrying on with their work.