[galaxyLogic]

That is a very good question. The discussion often circles around pro- and against FP. Proponents of "pure FP" seem to suggest that it is the best solution for everything always. Is it?

If not it would be nice to find discussion about when it is and when not and why? What is Haskell NOT the best solution for?

[valcron1000]

In my experience Haskell is a bad solution when you need fine control over memory usage and control flow. For those cases you should probably use something like C, C++ or Rust. For everything else I think is a viable option.

The hard part is understanding how you should model your problem. I found that you could classify problems into two big groups: pipelines like compilers or web services where you get an input, do some processing and produce an output, and graph problems like GUI applications where the system is a living thing and events change how components should behave. The first group is easy, the second not so much. Excel is the best example of this kind of system: an user describes relationships between cells and lets the runtime perform the appropriate side effects. Ideally we would be able to write arbitrary programs using this same model.

[galaxyLogic]

It took me some time to understand how Haskell performs its "magic" but now I think I do. Anybody please correct me if I'm still getting it wrong...

Haskell main program in essence runs a big loop until it decides the program should exit. In Haskell syntax the loop is coded as a call to a tail-recursive function, which on a real hardware of course needs to be translated to do iteration. At the end of each loop it sets the new calculated value of the new state to be used during the next loop.

Using tail-recursion optimization it looks like our whole program executes a single top-level function-call of a recursive function, that is how you code it in Haskell.

This is great but somehow it feels like a trick. It is actually still doing iteration and state mutation internally while running. You could program that in an imperative language writing a top-level loop and iterating over it and modifying the statefull variables at the end of each loop.

And what happens if you cannot write your program as tail-recursive function? What if the problem domain requires you to use general recursion, which can not be optimized away like tail-recursion can?

[hither_shores]

No, that's not accurate. The final form of your Haskell program before it's compiled down to some assembly language (currently C-- or LLVM) is a graph reduction problem. The partially imperative part is the runtime system, which actually reduces the graph (= runs your program), but this is significantly more complicated than the sort of high level loop transformations you're talking about. Search "spineless tagless G-machine" for a (somewhat idealized) description of the core architecture of GHC.

[valcron1000]

What you are trying to describe is not Haskell the language but an implementation, and right now the de facto implementation is GHC. I really can't talk about how GHC works internally, but when it comes to the language you need to think your program as a value, just like `5` or `[1,2,3]` or `{ "name": "susan" }`. That is, effects are values, you compose those effects, and, your program being some kind of effect, is also a value. That's the reason behind `main :: IO ()`: your program is an `IO value`.

When it comes to "executing" your program, a runtime (like GHC's runtime) takes your program which is a value and actually performs the requested actions, dealing with mutation, loops, jumps, etc. This is a hack just as much as any other language dealing with variables instead of the stack, heap or registries directly.

In case you're interested, this idea of using some kind of interpreter which loops over your program and performs mutations and side effects is very popular in Scala: libraries like Cats and ZIO do this, allowing you to effectively get Haskell's IO. As far as I know, tail-recursion is not really needed for this, but when implemented on the runtime it allows the end user to write loops using recursion without blowing up the stack.

[galaxyLogic]

Right. I was thinking of how to write a game in Haskell or similar. I need to write some kind of loop to keep my program/game running indefinitely. And I need to avoid "blowing up" the stack by writing my program as a set of calls to tail-recursive functions. No?

The program must keep the previous state somewhere and then modify it based on the player's inputs and again store that new state somewhere somehow so it is remembered when next input comes. If loops which keep and modify state are not allowed, then I don't see how it could be done without tail-recursive functions.