For example, I got into Ruby via Rails, because Rails lets you quickly prototype simple web apps. So I could go from "I wish I had an app that does X" to actually building it, deploying it and sharing it with others. What would a similar "learning flow" look like in Haskell? (doesn't have to be web-based)
Put another way, when I come across a problem, how do I recognize it as the type of problem that is best solved using Haskell, vs. an imperative language?
I came from Python to Erlang / Scheme / Haskell and at this point I would answer your question,
> What kind of things can I build with Haskell?
With: Everything.
We use Haskell in production at Plum for our REST APIs, job schedulers, web applications, AWS service interfaces, a static site compiler, DB modeling, command line utilities, etc...
We also use it for two CLI utilities that are cross-compiled for the ARM9 on our IoT product.
I consider Haskell to be superior to any of the dynamically typed languages when writing production-level code, it's cleaner, safer, easier to maintain, easier to refactor, and much more fun IMHO.
[EDIT] I neglected the other part of your question, "What is the learning flow like?"
Definitely a bit rougher than Python or Ruby, I will not lie, but don't be discouraged. It simply means you need to do a bit more studying up-front first before you can tinker without being caught at every turn by the straight-jacket.
I would first go through Learn You a Haskell because it is pretty accessible and introduces the language basics well enough. Then study the type system. You must learn Haskell's type system and terminology before you can understand more advanced code.
This may be nitpicking but certain classes of programs cannot be realistically built with Haskell. Anytime you need to tightly control latency (soft realtime) won't really work since you have both a garbage collector and lazy evaluation. Memory constrained systems are pretty tough as well since you don't really get insight into allocation/deallocation, which also makes structuring your data into a particular memory layout tricky compared to C(++) for instance.
Not to say that Haskell isn't awesome. It should probably be used for more systems. It just can't be the "hammer" to make every problem into a nail.
That said, it's perfectly possible to write a DSL that handles scheduling, uses GHC's type machinery to track memory use and execution time, and have GHC generate a program that will generate C code that meets hard real-time guarantees. In fact, someone wrote it, it's available on hackage (https://hackage.haskell.org/package/atom) and my understanding is that it's used in production for control software in the auto industry.
It was slow at first, and there was a lot of "now what's this arrow doing here?", but I would go back to the books when I had questions and eventually things started making sense. That won't work for everyone, but maybe it will work for you too?
The folks in #haskell are generally pretty helpful, and delightfully easy to troll.
[2]: http://www.stephendiehl.com/llvm/ [1]: http://yannesposito.com/Scratch/en/blog/Haskell-the-Hard-Way...
There's a reasonable perspective that says the type of problem that's best solved in a language is a problem for which there's good library support. For that, I'd probably recommend scanning https://github.com/Gabriel439/post-rfc/blob/master/sotu.md
But in my experience, where Haskell really shines is sketching out operations on some type that I know I'm going to get wrong - substantially or subtly - my first many tries. When I go to fix it, the compiler helps me find everything I need to change in tandem, and feel ahead to find inconsistencies in my model before I get there in the code. One example of this kind of problem is compilers - where we parse into an AST, transform the AST, and then produce other things from the AST. As development goes on, the AST evolves, and you have a lot of help knowing what needs to change to match and what you can ignore. I've recently been doing this for SQL (though with the goal of analyzing the queries, not producing an executable).
https://pbrisbin.com/posts/developing_web_applications_with_...
You might want to try using stack instead of cabal though to ensure you avoid any dependency issues. I think cabal will work fine these days, but I have been using stack/stackage for some time so can't guarantee it.
