Well there's C# / .NET, which ticks off all of those boxes, even the functional syntax is well supported since it added pattern matching and people write a lot of fluent functional style anyway with LINQ.
It also interops nicely with F#, so you can write a pure functional library in F# and call it from a C# program in the "functional core, imperative shell" style.
It has an incredibly solid runtime, and a good type system that balances letting you do what you want without being overly fussy.
It misses the frontend/backend symmetry and has too large a coupling to Microsoft and Windows in my head. I know that these days it's supposed to be cross platform, but every time I've tried to figure out how to install it I get lost in the morass of nearly-identical names for totally different platforms and forget which one I'm supposed to be installing on Linux these days.
That doesn't mean there's anything wrong with it and I've often thought to give it another shot, but it's not a viable option right now for me because it's been too hard to get started.
>but every time I've tried to figure out how to install it I get lost in the morass of nearly-identical names for totally different platforms and forget which one I'm supposed to be installing on Linux these days.
I realize Microsoft is terrible at naming things, but for .NET/C# it's really not that hard these days. If you want to use the new, cross platform .NET on Linux then just install .NET 8 or 9.
New versions come out every year, with the version number increasing by one each year. Even numbered versions are LTS, odd numbered releases are only supported for about a year. This naming scheme for the cross-platform version of .NET has been used since .NET 5, almost 5 years ago, it's really not too complicated.
Fair enough, I guess I haven't looked in the last few years. The last time that I did a search for .NET there were about five different names that were available and Mono still turned up as the runtime of choice for cross platform (even though I knew it wasn't any more).
To clear things up for you a bit more (hopefully, or I'll just make it worse):
Any legacy .NET projects are made with .NET Framework 4.x (4.8.1 8s the latest). So if it's 4.x, or called .NET Framework instead of just .NET, it's referring to the old one.
.NET Core is no longer used as a name, and hasn't been since .NET Core 3.1. They skipped .NET Core t completely (to avoid confusion with the old one vut I think they caused confusion with this decision instead) and dropped the Core for .NET 5. Some people will still call .NET 5+ .NET Core (including several of my coworkers) which I'm sure doesn't help matters.
Mono isn't 100% completely dead yet, but you'll have little if any reason to use it (directly). I think the Mono Common Language Runtime is still used behind the scenes by the newer .NET when publishing on platforms that don't allow JIT (like iOS). They've started adding AOT compilation options in the newest versions of .NET so I expect Mono will be dropped completely at some point. Unless you want to run C# on platforms like BSD or Solaris or other exotic options that Mono supports but the newer .NET doesn't.
It also interops nicely with F#, so you can write a pure functional library in F# and call it from a C# program in the "functional core, imperative shell" style.
It has an incredibly solid runtime, and a good type system that balances letting you do what you want without being overly fussy.