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Is Haskell Worth Learning in 2022? Yes, Haskell is worth learning in 2022 because functional languages like it are getting more popular among big companies like Facebook. Functional languages are typically ideal for big data and machine learning.
Out of mainstream languages, I consider C++ the hardest. Theorem-proving languages (like Agda or Coq) are harder than Haskell conceptually. Haskell isn't a hard language, but it takes time to learn its pattern and libraries (both standard and third-party).
As mentioned earlier, Python is easier than Haskell to learn. The learning curve for Haskell is steep, especially for those with no prior functional programming experience. In terms of library support, Python has more libraries and use-cases than Haskell.
Some of the Haskell alternatives which we will be discussing are Java, Lua, Dart, Scala, Kotlin, Haxe, Nim, Asp.net, etc.
I would like to broaden my knowledge of programming. (...) I thought I would pose the question here, along with several stipulations about the type of language I am looking for. Some are subjective, some are intended to ease the transition from Haskell.
Strong type system. (...) It also makes informally reasoning about the correctness of my program easier. I'm concerned with correctness, not efficiency.
Emphasis on recursion rather than iteration. (...)
You may be easing the transition a bit too much here, I'm afraid. The very strict type system and purely functional style are characteristic of Haskell and pretty much anything resembling a mainstream programming language will require compromising at least somewhat on one of these. So, with that in mind, here are a few broad suggestions aimed at retaining most of what you seem to like about Haskell, but with some major shift.
Disregard practicality and go for "more Haskell than Haskell": Haskell's type system is full of holes, due to nontermination and other messy compromises. Clean up the mess and add more powerful features and you get languages like Coq and Agda, where a function's type contains a proof of its correctness (you can even read the function arrow -> as logical implication!). These languages have been used for mathematical proofs and for programs with extremely high correctness requirements. Coq is probably the most prominent language of the style, but Agda has a more Haskell-y feel (as well as being written in Haskell itself).
Disregard types, add more magic: If Haskell is sorcery, Lisp is the raw, primal magic of creation. Lisp-family languages (also including Scheme and Clojure) have nearly unparalleled flexibility combined with extreme minimalism. The languages have essentially no syntax, writing code directly in the form of a tree data structure; metaprogramming in a Lisp is easier than non-meta programming in some languages.
Compromise a bit and move closer to the mainstream: Haskell falls into the broad family of languages influenced heavily by ML, any of which you could probably shift to without too much difficulty. Haskell is one of the strictest when it comes to correctness guarantees from types and use of functional style, where others are often either hybrid styles and/or make pragmatic compromises for various reasons. If you want some exposure to OOP and access to lots of mainstream technology platforms, either Scala on the JVM or F# on .NET have a lot in common with Haskell while providing easy interoperability with the Java and .NET platforms. F# is supported directly by Microsoft, but has some annoying limitations compared to Haskell and portability issues on non-Windows platforms. Scala has direct counterparts to more of Haskell's type system and Java's cross-platform potential, but has a more heavyweight syntax and lacks the powerful first-party support that F# enjoys.
Most of those recommendations are also mentioned in other answers, but hopefully my rationale for them offers some enlightenment.
I'm going to be That Guy and suggest that you're asking for the wrong thing.
First you say that you want to broaden your horizons. Then you describe the kind of language that you want, and its horizons sound incredibly like the horizons you already have. You're not going to gain very much by learning the same thing over and over.
I would suggest you learn a Lisp — i.e. Common Lisp, Scheme/Racket or Clojure. They're all dynamically typed by default, but feature some sort of type hinting or optional static typing. Racket and Clojure are probably your best bets.
Clojure is more recent and has more Haskellisms like immutability by default and lots of lazy evaluation, but it's based on the Java Virtual Machine, which means it has some odd warts (e.g. the JVM doesn't support tail call elimination, so recursion is kind of a hack).
Racket is much older, but has picked up a lot of power along the way, such as static type support and a focus on functional programming. I think you'd probably get the most out of Racket.
The macro systems in Lisps are very interesting and vastly more powerful than anything you'll see anywhere else. That alone is worth at least looking at.
From the standpoint of what suits your major, the obvious choice seems like a logic language such as Prolog or its derivatives. Logic programming can be done very neatly in a functional language (see, e.g. The Reasoned Schemer) , but you might enjoy working with the logic paradigm directly.
An interactive theorem proving system such as twelf or coq might also strike your fancy.
I'd advise you learn Coq, which is a powerful proof assistant with syntax that will feel comfortable to the Haskell programmer. The cool thing about Coq is it can be extracted to other functional languages, including Haskell. There is even a package (Meldable-Heap) on Hackage that was written in Coq, had properties proven about its operation, then extracted to Haskell.
Another popular language that offers more power than Haskell is Agda - I don't know Agda beyond knowing it is dependently typed, on Hackage, and well respected by people I respect, but those are good enough reasons to me.
I wouldn't expect either of these to be easy. But if you know Haskell and want to move forward to a language that gives more power than the Haskell type system then they should be considered.
As you didn't mention any restrictions besides your subjective interests and emphasize 'rewarding to learn' (well, ok, I'll ignore the static typing restriction), I would suggest to learn a few languages of different paradigms, and preferably ones which are 'exemplary' for each of them.
Especially Lisps (CL not as much as Scheme) and Prolog (and Haskell) embrace recursion.
Although I am not a guru in any of these languages, I did spend some time with each of them, except Erlang and Forth, and they all gave me eye-opening and interesting learning experiences, as each one approaches problem solving from a different angle.
So, though it may seem as if I ignored the part about your having no time to try a few languages, I rather think that time spent with any of these will not be wasted, and you should have a look at all of them.
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